厚朴酚、和厚朴酚基态与激发态的电离常数及光谱性质差异的研究

Study on the Difference of Dissociation Constants and Spectroscopic Property between Magnolol and Honokiol in Ground and Excitation States

通过对比厚朴酚、和厚朴酚的紫外光谱和荧光光谱在pH为-0．3～14．7时的变化情况，发现二者的光谱性质存在显著差异，从基态、激发态的电离平衡解释了其原因，探讨了取代基异构对厚朴酚与和厚朴酚基态、激发态电离平衡的影响。实验发现，厚朴酚具有较强的光酸性，水溶液中，当pH〉0时，厚朴酚只显示出1价阴离子HA^-＊的发射（λcm=400nm），采用荧光滴定法得到厚朴酚基态、激发态的电离常数分别为：pKa1^＊=0．57，pKa1=7．54、pKa2=14．38。而和厚朴酚的光酸性较厚朴酚小6个pK单位，其吸收和荧光光谱随pH的变化情况较厚朴酚复杂，采用荧光滴定法测定其pKa1^＊约为6．5。

The spectral characteristics of magnolol and honokiol were investigated in a wide pH range by using absorption spectrophotometer and fluorescence spectrophotometer, respectively, and a great spectral difference was found between them. The spectral mechanisms of magnolol and honokiol were investigated based on the dissociation equilibriums of them in the ground and excitation states, and the effect of substituent geometric conformation on the ionization equilibrium of magnolol and honokiol in both the ground and excitation states were discussed. The results showed that magnolol possesses strong photoacidity, and only a emission band of HA^- ＊ at 400 nm appeared in aqueous solution with pH more than 0. The dissocitation constants of magnolol in ground and excitation states were determined by fluorescence titration, in which pKa1^ ＊ , pKa1 and pKa2 were 0.57, 7.54 and 14.38, respectively. Comparing with magnolol, honokiol possesses an exceptional weak photoacidity with pKa1^ ＊ of 6. 5. Honokiol displays more complex spectral characteristics dependent on the change of pH.