Solution Visible Difference Spectral Properties of Fe3+-L-Amino Acid Complexes at pH 6.60

Solution visible difference absorption spectral properties of the complexes of Fe3+ with different L-amino acids, N-acetyl-L-amino acids, L-amino acid methyl esters, O-phospho-L-serine, phosvitin, and conalbumin were studied at pH 6.60 in the range 350-750 nm. There were extensive similarities in the spectral properties of various Fe3+-amino acid complexes but considerable variation in the differential molar absorptivities (delta epsilon, M-1 cm-1) of the absorption bands (lambda max, nm) calculated for different Fe3+-amino acid complexes. In general, N-acetyl-L-amino acid derivatives showed much higher affinity to complex with Fe3+ than either L-amino acids or their methyl ester derivatives. The spectral data indicated that coordination of Fe3+ to an oxygen is more strongly favored than that to the nitrogen group of the amino acid. alpha-Amino nitrogens in amino acids, however, appear to lack an affinity for Fe3+ ions, as do hydroxyl groups of L-serine and L-threonine. The spectra of the complexes of Fe3+ with various amino acids and their derivatives were used to make the following spectral band assignments: The positive peaks at 422-424, 470-472, and 571-575 nm and the positive shoulder at 493-494 nm, as in the case of various L-amino acids (except L-tyrosine) and their N-acetylated derivatives, were assigned to yellow complexes of Fe3+ with carboxyl oxygens, although the epsilon-amino nitrogen of lysine, the guanidino nitrogen of arginine, and the imidazole nitrogen of histidine may also be involved in the Fe3+-ligand bonding. The single positive peak at 485-493 nm as in the case of L-tyrosine and N-acetyl-L-tyrosinamide was assigned to reddish brown complexes of Fe3+ with phenolate oxygens. The negative absorption band at 416-420 nm as in the case of O-phospho-L~serine and phosvitin was assigned to rust colored complexes of Fe3+ with phosphoseryl groups. The complexes of Fe3+ involving both carboxyl and phenolate oxygens as in the case of N-acetyl-L-tyrosine and conalbumin, however, had a characteristic spectrum with positive peaks at 473 and 493 nm and positive shoulders at 430 and 570 nm.