CONFORMATIONAL STUDIES ON PEPTIDES CONTAINING ENANTIOMERIC ALPHA-METHYL ALPHA-AMINO-ACIDS .1. DIFFERENTIAL CONFORMATIONAL PROPERTIES OF (R)-2-METHYLASPARTIC AND (S)-2-METHYLASPARTIC ACID

The conformational properties of four model peptides of the general formula Ac-Tyr-Xaa-Yaa-Zaa-Ala-Lys-Glu-Ala-Ala-Glu-Lys-Ala-Zaa-Yaa-Xaa-Lys-NH, (Xaa-Yaa-Zaa=Ala-Ala-(R)-Asp(2-Me), 1; Ala-Ala-(S)-Asp(2-Me), 2; Ala-Aib-Asp, 3; Ala-Ala-Asp, 4; Asp(2-Me) = 2-methylaspartic acid; Aib = 2-aminoisobutyric acid) were studied by CD spectroscopy in solution, to evaluate the helix-inducing potential of enantiomerically pure 2-methylaspartic acid as a function of its chirality at C(2). At neutral pH and 1-degree, all peptides exhibit significant helix formation in aqueous solution. the degree of helicity increasing in the order 4 < 3 almost-equal-to 2 < 1. Lowering the pH to 2 results in a dramatic increase in helicity for peptide 1, while the diastereoisomeric peptide 2 now exists in a predominantly unordered conformation. Helix induction by protonated (R)-Asp(2-Me) exceeds Aib-induced helix formation in peptide 3, and the helix content of 1 in aqueous solution at pH 2 is comparable to the degree of helicity in the strongly helix-inducing solvent 2,2,2-trifluoroethanol.