1H and 13C nmr studies of cyclic and linear dipeptides containing threonyl, seryl, aspartyl, and histidyl residues

Abstract

AbstractThe side‐chain conformations of D‐ orL‐ Thr, D‐ or L‐Ser, L‐Asp, and L‐ His residues in cyclic and linear dipeptides in D2O or in DMSO‐d6 are deduced from vicinal (1H,1H) and (13C, 1H) coupling constants. Vicinal (13C, 13C) coupling constants strongly depend on substituents and cannot be used without a more sound analysis. In cyclic dipeptides, the Thr and Ser side chains are folded above the DKP ring, with χ1 near 60°. The L‐Asp side chain interacts more specifically with peptide bonds (χ1 near 300°). The L‐ His side chain is more flexible and its conformation depends on the proximity of a second side chain and on solute‐solvent interactions. In all cases, this side chain is not completely folded. In linear dipeptides, the conformation of a C‐terminal L‐His residue is mainly influenced by the end carboxylic group. On the other hand, a N‐terminal L‐His residue interacts more easily with a neighboring L‐Asp residue. In aqueous solution, the imidazole pKa depends on the proximity of terminal and lateral charged groups but does not reveal any specific interaction in cyclic dipeptides. A comparison between the conformations of cyclic peptides observed in solution, in the crystalline state and calculated by empirical methods, allows one to point out the discriminating role of the packing in crystals, and of solute‐solvent interactions in solution.