Proton transfer reactions in aqueous solutions of pyridoxamine phosphate

UV-visible and 13C NMR measurements described in the literature and our 31P NMR measurements support the following mechanism of proton transfer reactions in aqueous solutions of pyridoxamine phosphate: Only the tautomeric equilibrium between neutral form, AN, and zwitterion, AZ, which is analogous to the tautomeric equilibrium of 3-hydroxypyridine in aqueous solution, is important, and that equilibrium does not change upon the dissociation of the second phosphate proton. With these simplifying assumption, we have simulated the relaxation spectrum of the proton transfer reactions of pyridoxamine phosphate in water using parameters from analogous reactions and compared it with our ultrasound and temperature jump measurements. We have found that the relaxation process measured by the temperature jump experiment is mainly caused by the overall reaction AN=AZ (or AN-=AZ-) and the ultrasound absorption at the isoelectric point between pK2 and pK3 is mainly caused by the overall reaction $$A^{\text{ + }} + {\text{x}} A_N^ - + \left( {1 - {\text{x}}} \right)A_Z^ - = {\text{y}}A_N + \left( {2 - {\text{y}}} \right)A_Z , 0 \leqq {\text{x}} \leqq {\text{1,}} {\text{0}} \leqq {\text{y}} \leqq 2$$ .