A (1)H and (13)C NMR Study on the Role of Salt-Bridges in the Formation of a Type I β-Turn in N-Acetyl-L-Asp-L-Glu-L-Lys-L-Ser-NH(2).

Abstract The conformation of the tetrapeptide N-Acetyl-Asp(7)-Glu(8)-Lys(9)-Ser(10)-NH(2), a fragment of the type I collagen α-1 chain N-telopeptide, has been studied by (1)H and (13)C NMR and circular dichroism spectroscopy. The spectroscopic evidence, based on two-dimensional, phase-sensitive NMR techniques such as COSY, ROESY, proton-carbon shift correlation and selective COLOC, indicates a strong dependence of the conformation on the experimental conditions. In CD(3)OH/H(2)O (60/40) at ca. neutral pH the tetrapeptide forms a β-turn, stabilized by a hydrogen bond between NH(S(10)) and CO(D(7)) and a strong salt-bridge between COO(-)(E(8)) and NH(3) (+)(K(9)). The β-turn is type I and appears to coexist with a non-hydrogen-bonded structure. The coexistence of these two conformers is proven by proton NMR data such as NH-NH ROEs, reduced NH-H(α)(E(8)) coupling constant NH(E(8)) low-field shift and the temperature coefficient of NH(S(10)), whereas the conclusion regarding the salt-bridge is based on (13)C results. In the same solvent, at a pH below the pKa of the carboxyl groups, no evidence for a conformation other than extended can be found. In aqueous solution at approximately neutral pH, evidence for the E(8)-K(9) charge interaction is observed, but not for a hydrogen bond anywhere in the molecule.