Schiff bases and geminal diamines derived from pyridoxal 5'-phosphate and diamines

The reactions of the diamines 1,2-diaminoethane, 1,3-diaminopropane, and 1,4-diaminobutane with pyridoxal 5'-phosphate to form Schiff bases and cyclic geminal diamines have been investigated by ultraviolet-visible and ¹H NMR spectroscopy. The formation constants, pk_a values, and spectra of individual ionic species have been evaluated from the pH and concentration dependence of the spectra, and the formation constants have been evaluated independently from quantitative NMR data. The well-known system alanine:pyridoxal phosphate has been evaluated by the same techniques for purposes of comparison. Resolution of the absorption spectra into components by use of log normal distribution curves together with chemical shift values has permitted evaluation of cyclization constants for two different states of protonation of the Schiff bases of the diamines and also of tautomerization constants. Cyclization occurs readily for the diaminopropane system above pH 8 but not as readily for the diaminoethane system. For the latter, tautomerization of the ketoenamine form of the Schiff base to an enolimine form occurs to a major extent in the pH region 6–9. It is proposed that in this same pH region ˜13% of the 1:1 complex may be a carbinolamine species. The data are discussed in terms of the mechanism of transimination in enzymic catalysis.