CONFORMATIONAL ANALYSIS OF AROMATIC AMINO ACIDS BY X-RAY CRYSTALLOGRAPHY

A systematic analysis of 48 crystal structures of aromatic amino acids or amino acid residues with tyrosine or phenylalanine in their molecular make up reveals that there are three basic energy minimum conformations which these molecules adopt and which, in this statistical sample, can be further subdivided into subsets. These groups are bounded by the interactions of the three rotational parameters X1, X2 and o1 which describe the amino acid backbone. The three major classes A, B and C are defined as the structures having the three staggered positions of X1 (60°, 180° and 300°, respectively). Class B (X1 = 180°) can be subdivided into two subsets B1 and B2 by means of the correlation between χ2 and o1. The statistical distribution of this sample (A:B:C: 23:31:46%) agrees better with the theoretical distribution from energy minimization calculations (19:36:45%) than does the distribution observed in proteins (4:29:67%). Also, it is observed that, in those structures which contain an ethyl ester, the latter maintains a fixed orientation relative to the amino acid backbone.