UV, IR spectrophotometry and NMR spectroscopy were carried out to elucidate the electronic mechanism of 1-ascorbic acid stabilization against anaerobic degradation in niacinamide-ascorbate complex. The decreased niacinamide solubility in isopropyl alcohol, the increased molar absorbance of the complex at 262 nm, the reduced force constant of the carbonyl group of 1-ascorbic acid moiety in the complex resulted approximately 7 cm-1 shift toward lower held in IR were observed. The NMR signals of niacinamide, I-ascorbic acid and the complex were assigned and the significant chemical shifts were also demonstrated. A predominant resonance structure was proposed on the basis of experimental findings which revealed the driving force of nucleophilic attack at carbonyl carbon of 1-ascorblc acid was drastically weakened by the complex formation.
In prior publication l, the stabilization of 1-ascorbic acid in anaerobic degradation due to the formation of niacinamide-ascorbic acid complex through pH range of 3.8 to 7.O was demonstrated kinetically. It was postulated that the stabilization effect of complex formation was perhaps a combination of steric and electronic mechanism. It was also observed that niacinamide-ascorbate complex would cause riboflavin photodegradation2, while photolysis of riboflavin was normally surpressed by l-ascorbic acid3. Niacinamide-ascorbate showed a higher critical relative humidity than that of simple mixture and much stable to moisture stress than 1-ascorbic acid when mixed with vitamin B group4. The instability of vitamin preparations containing 1-ascorbic acid had been a challenge to pharmaceutical formulations, however, niacinamide-ascorbate would have its place in multivitamin preparations. This communication presented the result from solubility, UV, IR and NMR study of the structure of niacinamide-ascorbate complex in order to clarify the mechanism of stabilization which had not yet been demonstrated. 利用UV.IR.NMR. 解明Niacinamide-Ascorbate複合體之構造及複合體中1-Ascorbic Acid抗拒厭氧性分解之安定化機序。根據實驗結果之解析獲知，在複合體中1-Ascorbic Acid之C3´上之O-H基和Niacinamide之Pyridine N間形成部分結合鍵，導致1-Ascorbic Acid分子中電荷分佈之改變，產生共鳴構造，弱化Cl´之部份陽極性，抗拒水解反應之進行，此為安定化之電性機序。