A potent pyridoxal model capable of promoting transamination and .BETA.-elimination of amino acids.

概要

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N-Dodecyipyridoxal chloride (1a) has been synthesized as a potent pyridoxal 5′-phosphate model. The equilibrium constant for Schiff base formation of 1a with phenylalanine (21000 M−1) in 3 mM hexadecyltrimethylammonium chloride (CTACl) micelle is over a thousand times larger than that for the N-methyl counterpart (1b). The Schiff base of 1a and phenylalanine exists over a broad pH range of 5–11 in two ionic forms, HSB and SB−, that differ in the protonation state at the azomethine group. These Schiff bases undergo transamination at 30 °C in the absence of a metal ion at considerably different rates; 1.33×10−3 and 1.5×10−5 s−1 for HSB and SB−1, respectively. Likewise, the former species exhibited 11 times higher reactivity in the β-elimination of O-phosphonoserine catalyzed by 1a in 3 mM CTACl at 40 °C. These results indicate unambiguously the importance of a positive charge on the azomethine nitrogen as well as the one residing on the pyridine nitrogen in vitamin B6 catalysis. The kinetic α-deuterium isotope effect of 5.1 in the transamiantion of phenylalanine with 1a at pH 7.7 and 30 °C revealed that α-hydrogen abstraction takes place in the transition state.
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