As a structural basis for the specificity of protein-nucleic acid interactions, the elementary interactions between nucleic acid base (A, G, C, U or T) and amino-acid side chain (hydroxyl, imidazolyl, carboxyl, carbamoyl, phenyl, hydroxyphenyl or indolyl group) were extensively investigated by using model crystals that contain both components. Interaction patterns between complementary base pair and amino acid side chain were also examined with the ternary model systems newly designed. Several characteristics are observed in the model crystals ; for instance, stabilization of stacking between the bases and histidine by the protonation of the latter, common hydrogen bonding patterns between the bases and hydroxyl or carboxyl group by reversion of donor-acceptor relationship or ionization. UV, NMR and CNDO/2 calculation suggest energetistic aspects of these interactions. A structural constraint is derived between the hydrogen bonds and the secondary structural fitting of α-helical segment into the major groove of B-DNA. In terms of the elementary interactions, stereochemical interpretations are given for the binding of
cro repressor to operator, enzymatic reaction of RNAase and initiation and elongation in the assembly of TMV.
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