Abstract
Ultraviolet spectral studies of pyridoxal-homocysteine (PAL-HCySH) in methanol were carried out at various media. The absorption bands were assigned and then the equilibria among the various molecular species in the methanol were found. Dissociation constants obtained by spectrophotometric method showed pKa_1=2.5,pKa_2=5.2,and pKb=2.8. Pyridoxal (PAL) is rapidly decomposed by exposure to light, particulary in neutral aqueous solution, while PAL-HCySH is relatively stable in the solution. Photodecomposition of PAL-HCySH would occur in the aqueous solution, because of the decomposition of PAL produced by the hydrolysis. The decomposition would be avoided in methanol, though the PAL was rapidly inactivated by exposure to light in alkaline methanol solution. PAL and PAL-HCySH in solid state were not decomposed by sunlight.
