Calcium and praseodymium complexes in solution. 1H n.m.r. conformational study of the model tetrapeptide acetyl-aspartyl-valyl-aspartyl-alanine.

The synthetic tetrapeptide acetyl-aspartyl-valyl-aspartyl-alanine (Ac-DVDA) is a model of the calcium binding site of proteins such as carp parvalbumin, thermolysin and calmodulin. 1H n.m.r. spectra of the tetrapeptide are presented and assigned for D2O and DMSO solutions to determine the conformational mobility. The resonance of the two aspartyl side chains could be completely analysed and the vicinal coupling (C alpha H-C beta H and NH-C alpha H) indicated that the free peptide has considerable conformational mobility. The Ca(II) complex generates a different 1H n.m.r. spectrum for the aspartyl resonances at neutral pH. The solution conformation of Pr(III) complex of Ac-DVDA has been investigated using induced chemical shifts. The observed trends in the magnitude of the shift ratios and the rotamer population suggest that the metal ion binds predominantly to both carboxylates of two aspartyl residues in a bidentate fashion. We discuss the consistency of the differentiated spectra for aspartyl residues in the complex with the stepwise binding of Ca2+ to the carrier.