Induced cotton effects for complexes of magnesium porphyrin with D- and L-proline, L-serine, L-threonine and L-tryptophan

Abstract

Circular dichroism and optical rotatory dispersion results are reported for solutions of magnesium protoporphyrin and magnesium mesoporphyrin containing the amino acids, D and L-proline, L-serine, L-threonine and L-tryptophan. The observed induced Cotton effects are attributed to six-coordinate magnesium porphyrin (amino acid) 2 species. The Cotton effects are most pronounced for proline and the related pair of amino acids, serine and threonine. In the former case, the observed spectra are inverted with respect to those for other amino acids of the same chirality. It is suggested that this arises from the formation of an additional chiral centre at the ring nitrogen atom due to the coordination of proline to magnesium via this atom. Steric interactions produce an opposite chirality at nitrogen to that existing at the asymmetric carbon atom. For L-serine and L-threonine H-bonding of the -OH groups with the -COO − porphyrin side chains is proposed as the localising effect that produces the observed induced Cotton rotation. For L-tryptophan, where a weaker CD/ORD effect is observed, stacking of the aromatic rings of the indole and porphyrin groups is considered as a possible localising interaction.