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Figure 3
Detection of twinning and estimation of the twin fraction α. Top row, cumulative intensity difference plot of the intensity difference of local pairs of intensities that are not twin-related |L| {L = [I(h1) − I(h2)]/[I(h1) + I(h2)]} against the cumulative probability distribution N(L) of the parameter L (Padilla & Yeates, 2003BB26). The expected plots for untwinned and twinned acentric data (red) and the calculated plots for the SIVmac239 Nefcore–TCRζ polypeptide data (blue) are shown. Middle row, estimation of the twin fraction α by Britton plot analysis (Britton, 1972BB5). The percentage of negative intensities after detwinning is plotted as a function of the assumed value of α. Overestimation of the twin factor α results in an increase in the percentage of negative intensities. The estimated value of α is extrapolated from the linear fit (dashed line). Bottom row, estimation of the twin fraction α using the H plot (Yeates, 1988BB42). The cumulative fractional intensity difference of acentric twin-related intensities H {H = |I(h1) − I(h2)|/[I(h1) + I(h2)]} is plotted against H. The initial slope (dashed line) of the distribution is a measure of α. The expected slopes for the indicated twin fractions 0.0–0.4 are shown (dotted lines).

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ISSN: 2059-7983
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