research papers
Describing smallangle scattering profiles by a limited set of intensities
^{a}Department of Structural Biology, Jacobs School of Medicine and Biomedical Sciences, University at Buffalo, NY 14203, USA
^{*}Correspondence email: tdgrant@buffalo.edu
Smallangle scattering (SAS) probes the size and shape of particles at low resolution through the analysis of the scattering of Xrays or neutrons passing through a solution of particles. One approach to extracting structural information from SAS data is the indirect Fourier transform (IFT). The IFT approach parameterizes the realspace pair distribution function [P(r)] of a particle using a set of basis functions, which simultaneously determines the scattering profile [I(q)] using corresponding reciprocalspace basis functions. This article presents an extension of an IFT algorithm proposed by Moore [J. Appl. Cryst. (1980), 13, 168–175] which used a trigonometric series to describe the basis functions, where the realspace and reciprocalspace basis functions are Fourier mates. An equation is presented relating the Moore coefficients to the intensities of the SAS profile at specific positions, as well as a series of new equations that describe the size and shape parameters of a particle from this distinct set of intensity values. An analytical realspace regularizer is derived to smooth the P(r) curve and ameliorate systematic deviations caused by series termination. Regularization is commonly used in IFT methods though not described in Moore's original approach, which is particularly susceptible to such effects. The algorithm is provided as a script, denss.fit_data.py, as part of the DENSS software package for SAS, which includes both command line and interactive graphical interfaces. Results of the program using experimental data show that it is as accurate as, and often more accurate than, existing tools.
Keywords: smallangle scattering; indirect Fourier transform; solution scattering; pair distribution function.
1. Introduction and overview
Smallangle scattering (SAS) yields structural information at low resolution about the size and shape of particles in solution. Xrays or neutrons scattering from freely tumbling particles in solution exhibit rotational averaging in I(q), where q is the momentum transfer [q = (4π/λ)sinθ, where θ is half the scattering angle and λ is the wavelength of the incident radiation], is determined by its 3D scattering length density function, and thus SAS profiles can be calculated directly from known atomic structures. However, due to the spherical averaging of the intensities, the inverse problem of calculating a unique 3D structure from SAS profiles is not possible. Nonetheless, structural information describing global properties of size and shape can be obtained through analysis of the SAS profile.
resulting in isotropic scattering profiles collected on 2D detectors. This rotational averaging results in the loss of information describing the 3D structure of the particle. The scattering of a moleculeWhile unique 3D realspace information cannot be obtained directly from a SAS profile, a Fourier transform of the reciprocalspace intensity profile yields the set of pair distances in the particle, known as the pair distribution function or P(r). However, due to limitations caused by the termination of higherorder scattering data to a finite q range, uncertainties in intensity measurements and systematic errors, direct calculation of the Fourier transform yields P(r) functions with large systematic deviations (Glatter, 1977; Moore, 1980; Hansen & Pedersen, 1991; Svergun, 1992; Svergun & Pedersen, 1994). One popular approach to extracting this structural information from SAS profiles is the indirect Fourier transform (IFT) proposed by Glatter (1977). In this approach, a set of basis functions is used to parameterize the P(r) function. The weights of these basis functions are then adjusted to optimize the fit of the corresponding intensity function to the experimental scattering profile.
One such IFT algorithm proposed by Moore (1980) takes advantage of information theory (Shannon, 1948) to describe a set of basis functions defined by the maximum particle dimension D. Moore uses a trigonometric series to define a function Q(r) = P(r)/r. This definition resulted in a convenient relationship between the realspace Q(r) and the reciprocalspace U(q) = qI(q), where the two are Fourier mates. Key to Moore's approach (and other IFT methods; Glatter, 1977; Svergun, 1992) is that the coefficients of the series terms define both the realspace and reciprocalspace profiles, using the appropriate basis functions. Least squares can be used to determine the coefficients and the associated standard errors by minimizing the fit to the experimental scattering profile (full details are given in Section S1 of the supporting information). This approach has the advantage of providing the necessary information on the variances and covariances of the coefficients to determine the errors on each coefficient. Moore showed, using Shannon information theory, that the number of coefficients that can be determined from the data is the number of independent pieces of information that the data are able to describe about the particle. Moore derived a series of equations relating the coefficients to commonly used SAS parameters such as the intensity I(0), the R_{g} and the average vector length , along with error estimation for each parameter. One advantage of Moore's approach over others is that a separate regularizing function is not explicitly required to smooth the P(r) curve due to the use of the sine series (Moore, 1980). However, in practice with experimental data, it has been found that Moore's approach is often more susceptible to large oscillations in the P(r) curve due to series termination (Svergun & Pedersen, 1994; Hansen & Pedersen, 1991), probably because of the lack of a regularizing function. Such regularizing functions have been shown to be effective at smoothing the P(r) curves calculated using Moore's approach (Tully et al., 2021; Rambo, 2021).
