Copper(II) binding by fragments of alpha-synuclein containing M1-D2- and -H50-residues; a combined potentiometric and spectroscopic study

Stability constants and ligand donor sets of the copper(II) complexes of the NH2-29-56(L1)(AA30GKTKEGVLYV40GSKTKEGVVH50GVATVA56-NH2), NH2-M29-D30-56(L2) and Ac-M29-D30-56(L3) fragments of alpha-synuclein were determined in aqueous solution for 1 : 1 metal-to-ligand molar ratio in the pH range 2.5-10.5. The tyrosine residue in the 39th position of the alpha-synuclein fragments does not take part in the coordination of the metal ion. The potentiometric and spectroscopic data (UV-Vis, CD, EPR) show that acetylation of the amino terminal group induces significant changes in the coordination properties of the L3 fragment compared to that of the L2 peptide. When the amino group is blocked (L3) the imidazole nitrogen of the histidine residue acts as an anchoring site and at higher pH the 3N {N(Im),2N-} and 4N {N(Im),3N-} complexes are formed. The L1 peptide at physiological pH forms in equilibrium 3N {NH2,N-,CO,N(Im)} and 4N {NH2,2N-,N(Im)} complexes. For the L2 peptide the coordination of the copper(II) ions starts from the N-terminal Met residue and with increasing of pH the Asp residue in second position of amino acid sequence coordinates and stabilizes significantly the 2N complex as a result of chelation through the beta-carboxylate group. At physiological pH the 3N {NH2,N-,beta-COO-,N(Im)} coordination mode dominates. At pH above 6 the results for the L2 fragment suggest the formation of 3N and 4N complexes (in equatorial plane) and the involvement of the lateral NH2 group of Lys residue in the axial coordination of Cu(II) ion. In CD spectra sigma (epsilon-NH2-Lys) --> Cu(II) charge transfer transition is observed. The stability constants for the L2 fragment of alpha-synuclein of the 4N {NH2,2N-,N(Im)} and {NH2,3N-} complexes are higher about 1.5 and 0.7 orders of magnitude, respectively, by comparison to those of the L1 peptide. This increase may be explained by the involvement of the epsilon-NH2 group of Lys residue in the coordination sphere of metal ion.