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Some tripeptides obtained by enzymic digestion of caseins possess immunomodulating properties. In order to correlate activity and structure, X-ray analysis has been applied to two of them Leu-Leu-Tyr and Gly-Leu-Phe.

A new translation function T(t) is defined, whose main peak is expected to give directly the absolute position t = t o of a known molecular configuration (the isostructural model) of known orientation in an unknown crystal structure. T(t) combines all required information: crystal symmetry, molecular steric properties of the model and experimental data (observed structure factors F o) leading to more reliable results: T(t) = TO(t)/O(t), where TO(t) measures the degree of agreement between F o and Fc(0, while O(t) is a function measuring intermolecular overlap. TO and O can both be computed efficiently by the possibility of their expansion in Fourier series, and the use of FFT. A theoretical study followed by application to a simple synthetic structure cast light upon ambiguities and failures [e.g. Bott & Sarma (1976). J. Mol. Biol. 106, 1037-1046 encountered in using translation functions previously described [e.g. Crowther & Blow (1967). Acta Cryst. 23,[544][545][546][547][548]. The crystal structure of the high-temperature form of hen-egg-white lysozyme was determined at 6 A resolution by the application of this fast translation function and compared with the equivalent structure obtained by the isomorphous replacement method.

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