Donetsk National Technical University, 58 Artema Str., 83000 Donetsk, Ukraine.
J Phys Chem B. 2009 Dec 31;113(52):16557-67. doi: 10.1021/jp907751z.
The semiempirical quantum-chemical PM3 method is used to calculate the thermodynamic parameters of clusterization of the S-form of alpha-amino acids with the general composition C(n)H(2n+1)CHNH(2)COOH (n = 5-15) at 278 and 298 K. It is shown that six stable conformations of monomers exist, for which the thermodynamic parameters (enthalpy and Gibbs' energy) of the formation and absolute entropy are calculated. The correlation dependencies of the calculated parameters on the alkyl chain length are found to be linear. The structures of the monomers are used to build larger clusters (dimers, tetramers, hexamers). For all small clusters (comprised of two to six molecules), the thermodynamic parameters of formation and clusterization are calculated. It is shown that for tetramers and hexamers the enthalpy, entropy, and Gibbs' energy of clusterization are linearly dependent on the alkyl chain length, whereas for the dimers these dependencies are stepwise. The thermodynamic characteristics of clusterization of associates tilted by angles of 9 and 30 degrees with respect to the normal to the interface are calculated. It is shown that the 30 degrees angle orientation is more energetically advantageous for this class of compounds. The geometric parameters of the unit cell characteristic for the infinite 2D film which corresponds to the most advantageous conformation of the monomer were calculated using the PM3 parametrization to be a = 4.57-4.71 A and b = 5.67-5.75 A, with the angle between the axes theta = 100-103 degrees . These values agree well with the available experimental data. Spontaneous clusterization of alpha-amino acids at the air/water interface at 278 K takes place if the alkyl chain length exceeds 11-12 carbon atoms, whereas for 298 K this clusterization threshold corresponds to 13-14 carbon atoms in the alkyl chain, also in agreement with the experimental data.
采用半经验量子化学 PM3 方法计算了 S 型α-氨基酸（通式为 C(n)H(2n+1)CHNH(2)COOH，n = 5-15）在 278 和 298 K 下的聚集热力学参数。结果表明，单体存在 6 种稳定构象，并计算了其形成的热力学参数（焓和吉布斯自由能）和绝对熵。发现计算参数与烷基链长度的相关关系呈线性。利用单体结构构建了更大的聚集体（二聚体、三聚体、六聚体）。计算了所有小分子聚集体（由 2 到 6 个分子组成）的形成和聚集热力学参数。结果表明，对于三聚体和六聚体，聚集焓、熵和吉布斯自由能与烷基链长度呈线性关系，而对于二聚体，这些关系呈阶跃式。计算了与界面法线成 9 和 30 度角倾斜的缔合体的聚集热力学特性。结果表明，30 度角取向对这类化合物更为有利。使用 PM3 参数化，计算了与单体最有利构象相对应的无限 2D 膜的单位晶格的几何参数，结果为 a = 4.57-4.71 A 和 b = 5.67-5.75 A，轴之间的夹角 theta = 100-103 度。这些值与现有的实验数据吻合较好。在 278 K 下，α-氨基酸在空气/水界面上的自发聚集发生在烷基链长度超过 11-12 个碳原子的情况下，而在 298 K 下，烷基链长度为 13-14 个碳原子时，也会发生这种聚集，这与实验数据一致。
