Preparations and Characterizations of Novel N, N′-Ethylene-Bridged-(S)-Histidyl-(S)-Tyrosine Derivatives and Their Copper(II) Complexes as Models of Galactose Oxidase

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Abstract
Structurally constrained ethyl (2S)-3-(4-hydroxyphenyl)-2-{[(3S)-3-(imidazole-4-yl)-methyl]-2-oxo-piperazine-1-yl}propionate (N,N′-ethylene-bridged-(S)-histidyl-(S)-tyrosine ethyl ester, eHY-OEt) was obtained by a one-pot reaction from (S)-histidine methyl ester and (S)-tyrosine ethyl ester with glyoxal. Four ligands, (Boc-Gly-eHY-OEt (HL1), Boc-Gly-eHY-NH2 (HL2), Ac-eHY-OEt (HL3), and Ac-eHY-NH2 (HL4), where Boc-Gly is t-butyloxycarbonyl-glycyl and Ac is acetyl) were derived from eHY-OEt. Also, corresponding copper(II) complexes, in which only amino acid side chains coordinate to the copper(II) ion, [Cu(L1-)2(H2O)] 1, [Cu(L2-)2(H2O)] 2, [Cu(L3-)2(H2O)] 3, and [Cu(L4-)2(H2O)] 4, were prepared and characterized by elemental analyses and the FAB-MS spectra. The conformational behaviors were confirmed by the 1H NMR spectra. UV-visible-NIR, CD and ESR spectra showed that copper(II) complexes are mononuclear square-pyramidal structures in both the solid and solution states. Cyclic voltammetry of HL1 and 1 showed irreversible ligand oxidation. The acid-dissociation constants of HL4 were obtained as 6.38 (pKa1) and 9.67 (pKa2) by potentiometric titration, and the complex formation constants of 4 were obtained as 7.16 (log K1) and 3.71 (log K2). Furthermore, thermogravimetric analyses (TGA) and differential thermal analyses (DTA) of 1 and 2 were measured.

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