Dynamic and 2D NMR studies on hydrogen-bonding aggregates of cholesterol in low-polarity organic solvents.
Dynamic and 2D NMR studies on hydrogen-bonding aggregates of cholesterol in low-polarity organic solvents.
复制标题
DOI：
10.1021/jp062607t
复制
发表时间：
2006-08
期刊：
影响因子：
--
通讯作者：
Cristiano Giordani;Chihiro Wakai;E. Okamura;N. Matubayasi;M. Nakahara
中科院分区：
文献类型：
--
作者：
Cristiano Giordani;Chihiro Wakai;E. Okamura;N. Matubayasi;M. Nakahara
通过改变杂质水浓度(~gt；2 mm)和温度(30-50℃)，在1-700 mm的正辛醇、氯仿和环己烷中测量了正常(非重氢)和重氢胆固醇-d(6)(C26和C27甲基重氢)的自扩散系数(D)。分别使用600 MHz和92 MHz的脉冲场梯度自旋回波(PGSE)(1)H和(2)H核磁共振。在30℃下，当杂质水可以忽略时，在正辛醇、氯仿和环己烷中，从D值和溶剂粘度20 mm处获得的流体动力学半径(R)分别为5.09、7.07和6.17 A。正辛醇的R值是最小的，与胆固醇分子的平均分子轴长度相当。在正辛醇中，R不随浓度变化而变化，而在氯仿中，R较大，且几乎与胆固醇浓度呈线性增加。在最高浓度为700 mm时，当杂质水处于可忽略和饱和浓度时，氯仿中的R分别为13.5A和16.7A。R值大于氢键正辛醇的R值，表明胆固醇通过氢键形成聚集体。通过比较氯仿和正辛醇中的NOESY光谱，确定了聚集体结构。NOESY分析表明，与正辛醇相比，氯仿中存在一个额外的交叉峰(C4-C19)。因为与交叉峰相关的碳原子靠近羟基(C3-OH)，所以胆固醇分子被认为不是堆积在一起的，但发现在聚集体中是以OH为中心的。在C2、C3、C4和C6位置测得的较大旋转流体动力学半径也支持这一点。这表明聚集体的形成是由胆固醇分子之间的氢键驱动的。
Self-diffusion coefficients (D) are measured for normal (nondeuterated) and deuterated cholesterol-d(6) (C26 and C27 methyl groups deuterated) in 1-octanol, chloroform, and cyclohexane at concentrations of 1-700 mM by varying the impurity water concentration (>2 mM) and temperature (30-50 degrees C). The pulsed field gradient spin-echo (PGSE) (1)H and (2)H NMR were used, respectively, at 600 and 92 MHz. At 30 degrees C, the hydrodynamic radius (R) obtained at 20 mM from the D value and solvent viscosity is 5.09, 7.07, and 6.17 A, respectively, in 1-octanol, chloroform, and cyclohexane when the impurity water is negligible. The R value in 1-octanol is the smallest and comparable with the average length of the molecular axes for the cholesterol molecule. In 1-octanol, R is invariant against the concentration variation, whereas in chloroform, R is larger and increases almost linearly with cholesterol concentration. At the highest concentration, 700 mM, the R in chloroform is 13.5 and 16.7 A, respectively, when the impurity water is at negligible and saturated concentrations. The R value larger than that in hydrogen-bonding 1-octanol indicates that cholesterol forms an aggregate through hydrogen bonding. The aggregate structure is confirmed by comparing NOESY spectra in chloroform and 1-octanol. The NOESY analysis reveals the presence of one extra cross peak (C4-C19) in chloroform compared to 1-octanol. Because the carbon atoms related to the cross peak are close to the hydroxyl group (C3-OH), cholesterol molecules are considered to be not piled but are found to be OH-centered in the aggregate. This is supported also by larger rotational hydrodynamic radii measured on cholesterol deuterated at positions C2, C3, C4, and C6. This shows that the aggregate formation is driven by the hydrogen-bonding between cholesterol molecules.