FOLDED AND EXTENDED STRUCTURES OF HOMOOLIGOPEPTIDES FROM ALPHA,ALPHA-DIALKYLATED ALPHA-AMINO-ACIDS - AN INFRARED-ABSORPTION AND HYDROGEN NUCLEAR MAGNETIC-RESONANCE STUDY

The conformational preferences of the N- and C-protected homopeptides from a,a-di-n-propylglycine (to the pentapeptide) in the solid state and in chloroform solution have been assessed by using infrared absorption and 'H nuclear magnetic resonance. A comparison is made with the conformations adopted by the corresponding series from a-aminoisobutyric acid, also dialkylated at the a-carbon, and from L-norvaline, in which the single alkyl side chain is the same as those in a,a-di-n-propylglycine. The highest r-norvaline homopeptides exhibit a significant tendency for adopting an intermolecularly H-bonded beta-structure, in contrast to the a,a-di-n-propylglycine and a-aminoisobutyric acid peptides where intramolecular H-bonding is the dominating factor. The likely absence of a conformational transition with increasing main-chain length and the exceptional structural stability upon heating of all the a,a-di-n-propylglycine homopeptides represent an additional relevant finding of the present work.