Confromational Studies on Peptides Containing Enantiometric α‐Methyl α‐Amino Acids. Part I. Differential conformational properties of ( R )‐ 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 2 (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°, all peptides exhibit significant helix formation in aqueous solution, the degree of helicity increasing in the order 4 3 ≈ 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.