Here we extend Moore's derivation to relate the Moore coefficients to specific intensity values such that each term in the series is now weighted by a corresponding intensity, termed I_{n} (Section S1 in the supporting information). We present equations for calculating a variety of commonly used SAS parameters and their associated errors from the I_{n} values. Additionally, we derive a modified equation for leastsquares minimization taking into account an analytical regularization of the P(r) curve. We provide opensource software with convenient interfaces for performing all of the presented calculations, including a novel approach to estimating parameters sensitive to systematic errors. Finally, we describe the results using both simulated and real experimental data and compare with current stateoftheart software tools.
2. Theoretical background
2.1. Extension of Moore's IFT
Moore's use of Shannon information theory to define I(q) resulted in a selection of q values, namely q_{n} = nπ/D, termed `Shannon channels' (Feigin & Svergun, 1987; Svergun & Koch, 2003; Rambo & Tainer, 2013). The intensities at q_{n}, i.e. I_{n} = I(q_{n}), therefore become important values as they determine the Moore coefficients a_{n} and thus similarly can be used to describe completely the lowresolution size and shape of a particle obtainable by SAS. In Section S1 we derive the mathematical relationship between I_{n} and a_{n} which results in the following general equation for I(q) as a function of the intensity values at the Shannon points:
Defining basis functions B_{n} as
I(q) can now be expressed as a sum of the basis functions B_{n} weighted by the intensity values at q_{n},
As in Moore's original approach, the B_{n} functions are determined by the maximum dimension of the particle D. B_{n} values for D = 50 Å are illustrated in Fig. 1. The P(r) function can be represented using the series of I_{n} values as
(Section S1) or by defining realspace basis functions S_{n} as follows:
Least squares can be used to determine optimal values for each I_{n} from the oversampled experimental SAS profile, along with error estimates for each, taking into account the variances and covariances of the coefficients. These terms can then be used to calculate the corresponding I(q) and P(r) curves using equations (1) and (4) and the associated errors (Section S1).
The maximum particle dimension D is required for determining the q_{n} values associated with the I_{n} values. Estimates for the true value of D that are too small will result in B_{n} values that lack sufficiently high frequencies for the adequate reconstruction of I(q). Estimates of D that are too large will result in overfitting the data. Moore found that testing increasing values of D yielded improved fits to the experimental I(q) function and used χ^{2} (Section S1) to estimate the true value of D by selecting the smallest D value that minimizes χ^{2} while avoiding larger D values that result in overfitting (Moore, 1980). An alternative method is to estimate D from the P(r) curve by first guessing a reasonable value for D, such as 3.5R_{g} or larger, fit I(q) and calculate the P(r) curve, and then estimate the true value of D on the basis of where P(r) gradually falls to zero.
2.2. Derivation of parameters from I_{n} values
Similarly to what Moore described for the a_{n} coefficients, since the I_{n} values contain all the information present in I(q), quantities that can be derived from I(q) can also be derived directly from the I_{n} values. For example, to determine the intensity I(0), we take the limit of equation (1) as q approaches zero to yield
Equation (7) demonstrates a simple relationship between the of a particle and the I_{n} values. Note that the particle dimension D is not explicitly present in equation (7). Fig. 2 illustrates the relationship between the I_{n} values and I(0).
