Protonation, binary and ternary Cu(II) complex formation constants of N-methyl-L-histidine (N-MH) and N-a-methyl-L-histidine (N-a-MH) have been measured through potentiometric titrations. UV/VIS spectra have been recorded for both the binary systems; the Cu(II)/N-a-MH complexes have been investigated by means of Fast Atom Bombardment mass spectroscopy as well. The data on protonation equilibria of N-MH and N-a-MH show the existence of a difference in basicity between the respective nitrogen atoms, thus giving information on the tautomerism involving the imidazole residue of L-histidine (L-His) in the physiological pH range. The complexation behaviour of N-MH is quite similar to that of underivatized L-His: the same species are formed and their log beta values are very close. On the contrary, the presence of a methyl substituent on the N alpha atom deeply affects the complexing properties of the ligand. N-a-MH cannot act as a tridentate ligand for steric reasons and tends to form polynuclear complexes.
Binary and ternary Cu(II) complexes of Ntau- and Npi-methyl-L-histidine in aqueous solution
REMELLI, Maurizio;PULIDORI, Fernando
1994
Abstract
Protonation, binary and ternary Cu(II) complex formation constants of N-methyl-L-histidine (N-MH) and N-a-methyl-L-histidine (N-a-MH) have been measured through potentiometric titrations. UV/VIS spectra have been recorded for both the binary systems; the Cu(II)/N-a-MH complexes have been investigated by means of Fast Atom Bombardment mass spectroscopy as well. The data on protonation equilibria of N-MH and N-a-MH show the existence of a difference in basicity between the respective nitrogen atoms, thus giving information on the tautomerism involving the imidazole residue of L-histidine (L-His) in the physiological pH range. The complexation behaviour of N-MH is quite similar to that of underivatized L-His: the same species are formed and their log beta values are very close. On the contrary, the presence of a methyl substituent on the N alpha atom deeply affects the complexing properties of the ligand. N-a-MH cannot act as a tridentate ligand for steric reasons and tends to form polynuclear complexes.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
