Shiraga K, Suzuki T, Kondo N, Ogawa Y
Graduate School of Agriculture, Kyoto University, Kitashirakawa-oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan.
J Chem Phys. 2014 Dec 21;141(23):235103. doi: 10.1063/1.4903544.
Water conformation around hydrophobic side chains of four amino acids (glycine, L-alanine, L-aminobutyric acid, and L-norvaline) was investigated via changes in complex dielectric constant in the terahertz (THz) region. Each of these amino acids has the same hydrophilic backbone, with successive additions of hydrophobic straight methylene groups (-CH2-) to the side chain. Changes in the degree of hydration (number of dynamically retarded water molecules relative to bulk water) and the structural conformation of the water hydrogen bond (HB) network related to the number of methylene groups were quantitatively measured. Since dielectric responses in the THz region represent water relaxations and water HB vibrations at a sub-picosecond and picosecond timescale, these measurements characterized the water relaxations and HB vibrations perturbed by the methylene apolar groups. We found each successive straight -CH2- group on the side chain restrained approximately two hydrophobic hydration water molecules. Additionally, the number of non-hydrogen-bonded (NHB) water molecules increased slightly around these hydrophobic side chains. The latter result seems to contradict the iceberg model proposed by Frank and Evans, where water molecules are said to be more ordered around apolar surfaces. Furthermore, we compared the water-hydrophilic interactions of the hydrophilic amino acid backbone with those with the water-hydrophobic interactions around the side chains. As the hydrophobicity of the side chain increased, the ordering of the surrounding water HB network was altered from that surrounding the hydrophilic amino acid backbone, thereby diminishing the fraction of NHB water and ordering the surrounding tetrahedral water HB network.
通过太赫兹（THz）区域复介电常数的变化，研究了四种氨基酸（甘氨酸、L-丙氨酸、L-氨基丁酸和L-正缬氨酸）疏水侧链周围的水构象。这些氨基酸中的每一种都具有相同的亲水主链，其侧链上依次添加了疏水直链亚甲基（-CH2-）。定量测量了与亚甲基数量相关的水合程度（相对于大量水的动态受阻水分子数量）和水氢键（HB）网络的结构构象变化。由于太赫兹区域的介电响应代表亚皮秒和皮秒时间尺度上的水弛豫和水HB振动，这些测量表征了被亚甲基非极性基团扰动的水弛豫和HB振动。我们发现侧链上每连续一个直链-CH2-基团会束缚大约两个疏水水合水分子。此外，在这些疏水侧链周围，非氢键结合（NHB）水分子的数量略有增加。后一结果似乎与Frank和Evans提出的冰山模型相矛盾，在该模型中，据说水分子在非极性表面周围更加有序。此外，我们比较了亲水氨基酸主链的水-亲水相互作用与侧链周围的水-疏水相互作用。随着侧链疏水性的增加，周围水HB网络的有序性与亲水氨基酸主链周围的不同，从而减少了NHB水的比例，并使周围的四面体水HB网络有序化。