The q data points or by integration of the P(r) function. Equation (7) provides an alternative method of measuring the of a particle directly from the data through the sum of the I_{n} values. While equation (7) is defined as a sum from n = 1 to infinity, typical experimental setups only provide data for the first 10–30 Shannon channels, depending on the size of the particle. Thus in practice equation (7) yields an estimate of the rather than an exact measurement. However, since the vast majority of the scattering intensity present in the profile occurs within these 10–30 Shannon channels, equation (7) should provide an accurate estimate of the for most particles and experimental setups.
of a particle is not directly measured in an experiment due to its coincidence with the incident beam and is thus typically estimated as an extrapolated value from lowOther parameters can be similarly derived (Section S2). For example, R_{g} can be estimated from the I_{n} values as
where
Another parameter describing particle size is the average vector length in the particle , which can be estimated from the I_{n} values as
where
The Porod invariant Q is defined as the integrated area under the (Porod, 1982), which can be described in terms of the I_{n} values as
The Porod volume can then be calculated using the Porod invariant (Section S2) (Porod, 1982). The Porod volume is commonly used to estimate molecular weight for globular biological macromolecules. More recently, Rambo & Tainer (2013) derived a new SAS invariant termed the volume of correlation, V_{c}, with units of length^{2} and which is related to the correlation length of the particle ℓ_{c}. V_{c} can be used to estimate the molecular weight for macromolecules that may be either globular or flexible (Rambo & Tainer, 2013). V_{c} can be estimated from the I_{n} values as
where Si(nπ) is the Sine integral. The correlation length can similarly be calculated as
Since the variances and covariances of the I_{n} values are known from the leastsquares minimization, error propagation can be used to determine the associated uncertainties for each of the parameters described above (Section S2).
2.3. Regularization of P(r)
The original IFT proposed by Glatter (1977) and other IFTs (Svergun, 1992; Vestergaard & Hansen, 2006) make use of regularization of the P(r) curve, similar to the general method of Tikhonov regularization (Tikhonov & Arsenin, 1977). The goal is to use the knowledge that P(r) functions are smooth for most particle shapes to generate curves that are free of strong oscillations from series termination and are relatively stable to statistical errors. Rather than minimize χ^{2} directly, a new function T is minimized, taking into account the smoothness of the P(r) curve according to equation (15):
where S is the regularizing function, which can take different forms, and α is a Lagrange multiplier that acts as a weight to determine the strength of the smoothing. Larger α leads to a smoother P(r) function but may result in a worse fit of I(q) to the experimental data. The IFT method used by Moore has been shown to be more susceptible than other IFT methods to oscillations in the P(r) curve (Hansen & Pedersen, 1991; Svergun & Pedersen, 1994), most likely due to the lack of a regularizing function. We provide a detailed derivation of an analytical regularization of P(r) using I_{n} values in Section S3.
As for other similar IFT methods utilizing regularization, a suitable choice of α must be found to optimize the smoothness of the P(r) curve and the fit to the experimental data. Various methods for selecting the optimal value for α have been proposed, including via point of inflection (Glatter, 1977), Bayesian methods (Vestergaard & Hansen, 2006) and using perceptual criteria (Svergun, 1992). We describe our approach in Section 2.4 below.
Equation (3) assumes a sum from n = 1 to infinity. However, data are only collected to the maximum q value allowed by the experiment, q_{max}. The lack of data for q > q_{max} implicitly corresponds to setting the I_{n} values to zero for those data points where n > n_{max} [where n_{max} = int(q_{max}D/π), i.e. the largest index in the series]. The regularization often results in poorer fits of the intensity profile at higher experimental q values with increasing α due to this implicit bias of I_{n} values for n > n_{max} towards zero. In order to remove this bias and allow for the I_{n} values at n > n_{max} to be unrestrained, I_{n} values for n > n_{max} are allowed to float (calculated up to 3n_{max}). Note that the number of Shannon channels that can be reliably extracted from the data is dictated largely by the quality of the data in addition to the q range, as described by Konarev & Svergun (2015).
2.4. Implementation
Tools for performing the leastsquares fitting of I_{n} values to experimental data, calculation of parameters and errors, and regularization of P(r) have been developed using Python, NumPy and SciPy (Harris et al., 2020; Virtanen et al., 2020) and are provided open source through the DENSS suite of SAS tools (Grant, 2018; https://github.com/tdgrant1/denss). The primary interface to use this algorithm is the denss.fit_data.py Python script. To enable ease of use, in addition to the command line interface, an interactive graphical user interface (GUI) (Fig. 3) has been developed using the Matplotlib package (Hunter, 2007).
