Coenzyme models. Part 29. On the unusual spectroscopic behaviour of 10-dodecyl-3-methylisoalloxazine (amphiphilic flavin analogue) in aqueous solution
Coenzyme models. Part 29. On the unusual spectroscopic behaviour of 10-dodecyl-3-methylisoalloxazine (amphiphilic flavin analogue) in aqueous solution
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辅酶模型。
DOI：
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发表时间：
1982
期刊：
影响因子：
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通讯作者：
F. Yoneda
中科院分区：
文献类型：
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作者：
S. Shinkai;A. Harada;Y. Ishikawa;O. Manabe;F. Yoneda
在室温水溶液中，标题异咯嗪（3）的可见吸收峰（S1）给出了良好分辨的三带精细结构，据信该结构仅出现在非极性溶剂或酶促疏水口袋中，其中异咯嗪没有氢键作用。另一方面，10-乙基-3-甲基异咯嗪（1）和另一种两亲性异咯嗪，10-丁基-3-十六烷基异咯嗪（2）没有显示出这种精细结构。我们观察到：（1）当加入有机溶剂时，（吡啶或乙醇）或表面活性剂高于临界胶束浓度，（ii）在u. v.区域（S2）中的另一个吸收峰的蓝移不发生，和（iii）水溶液（3）的荧光强度远小于水溶液（1）的荧光强度，但在加入有机溶剂或表面活性剂胶束时增加。这些结果可以用（3）在水溶液中的聚集-解聚集平衡来解释，水溶液（3）的精细结构归因于异咯嗪分子的“堆积”缔合。通过研究N-苄基-1，4-二氢烟酰胺（BzIMH）和几种硫醇的水解和氧化反应，估算了（3）在水溶液中的反应活性。在大多数情况下，（3）的速率常数比（1）小几个数量级，但通过添加有机溶剂或十六烷基三乙基溴化铵胶束显着提高。结果表明，异咯嗪介导的反应，主要涉及极化过渡态，是非常不利的聚集（3）的疏水区域。这些新发现对黄酮蛋白的化学具有重要意义，因为黄酮蛋白结合位点的周围环境经常根据结合的黄酮辅酶吸收光谱的变化进行讨论。
At room temperature in aqueous solution, the visible absorption peak (S1) of the title isoalloxazine(3) gave a well resolved, three-band fine structure which is believed to appear only in apolar solvents or in enzymatic hydrophobic pockets where isoalloxazine is free of hydrogen-bonding effects. On the other hand, 10-ethyl-3-methylisoalloxazine (1) and another amphiphilic isoalloxazine, 10-butyl-3-hexadecylisoalloxazine (2) did not show this fine structure. We have observed that (i) the fine structure disppears upon addition of organic solvents (pyridine or ethanol) or surfactants above the critical micelle concentration, (ii) the blue shift of another absorption peak in the u.v. region (S2) does not occur, and (iii) the fluorescence intensity of aqueous (3) is much smaller than that of aqueous (1) but increases upon addition of organic solvents or surfactant micelles. These results are explicable on the basis of an aggregation-deaggregation equilibrium of (3) in aqueous solution, and the fine structure of aqueous (3) is attributed to the ‘stacking’ association of the isoalloxazine molecules. The reactivity of (3) in aqueous solution has been estimated through an investigation of the hydrolytic decomposition and oxidation of N-benzyl-1,4-dihydronicotinamide (BzIMH) and several thiols. In most cases, the rate constants for (3) were smaller by a few orders of magnitude than those for (1) but were significantly enhanced by the addition of organic solvents or cetyltriethylammonium bromide micelle. The results indicate that the reactions mediated by isoalloxazine, which mostly involve polarised transition states, are extremely unfavourable in the hydrophobic region of aggregated (3). These novel findings have significant implications for the chemistry of flavoproteins, because the surroundings of the binding sites of flavoprotelns have frequently been discussed in terms of the change in the absorption spectra of bound flavin coenzymes.