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Bond-length and bond-angle parameters are derived from a statistical survey of X-ray structures of small compounds from the Cambridge Structural Database. The side chains of the common amino acids and the polypeptide backbone were represented by appropriate chemical fragments taken from the Database. Average bond lengths and bond angles are determined from the resulting samples and the sample standard deviations provide information regarding the expected variability of the average values which can be parametrized as force constants. These parameters are ideally suited for the refinement of protein structures determined by X-ray crystallography since they are derived from X-ray structures, are accurate to within the deviations from target values suggested for X-ray structure refinement and use force constants which directly reflect the variability or uncertainty of the average values. Tests of refinement of the structures of BPTI and phycocyanin demonstrate the integrity of the parameters and comparisons of equivalent refinements with XPLOR parameters show improvement in R-factors and geometry statistics.
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