2.4.1. Automatic estimation of D
To assist users, upon initialization of the script the experimental data are loaded and estimates of D and α are automatically calculated. To estimate D automatically, an initial estimate of D is calculated that is likely to be significantly larger than the actual D. This subsequently enables a more accurate estimation of D where P(r) falls to zero. An initial value of D = 7R_{g} is used as this should ensure a large enough value given a variety of particle shapes (Petoukhov et al., 2007; Grant et al., 2015). An initial rough estimate of R_{g} is first calculated using the Guinier equation (Guinier et al., 1955) with the first 20 data points. In cases where that estimate fails (e.g. due to excessive noise or a positive slope of the Guinier plot), the Guinier peak method is instead used (Putnam, 2016). The I_{n} values are then calculated from the experimental data using the regularized leastsquares approach outlined in Section S3, setting α = 0 to optimize the fit to the data. After the initial I_{n} values have been calculated, the corresponding P(r) function often suffers from severe ripples caused by Fourier termination effects due to the finite range of data, as described above, making it difficult to estimate D where P(r) falls to zero. To alleviate this effect, a Hann filter, which is a type of Fourier filter (Blackman & Tukey, 1958), is applied to remove the Fourier truncation ripples from P(r). D is then calculated from this filtered P(r) curve as the first position r where P(r) falls below 0.01P_{max} after the maximum, where P_{max} is the maximum value of the filtered P(r). This new D value is then used to recalculate the I_{n} values for the best fit to the experimental scattering profile. In addition to automatically estimating D directly from the data, users can manually enter an initial estimate of D to begin with.
2.4.2. Automatic estimation of α
Next, the optimal α is estimated, which yields I_{n} values corresponding to a smooth P(r) function while still resulting in a calculated I(q) curve that fits the experimental data. First, the best χ^{2} value possible is calculated by setting α = 0 and using the D value estimated in the previous step. Then, various values of α are scanned, from 10^{−20} to 10^{20} in logarithmic steps of 10^{1}. This wide range is used to accommodate a variety of different scattering profiles covering a range of signaltonoise values. At each step the χ^{2} is calculated. The optimal α is chosen by interpolating where , i.e. where χ^{2} rises to 10% above the best possible value.
2.4.3. Interface
The GUI mode of the script displays a plot of the intensities on a semilog y axis and plots the experimental data I_{e}(q) and the initial fit I_{c}(q), calculated from the I_{n} values at the experimental q (Fig. 3). The script additionally calculates I_{c}(q) at q values extrapolated to q = 0. Users can alternatively provide a set of desired q values to calculate I_{c}(q) as an ASCII text file when starting the program. The residuals, [I_{e}(q_{i}) − I_{c}(q_{i})]/σ_{i}, are also displayed to assist in assessing the quality of the fit. Next to the plot of intensities, the P(r) curve calculated from the I_{n} values is also displayed. In addition to input text boxes for manually entering new D and α values in the GUI, interactive sliders are available to change the D and α values, which automatically update the plots as they are adjusted. Users can also change the beginning and ending data points if desired, to remove outlier data points that often occur at either end of the experimental profile, or disable the calculation of intensities for q > q_{max}. Several of the parameters described above, including I(0), R_{g}, , V_{p}, V_{c} and ℓ_{c}, along with associated uncertainties, are calculated from the I_{n} values and displayed in the GUI. These parameters are updated interactively whenever D or α are changed.
