STRUCTURAL VERSATILITY OF PEPTIDES CONTAINING C-ALPHA,ALPHA-DIALKYLATED GLYCINES - CONFORMATIONAL ENERGY COMPUTATIONS, IR ABSORPTION AND H-1-NMR ANALYSIS OF 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID HOMOPEPTIDES

Conformational energy computations on the 1-aminocyclopropane-1-carboxylic acid mono-, di-, and tripeptide amides, Ac-(Ac3c)n-NHMe (n = 1-3), indicate that this C alpha, alpha-dialkylated, cyclic alpha-amino acid residue is conformally restricted and that type-I(I') beta-bends and distorted 3(10)-helices are particularly stable conformations for the di- and tripeptide amides, respectively. The results of the theoretical analysis are in agreement with those obtained in an i.r. absorption and 1H n.m.r. investigation in chloroform solution of Ac3c-rich tri- and tetrapeptide esters. A comparison is also made with the conclusions extracted from our previous work on peptides rich in Aib (alpha-aminoisobutyric acid), Ac5c (1-aminocyclopentane-1-carboxylic acid), and Ac6c (1-aminocyclohexane-1-carboxylic acid).