Vibrational spectroscopy of water in hydrated lipid multi-bilayers. I. Infrared spectra and ultrafast pump-probe observables
Vibrational spectroscopy of water in hydrated lipid multi-bilayers. I. Infrared spectra and ultrafast pump-probe observables
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DOI：
10.1063/1.3615717
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发表时间：
2011-08-21
影响因子：
4.4
通讯作者：
Skinner, J. L.
中科院分区：
文献类型：
--
作者：
Gruenbaum, S. M.;Skinner, J. L.
采用分子动力学模拟和混合量子/经典模型研究了双酰磷脂酰胆碱脂双层中水合水的振动光谱。实验测量了脂质多层水化水平对FTIR吸收光谱、各向同性和各向异性泵-探针衰减曲线的影响，并将其与实验结果联系起来。为此，我们使用三阶响应函数，它允许我们包括非高斯频率波动，非condon效应，分子旋转和波动振动寿命，我们认为所有这些对该系统都很重要。我们使用现有的跃迁频率和偶极子映射来计算响应函数。实验表明，在不同的分子环境中，HDO分子有两种不同的振动寿命。为了得到这些寿命，我们考虑了水合水氢键的一个简单的双居群模型。假设每个种群的寿命不同，然后我们计算各向同性泵-探针衰减，拟合实验得到每个水合水平的两个寿命。有了这些寿命，我们计算了FTIR光谱和泵探各向异性衰减作为水化的函数。因此，这种方法允许在统一的计算方案中对所有可观测值进行一致的计算。我们的理论结果与实验结果在定性上是一致的。脂相关OD基团的振动寿命系统地短于水相关OD基团的振动寿命，并且随着水合作用的减少，每个种群的寿命都增加，这与先前的分析一致。我们的理论FTIR吸收光谱成功地再现了实验观察到的随着脂质水合作用的减少而发生的红移，并且我们证实了之前的解释，即这种红移是由水的氢键到脂质磷酸基的结果。从泵-探针各向异性衰减中，我们证实，随着水化作用的减少，水分子的重定向运动明显减慢，在脂质羰基区域结合的水经历最慢的旋转。(C) 2011年美国物理研究所。(doi: 10.1063/1.3615717)
The vibrational spectroscopy of hydration water in dilauroylphosphatidylcholine lipid multi-bilayers is investigated using molecular dynamics simulations and a mixed quantum/classical model for the OD stretch spectroscopy of dilute HDO in H2O. FTIR absorption spectra, and isotropic and anisotropic pump-probe decay curves have been measured experimentally as a function of the hydration level of the lipid multi-bilayer, and our goal is to make connection with these experiments. To this end, we use third-order response functions, which allow us to include non-Gaussian frequency fluctuations, non-Condon effects, molecular rotations, and a fluctuating vibrational lifetime, all of which we believe are important for this system. We calculate the response functions using existing transition frequency and dipole maps. From the experiments it appears that there are two distinct vibrational lifetimes corresponding to HDO molecules in different molecular environments. In order to obtain these lifetimes, we consider a simple two-population model for hydration water hydrogen bonds. Assuming a different lifetime for each population, we then calculate the isotropic pump-probe decay, fitting to experiment to obtain the two lifetimes for each hydration level. With these lifetimes in hand, we then calculate FTIR spectra and pump-probe anisotropy decay as a function of hydration. This approach, therefore, permits a consistent calculation of all observables within a unified computational scheme. Our theoretical results are all in qualitative agreement with experiment. The vibrational lifetime of lipid-associated OD groups is found to be systematically shorter than that of the water-associated population, and the lifetimes of each population increase with decreasing hydration, in agreement with previous analysis. Our theoretical FTIR absorption spectra successfully reproduce the experimentally observed red-shift with decreasing lipid hydration, and we confirm a previous interpretation that this shift results from the hydrogen bonding of water to the lipid phosphate group. From the pump-probe anisotropy decay, we confirm that the reorientational motions of water molecules slow significantly as hydration decreases, with water bound in the lipid carbonyl region undergoing the slowest rotations. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3615717]