13C- and Water Proton-Nuclear Magnetic Relaxation of Cu(II)–Poly(D-glutamic acid) Complex in Aqueous Solution

Abstract

13C- and water proton-nuclear magnetic relaxation times were measured as functions of temperature and pH to study the interaction of Cu(II) with poly(D-glutamic acid) (PGA) in aqueous solution. In the pH region from 4.5 to 8, the addition of Cu(II) significantly influences the relaxation times of Cγ and Cδ carbons, while those of Cβ, Cα, and peptide C′ are not influenced. The relaxation times of Cγ and Cδ increase with increasing temperature, indicating that a fast exchange between the complexed and uncomplexed states occurs. The scalar relaxation is dominant to line broadening, suggesting that a significant amount of electron spin density is transferred from Cu(II) to Cγ Above pH 9, no paramagnetic effects of Cu(II) upon 13C spectra of PGA were observed. The water proton relaxation of Cu(II) aqueous solution is markedly enhanced by the addition of PGA at an acidic pH, but suppressed at an alkaline pH. These results show that carboxylate groups of PGA and water molecules are bound to Cu(II) at pH≤8, but excluded from Cu(II) at pH≥9.