Conformational studies of N‐acetyl‐N′‐methylamide derivatives of α‐aminobutyric acid, norvaline, and valine. I. Preferred conformations in solution as studied by ¹H‐nmr spectroscopy

Abstract

SynopsisThe 'H-nmr studies were extensively carried out to elucidate preferred conformations of dipeptides CH,C*O-X-NHCH,, with X = Abu, oval, and Val in various solvents. The vicinal 'H--'H coupling constants for the NH-CnH moiety and those around the C"-Cp bond in the articulated side chain provided the information regarding the average conformation of these molecules. The results indicate that transformation of skeletal conformations takes place in solution among conformers having similar dihedral angles, 8,,-,.,, in the Karplus expression.

Conformational studies of N‐acetyl‐N′‐methylamide derivatives of α‐aminobutyric acid, norvaline, and valine. II. Conformational energy calculations and comparison with experimental results

“…1 by using Eq. (1). Average values of JNH-COH can be easily calculated from the Boltzmann probabilities of given conformations.…”

“…The vicinal coupling constant J N H -c n H can be calculated for a given skeletal conformation by using a Karplus-type equation such as JNH--CaH = Acos21cp -60"l -Bcoslcp -6 0 ' 1 + Csin21cp -60'1 (1) with cp defined as the torsion angle around the N-C" bond. …”