Water at the surfaces of aligned phospholipid multibilayer model membranes probed with ultrafast vibrational spectroscopy.
Water at the surfaces of aligned phospholipid multibilayer model membranes probed with ultrafast vibrational spectroscopy.
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DOI：
10.1021/ja803252y
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发表时间：
2008-10-22
影响因子：
15
通讯作者：
Fayer, Michael D.
中科院分区：
文献类型：
--
作者：
Zhao, Wei;Moilanen, David E.;Fenn, Emily E.;Fayer, Michael D.
用超快偏振选择振动泵浦-探测光谱研究了磷脂双月桂酰磷脂酰胆碱(DLPC)多层膜的表面水动力学。实验是在不同的水化水平下进行的，x=2-16水分子/脂在37°C，水分子在膜表面上下约1 nm。为消除振动激发传递，对H2O中稀HOD的OD伸展模式进行了实验研究。在低水合水平下，DLPC双层体系中OD拉伸的FT-IR吸收光谱显示出相对于主体水的频率红移，这与在反胶束中的水纳米孔等体系中经常观察到的蓝移形成鲜明对比。X=4-16的光谱可以通过x=2的光谱和体水的叠加来再现。红外泵浦-探测测量表明，随着水合程度的降低，振动布居衰变(寿命)变长。用双指数函数对种群衰变进行了很好的拟合。作为OD拉伸频率的函数测量的布居衰变表明，在脂质双层的界面区域中存在两种主要类型的水分子。一个组分可能是疏水胆碱基团附近的笼状水团，另一个组分可能与主要与磷酸基团相关的水化水分子有关。随着水化程度的增加，这两个组分的振动寿命缩短，这表明与双分子层膨胀相关的两个组分的水化结构不断演变。从红外光谱得到的两组分的大小与从振动寿命测量得到的值的一致性进一步支持了双组分模型。振动布居衰变拟合还给出了磷酸盐结合水分子数和胆碱结合水分子数的估计值，当x从2增加到16时，它们的范围分别为1到4和1到12。随时间变化的各向异性测量给出了取向弛豫速率作为x的函数。各向异性衰减是双指数的。快分量几乎与x无关，并被解释为没有氢键重排而发生的小的取向涨落。随着水合程度的降低，较慢的成分会变得很长。这个分量是完全取向随机化速度的量度，这需要氢键重排，并根据跳跃重定向模型进行讨论。
The dynamics of water at the surface of artificial membranes composed of aligned multibilayers of the phospholipid dilauroyl phosphatidylcholine (DLPC) are probed with ultrafast polarization selective vibrational pump-probe spectroscopy. The experiments are performed at various hydration levels, x = 2 – 16 water molecules per lipid at 37 °C. The water molecules are ~1 nm above or below the membrane surface. The experiments are conducted on the OD stretching mode of dilute HOD in H2O to eliminate vibrational excitation transfer. The FT-IR absorption spectra of the OD stretch in the DLPC bilayer system at low hydration levels shows a red-shift in frequency relative to bulk water, which is in contrast to the blue shift often observed in systems such as water nanopools in reverse micelles. The spectra for x = 4 – 16 can be reproduced by a superposition of the spectra for x = 2 and bulk water. IR Pump-probe measurements reveal that the vibrational population decays (lifetimes) become longer as the hydration level is decreased. The population decays are fit well by biexponential functions. The population decays, measured as a function of the OD stretch frequency, suggest the existence of two major types of water molecules in the interfacial region of the lipid bilayers. One component may be a clathrate-like water cluster near the hydrophobic choline group and the other may be related to the hydration water molecules mainly associated with the phosphate group. As the hydration level increases, the vibrational lifetimes of these two components decrease, suggesting a continuous evolution of the hydration structures in the two components associated with the swelling of the bilayers. The agreement of the magnitudes of the two components obtained from IR spectra with those from vibrational lifetime measurements further supports the two component model. The vibrational population decay fitting also gives an estimation of the number of phosphate-associated water molecules and choline-associated water molecules from, which range from 1 to 4, and 1 to 12, respectively as x increases from 2 to 16. Time dependent anisotropy measurements yield the rate of orientational relaxation as a function of x. The anisotropy decay is biexponential. The fast component is almost independent of x, and is interpreted as small orientational fluctuations that occur without hydrogen bond rearrangement. The slower component becomes very long as the hydration level decreases. This component is a measure of the rate of complete orientational randomization, which requires hydrogen bond rearrangement and is discussed in terms of a jump reorientation model.
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