2.4.4. Calculation of V_{p}, V_{c} and ℓ_{c}
Particular care must be taken when estimating parameters that are sensitive to systematic errors in highq data points, such as V_{p}, V_{c} and ℓ_{c}. In practice, direct estimation of these parameters using the equations described above may yield unstable results, even with regularization. Porod's law is based on the assumption that all scattering comes from the surface of a particle, resulting in an asymptotic intensity decay proportional to q^{−4} (Porod, 1982), giving rise to the ability to estimate values such as the Porod volume V_{p}. In practice, shape scattering contributes significantly (Rambo & Tainer, 2011), as do systematic errors caused by inaccurate background subtraction (ManalastasCantos et al., 2021), resulting in poor estimation of these parameters without correction. To deal with this, many algorithms impose an artificial constant subtraction to force the Porod decay, which has proven effective at providing accurate estimates of particle volume (ManalastasCantos et al., 2021). However, different algorithms have different methods for calculating the constant to subtract and for determining the fitting region where these calculations are performed, and there is often subjectivity involved in selecting the appropriate `Porod region' (Rambo & Tainer, 2011; de Oliveira Neto et al., 2021). To avoid such issues with constant subtraction altogether, we have developed a different approach.
In our approach, we take advantage of the regularization provided above by intentionally oversmoothing using a large α. Oversmoothing has the effect of removing shape scattering while simultaneously enforcing a decay similar to Porod's law of q^{−4}, making the resulting scattering profile more consistent with the assumptions of the Porod law. To do this, we multiply α by a factor of 10, which in our tests with experimental data resulted in the most accurate and robust results (see Results section below). We also limit the q range to 8/R_{g}, which has previously been shown to be a reasonable cutoff for calculating Porod volume (ManalastasCantos et al., 2021; de Oliveira Neto et al., 2021). Note that this oversmoothing is only applied for calculation of the three parameters mentioned above and their associated errors and does not affect the actual fit of the scattering profile, P(r) curve or other parameters.
2.4.5. Output
Finally, upon exiting the script, the experimental data and calculated fit of the intensities are saved in a file, with the calculated parameter values saved in the header. The corresponding P(r) curve is also saved.
In addition to providing the denss.fit_data.py script as an interface to the algorithm described above, other scripts in the DENSS package also utilize this algorithm, including denss.py and denss.all.py, to allow automatic fitting of the data and estimation of D and α when using these programs for ab initio 3D density reconstructions.
3. Results
One of the few shapes for which an analytical scattering equation has been derived is the solid sphere (Rayleigh, 1910; Porod, 1982). Since the equation of scattering for a sphere is known exactly, the I_{n} values for a sphere can be calculated directly (Section S4), resulting in equation (16),
Note that the radius R of the sphere does not enter into equation (16). Interestingly, the odd I_{n} values for a sphere decay exactly as q^{−6} and the even I_{n} values decay exactly as q^{−4}. The decay of intensity at higher angles proportional to q^{−4} is described by Porod's law as mentioned above, generally an approximation for most globular particles but here derived analytically for a sphere for even I_{n} values.
All parameters outlined above, including R_{g}, volume etc., can be calculated analytically using equation (16), resulting in well known equations for solid spheres (Section S4). In Fig. 4 the scattering profile for a sphere of radius 25 Å with added Gaussian noise [I_{e}(q)] is shown with the fitted I_{n} values and the recovered I_{c}(q) profile. Eight Shannon points were used to fit the data, from which size parameters were calculated using the fitted I_{n} values, shown in Table 1. The I_{n} values can also be used to calculate the P(r) curve P_{c}(r), shown in Fig. 5 along with the exact P(r) curve for a sphere (Porod, 1982) (Section S4).

Data from publicly accessible databases for experimental SAS data, such as BIOISIS (https://www.bioisis.net) and SASBDB (Valentini et al., 2014), are particularly useful for verification and testing of algorithms such as that described here. To test denss.fit_data.py on experimental data sets, we downloaded two data sets from the benchmark section of the SASBDB online database, in particular SASDFN8 (apoferritin) and SASDFQ8 (bovine serum albumin) (Graewert et al., 2020). Automated estimates of D and α were suitable for accurate fitting and parameter estimation, as indicated by the plot of residuals and comparison with the published parameter values (Fig. 6). Best fits are achieved when setting α = 0, as expected, and increasing α results in smoother P(r) plots. Highquality fits and smooth P(r) curves can be obtained simultaneously with an appropriate α (Fig. 6), while setting α to too large a value results in poorer fits to the intensity profile. Similar to other IFT methods, a balance must be struck to select the optimal α value resulting in the smoothest P(r) function possible while still enabling a good quality fit of I(q).
