Kinetics and mechanism of transamination reaction of L-phenylalanine with hydrophobic pyridoxal in vesicular and micellar phases

Abstract

A hydrophobic pyridoxal derivative quaternized at the pyridyl nitrogen with a double-chain segment (PL ⁺2C₁₆) was embedded in the single-walled vesicle of NN-dihexadecyl-N^α-[6-(trimethyl-ammonio)hexanoyl]-L-alaninamide bromide (N⁺C₅Ala2C₁₆), and the pyridoxal moiety was fixed in the hydrogen-belt domain of the vesicle. While the transamination of L-phenylanine (L-Phe), a hydrophobic α-amino acid, with PL ⁺2C₁₆ in the vesicle and the hexadecyltrimethylammonium bromide (CTAB) micelle proceeded slowly to afford the pyridoxamine derivative (PM ⁺2C₁₆) and β-phenylpyruvic acid, addition of metal ions to the equilibrium mixture of the aldimine Schiff's base (ASB), PL ⁺2C₁₆, and L-Phe caused acceleration of the overall transamination rate. The transmination was most effectively catalysed by copper(II) ions in the N⁺C₅Ala2C₁₆ vesicle and the CTAB micelle. The catalytic activity of copper(II) ions was so enhanced as to allow significant accumulation of the carbanion chelate, derived from the ASB chelate by α-hydrogen removal, as an intermediate in the aldimine–ketimine isomerization. The reactivity of the overall copper(II)-catalysed transamination was greater in the vesicle than in the micelle and primarily controlled by the collapse ratio of the copper(II)–carbanion species as clarified by detailed kinetic analysis.