Malik Sheeba, Karmakar Smarajit, Debnath Ananya
Department of Chemistry, IIT Jodhpur, Jodhpur, Rajasthan, India.
Centre for Interdisciplinary Sciences, Tata Institute of Fundamental Research, Hyderabad, India.
J Chem Phys. 2023 Mar 21;158(11):114503. doi: 10.1063/5.0138681.
The slow relaxation of interface water (IW) across three primary phases of membranes is relevant to understand the influence of IW on membrane functions at supercooled conditions. To this objective, a total of ∼16.26μs all-atom molecular dynamics simulations of 1,2-dimyristoyl-sn-glycerol-3-phosphocholine lipid membranes are carried out. A supercooling-driven drastic slow-down in heterogeneity time scales of the IW is found at the fluid to the ripple to the gel phase transitions of the membranes. At both fluid-to-ripple-to-gel phase transitions, the IW undergoes two dynamic crossovers in Arrhenius behavior with the highest activation energy at the gel phase due to the highest number of hydrogen bonds. Interestingly, the Stokes-Einstein (SE) relation is conserved for the IW near all three phases of the membranes for the time scales derived from the diffusion exponents and the non-Gaussian parameters. However, the SE relation breaks for the time scale obtained from the self-intermediate scattering functions. The behavioral difference in different time scales is universal and found to be an intrinsic property of glass. The first dynamical transition in the α relaxation time of the IW is associated with an increase in the Gibbs energy of activation of hydrogen bond breaking with locally distorted tetrahedral structures, unlike the bulk water. Thus, our analyses unveil the nature of the relaxation time scales of the IW across membrane phase transitions in comparison with the bulk water. The results will be useful to understand the activities and survival of complex biomembranes under supercooled conditions in the future.
跨膜的三个主要相的界面水（IW）的缓慢弛豫与理解超冷条件下IW对膜功能的影响相关。为了实现这一目标，对1,2-二肉豆蔻酰-sn-甘油-3-磷酸胆碱脂质膜进行了总共约16.26微秒的全原子分子动力学模拟。发现在膜从流体相到波纹相再到凝胶相转变过程中，超冷驱动IW的异质性时间尺度急剧减慢。在流体相到波纹相再到凝胶相的转变过程中，IW在阿累尼乌斯行为中经历两次动态转变，由于氢键数量最多，在凝胶相时活化能最高。有趣的是，对于从扩散指数和非高斯参数导出的时间尺度，膜的所有三个相附近的IW都符合斯托克斯-爱因斯坦（SE）关系。然而，从自中间散射函数获得的时间尺度不符合SE关系。不同时间尺度下的行为差异是普遍存在的，并且是玻璃的固有属性。与本体水不同，IW的α弛豫时间中的第一个动态转变与局部扭曲的四面体结构中氢键断裂活化吉布斯自由能的增加有关。因此，与本体水相比，我们的分析揭示了跨膜相变时IW弛豫时间尺度的本质。这些结果将有助于未来理解复杂生物膜在超冷条件下的活性和存活情况。