To compare the parameter estimates with other software, we used DATGNOM from the ATSAS 3.0 package to estimate R_{g} and I(0), DATPOROD to estimate V_{p}, and DATVC to estimate V_{c} from these two data sets (ManalastasCantos et al., 2021). A comparison of parameter values calculated by DATGNOM/DATPOROD/DATVC and denss.fit_data.py is shown in Table 2. Overall, and very importantly for community standards, the values are similar for the two different methods [∼0.1% difference for R_{g} and I(0), and ∼3% difference for V_{p} and V_{c}]. To verify that the error bounds are estimated correctly, we followed the protocol outlined by ManalastasCantos et al. (2021) to use the DATRESAMPLE program to generate 1000 resampled scattering profiles from the two SASBDB data sets. This allows the calculation of parameters from each resampled profile and subsequently an estimate of the statistical errors based on the standard deviation of the parameter values, for comparison with the errors estimated by the programs. The results of this analysis are also shown in Table 2. The analysis shows that denss.fit_data.py produces similar or smaller statistical errors compared with the estimated errors, suggesting the estimated errors should be considered an upper bound and the statistical errors probably less, whereas the statistical errors appear to be underestimated by DATGNOM [note that only R_{g} and I(0) have estimated errors reported]. It is noteworthy that the statistical errors on R_{g} and I(0) are smaller from denss.fit_data.py (two to fivefold smaller) than from DATGNOM, while the statistical errors on V_{p} and V_{c} are about twofold smaller from DATGNOM/DATPOROD/DATVC.

The statistical errors described here are only based on resampling the scattering profile and do not account for systematic error that is likely to dominate. As discussed above, V_{p}, V_{c} and ℓ_{c} are particularly sensitive to systematic deviation. To test the algorithm for accuracy with experimental data, we calculated V_{p} values for 29 data sets from the Benchmark section of the SASBDB and used V_{p} to estimate the molecular weight (MW) of the particle (where MW = V_{p}/1.6). Fig. 7 shows a comparison of molecular weight values calculated using V_{p} estimates from denss.fit_data.py and DATPOROD with their expected values. Here, the expected value is taken from the expected molecular weight in the SASBDB entries calculated from the amino acid sequence. The median error from denss.fit_data.py is 8.7% and from DATPOROD is 18.0%. As expected, these real errors are in practice significantly larger than the <2% statistical or estimated errors in Table 2, confirming that systematic deviations dominate actual estimates of Porod volume from experimental data.
4. Discussion and conclusions
The approach outlined above is an extension of Moore's original description of SAS profiles using a trigonometric series with the advantage of replacing the nondescript Moore coefficients with specific intensity values. As such, this derivation is subject to all of the same requirements as Moore's, including the need for accurate intensity measurements for at least the first three Shannon channels to obtain reliable estimates of parameter values. We have described a derivation for performing regularization of the realspace P(r) curve analytically, and procedures for the automatic estimation of D and α values. We also present a novel approach for estimating parameters that are particularly sensitive to systematic deviations at high q values, such as V_{p}.
As in Moore's original approach, the use of leastsquares minimization for the derivation given here of a series of SAS parameters directly from the I_{n} values has enabled the estimation of uncertainties through error propagation while accounting for covariances in the data. The oversampling of the information content in the SAS profile effectively increases the signaltonoise ratio of each of the unique observations in the data, i.e. the I_{n} values. Additionally, the analytical regularization derived here simultaneously enables smooth P(r) curves and accurate fits to experimental data, all while providing error estimates for the I_{n} values and associated parameter calculations, accounting for covariances in the data. Using simulated and experimental data, we have shown that these methods yield parameter values describing the size and shape of particles that are as accurate as, and often more accurate than, existing tools.
The algorithm has been made available open source as a script called denss.fit_data.py, accessible on GitHub at https://github.com/tdgrant1/denss. The software can be run either from the command line or as an interactive GUI.
5. Related literature
The following additional references are cited in the supporting information: Fubini (1907); Tonelli (1909).
Supporting information
Additional derivations. DOI: https://doi.org//10.1107/S1600576722006598/vg5144sup1.pdf
Acknowledgements
The author thanks Drs Stephen Meisburger, Kushol Gupta and Robert Rambo for testing the software and for useful discussions.
