H-1 AND C-13 NMR RELAXATION INVESTIGATION OF THE CALCIUM COMPLEX OF BETA-ALANYL-L-HISTIDINE (CARNOSINE) IN AQUEOUS-SOLUTION

13C and 1H NMR chemical shifts, J couplings, and spin-lattice relaxation rates were measured for carnosine in aqueous solution at pH 7.0. A Dreiding model of the 'most-probable' conformation was built up, showing predominance of the g- rotamer and folding of the β-alanyl moiety towards the imidazole ring. The structural and dynamic picture was shown not to be severely changed by stepwise titration with Ca II ions. Exchange of carnosine among the bulk solution, a monomer complex (Ca2+ liganded by carboxy and carbonyl oxygens) and a dimer complex (Ca2+ liganded also by imidazole nitrogen) was suggested. The formation constant was evaluated at ca. 25 dm3 mol-1. A Dreiding model of the dimeric complex was also built up on the basis of measured cross-relaxation terms of proton pairs, showing occurrence of intermolecular interactions between peptide molecules mediated by CaII ions.