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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Charge State Dependence of Amino Acid Propensity at Water Surface: Mechanisms Elucidated by Molecular Dynamics Simulations
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Charge State Dependence of Amino Acid Propensity at Water Surface: Mechanisms Elucidated by Molecular Dynamics Simulations

机译：水表面氨基酸倾向的电荷状态依赖性：分子动力学模拟所阐明的机制

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Atmospheric aerosols contain a variety of compounds, among them free amino acids and salt ions. The pH of the aerosol droplets depends on their origin and environment. Consequently, compounds like free amino acids found in the droplets will be at different charge states, since these states to a great extent depend on the surrounding pH condition. In droplets of marine origin, amino acids are believed to drive salt ions to the water surface and a pH-dependent amino acid surface propensity will, therefore, indirectly affect many processes in atmospheric chemistry and physics such as for instance cloud condensation. To understand the surface propensity of glycine, valine, and phenylalanine at acidic, neutral, and basic pH, we used molecular dynamics (MD) simulations to investigate them at three different charge states in water. Their respective surface propensities were obtained by the means of a potential of mean force (PMF) in an umbrella sampling approach. Glycine was found to have no preference for the surface, while both valine and phenylalanine showed high propensities. Among the charge states of the surface-enriched ones, the cation, representing the amino acids at low pH, was found to have the highest affinity. Free energy decomposition revealed that the driving forces depend strongly on the nature of the amino acid and its charge state. In phenylalanine, the main factor was found to be a substantial entropy gain, likely related to the side chain, whereas in valine, hydrogen bonding to the functional groups leads to favorable energies and, in turn, affects the surface propensity. A significant gain in water-water enthalpy was seen for both valine and phenylalanine.
机译：大气气溶胶含有多种化合物，其中包括游离氨基酸和盐离子。气溶胶液滴的pH值取决于其来源和环境。因此，液滴中的游离氨基酸等化合物将处于不同的电荷状态，因为这些状态在很大程度上取决于周围的pH条件。在源自海洋的液滴中，氨基酸被认为会将盐离子推向水面，因此，依赖于pH值的氨基酸表面倾向将间接影响大气化学和物理中的许多过程，例如云凝结。为了了解甘氨酸、缬氨酸和苯丙氨酸在酸性、中性和碱性pH下的表面倾向性，我们使用分子动力学（MD）模拟来研究它们在水中的三种不同电荷状态。它们各自的表面倾向通过伞式取样法中的平均力势（PMF）获得。发现甘氨酸对表面没有偏好，而缬氨酸和苯丙氨酸都表现出较高的倾向性。在表面富集氨基酸的电荷态中，代表低pH值氨基酸的阳离子具有最高的亲和力。自由能分解表明，驱动力强烈依赖于氨基酸的性质及其电荷状态。在苯丙氨酸中，主要因素是大量的熵增益，可能与侧链有关，而在缬氨酸中，与官能团的氢键产生有利的能量，进而影响表面倾向性。缬氨酸和苯丙氨酸的水焓均显著增加。

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