Funding information
Support for this research was provided by the National Institute of General Medical Sciences of the National Institutes of Health (award No. R01GM133998) and by the National Science Foundation through the BioXFEL Science and Technology Center (award No. 1231306).
References
Blackman, R. B. & Tukey, J. W. (1958). Bell Syst. Tech. J. 37, 185–282. CrossRef Web of Science Google Scholar
Feigin, L. A. & Svergun, D. I. (1987). Structure Analysis by SmallAngle Xray and Neutron Scattering, 1st ed. New York: Plenum Press. Google Scholar
Fubini, G. (1907). Rom. Acc. L. R. (5), 16, 608–614. Google Scholar
Glatter, O. (1977). J. Appl. Cryst. 10, 415–421. CrossRef IUCr Journals Web of Science Google Scholar
Graewert, M. A., Da Vela, S., Gräwert, T. W., Molodenskiy, D. S., Blanchet, C. E., Svergun, D. I. & Jeffries, C. M. (2020). Crystals, 10, 975. Web of Science CrossRef Google Scholar
Grant, T. D. (2018). Nat. Methods, 15, 191–193. Web of Science CrossRef CAS PubMed Google Scholar
Grant, T. D., Luft, J. R., Carter, L. G., Matsui, T., Weiss, T. M., Martel, A. & Snell, E. H. (2015). Acta Cryst. D71, 45–56. Web of Science CrossRef IUCr Journals Google Scholar
Guinier, A., Fournet, G., Walker, C. & Yudowitch, K. (1955). SmallAngle Scattering of Xrays. Chichester: Wiley. Google Scholar
Hansen, S. & Pedersen, J. S. (1991). J. Appl. Cryst. 24, 541–548. CrossRef Web of Science IUCr Journals Google Scholar
Harris, C. R., Millman, K. J., van der Walt, S. J., Gommers, R., Virtanen, P., Cournapeau, D., Wieser, E., Taylor, J., Berg, S., Smith, N. J., Kern, R., Picus, M., Hoyer, S., van Kerkwijk, M. H., Brett, M., Haldane, A., del Río, J. F., Wiebe, M., Peterson, P., GérardMarchant, P., Sheppard, K., Reddy, T., Weckesser, W., Abbasi, H., Gohlke, C. & Oliphant, T. E. (2020). Nature, 585, 357–362. Web of Science CrossRef CAS PubMed Google Scholar
Hunter, J. D. (2007). Comput. Sci. Eng. 9, 90–95. Web of Science CrossRef Google Scholar
Konarev, P. V. & Svergun, D. I. (2015). IUCrJ, 2, 352–360. Web of Science CrossRef CAS PubMed IUCr Journals Google Scholar
ManalastasCantos, K., Konarev, P. V., Hajizadeh, N. R., Kikhney, A. G., Petoukhov, M. V., Molodenskiy, D. S., Panjkovich, A., Mertens, H. D. T., Gruzinov, A., Borges, C., Jeffries, C. M., Svergun, D. I. & Franke, D. (2021). J. Appl. Cryst. 54, 343–355. Web of Science CrossRef CAS IUCr Journals Google Scholar
Moore, P. B. (1980). J. Appl. Cryst. 13, 168–175. CrossRef CAS IUCr Journals Web of Science Google Scholar
Oliveira Neto, M. de, de Freitas Fernandes, A., Piiadov, V., Craievich, A. F., de Araújo, E. A. & Polikarpov, I. (2022). Protein Sci. 31, 251–258. Web of Science CrossRef PubMed Google Scholar
Petoukhov, M. V., Konarev, P. V., Kikhney, A. G. & Svergun, D. I. (2007). J. Appl. Cryst. 40(s1), s223–s228. Google Scholar
Porod, G. (1982). SmallAngle Xray Scattering, edited by O. Glatter & O. Kratky. London: Academic Press. Google Scholar
Putnam, C. D. (2016). J. Appl. Cryst. 49, 1412–1419. Web of Science CrossRef CAS IUCr Journals Google Scholar
