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

Abstract

Solution visible difference absorption spectral properties of the complexes of Fe 3+ 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 Fe 3+ -amino acid complexes but considerable variation in the differential molar absorptivities (Δe, M -1 cm -1 ) of the absorption bands (λ max , nm) calculated for different Fe 3+ -amino acid complexes. In general, N-acetyl-L-amino acid derivatives showed much higher affinity to complex with Fe 3+ than either L-amino acids or their methyl ester derivatives. The spectral data indicated that coordination of Fe 3+ to an oxygen is more strongly favored than that to the nitrogen group of the amino acid. α-Amino nitrogens in amino acids, however, appear to lack an affinity for Fe 3+ ions, as do hydroxyl groups of L-serine and L-threonine. The spectra of the complexes of Fe 3+ 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 Fe 3+ with carboxyl oxygens, although the e-amino nitrogen of lysine, the guanidino nitrogen of arginine, and the imidazole nitrogen of hisitidine may also be involved in the Fe 3+ -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 Fe 3+ 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 Fe 3+ with phosphoseryl groups. The complexes of Fe 3+ 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.