Rambo, R. (2021). ScatterIV – New Code Base for Scatter, https://github.com/rambor/scatterIV. Google Scholar
Rambo, R. P. & Tainer, J. A. (2011). Biopolymers, 95, 559–571. Web of Science CrossRef CAS PubMed Google Scholar
Rambo, R. P. & Tainer, J. A. (2013). Nature, 496, 477–481. Web of Science CrossRef CAS PubMed Google Scholar
Rayleigh, Lord (1910). Proc. R. Soc. London Ser. A, 84, 25–46. CrossRef Google Scholar
Shannon, C. E. (1948). Bell Syst. Tech. J. 27, 379–423. CrossRef Web of Science Google Scholar
Svergun, D. I. (1992). J. Appl. Cryst. 25, 495–503. CrossRef CAS Web of Science IUCr Journals Google Scholar
Svergun, D. I. & Koch, M. H. J. (2003). Rep. Prog. Phys. 66, 1735–1782. Web of Science CrossRef CAS Google Scholar
Svergun, D. I. & Pedersen, J. S. (1994). J. Appl. Cryst. 27, 241–248. CrossRef CAS Web of Science IUCr Journals Google Scholar
Tikhonov, A. N. & Arsenin, V. Y. (1977). Solutions of IllPosed Problems. New York: Winston. Google Scholar
Tonelli, L. (1909). Rom. Acc. L. R. (5), 18, 246–253. Google Scholar
Tully, M. D., Tarbouriech, N., Rambo, R. P. & Hutin, S. (2021). J. Vis. Exp. e61578. Google Scholar
Valentini, E., Kikhney, A. G., Previtali, G., Jeffries, C. M. & Svergun, D. I. (2014). Nucleic Acids Res. 43(D1), D357–D363. Google Scholar
Vestergaard, B. & Hansen, S. (2006). J. Appl. Cryst. 39, 797–804. Web of Science CrossRef CAS IUCr Journals Google Scholar
Virtanen, P., Gommers, R., Oliphant, T. E., Haberland, M., Reddy, T., Cournapeau, D., Burovski, E., Peterson, P., Weckesser, W., Bright, J., van der Walt, S. J., Brett, M., Wilson, J., Millman, K. J., Mayorov, N., Nelson, A. R. J., Jones, E., Kern, R., Larson, E., Carey, C. J., Polat, İ., Feng, Y., Moore, E. W., VanderPlas, J., Laxalde, D., Perktold, J., Cimrman, R., Henriksen, I., Quintero, E. A., Harris, C. R., Archibald, A. M., Ribeiro, A. H., Pedregosa, F., van Mulbregt, P., Vijaykumar, A., Bardelli, A. P., Rothberg, A., Hilboll, A., Kloeckner, A., Scopatz, A., Lee, A., Rokem, A., Woods, C. N., Fulton, C., Masson, C., Häggström, C., Fitzgerald, C., Nicholson, D. A., Hagen, D. R., Pasechnik, D. V., Olivetti, E., Martin, E., Wieser, E., Silva, F., Lenders, F., Wilhelm, F., Young, G., Price, G. A., Ingold, G., Allen, G. E., Lee, G. R., Audren, H., Probst, I., Dietrich, J. P., Silterra, J., Webber, J. T., Slavič, J., Nothman, J., Buchner, J., Kulick, J., Schönberger, J. L., de Miranda Cardoso, J. V., Reimer, J., Harrington, J., Rodríguez, J. L. C., NunezIglesias, J., Kuczynski, J., Tritz, K., Thoma, M., Newville, M., Kümmerer, M., Bolingbroke, M., Tartre, M., Pak, M., Smith, N. J., Nowaczyk, N., Shebanov, N., Pavlyk, O., Brodtkorb, P. A., Lee, P., McGibbon, R. T., Feldbauer, R., Lewis, S., Tygier, S., Sievert, S., Vigna, S., Peterson, S., More, S., Pudlik, T., Oshima, T., Pingel, T. J., Robitaille, T. P., Spura, T., Jones, T. R., Cera, T., Leslie, T., Zito, T., Krauss, T., Upadhyay, U., Halchenko, Y. O. & VázquezBaeza, Y. (2020). Nat. Methods, 17, 261–272. Web of Science CrossRef CAS PubMed Google Scholar
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