Quantum-Chemical Analysis of Thermodynamics of Two-Dimensional Cluster Formation of α-Amino Acids at the Air/Water Interface

Abstract

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.

The quantum-chemical approach to calculations of thermodynamic and structural parameters of formation of fatty acid monolayers with hexagonal packing at the air/water interface

“…The series of previous works about thermodynamic parameter calculations of substituted alkanes at the air/water interface demonstrate that the parameter values calculated within the quantum-chemical semiempiric PM3 method (Mopac 2000) reproduce the corresponding experimental parameters 17,[23][24][25][26][27][28][29][30] with a good level of accuracy. As the thermodynamic parameters of the formation of monomers and dimers and the clusterization (for the Dimers 1) are very close to the parameters calculated previously using Mopac 2000, 17 it allows us to conclude that thermodynamic parameters of monomers and clusters of the substituted alkanes (particularly, carboxylic acids), calculated within Mopac 2012, describe the experimental parameters with the same accuracy.…”

Theoretical Description of Mixed Film Formation at the Air/Water Interface: Carboxylic Acids–Alcohols

“…17,18 However, there are few theoretical papers modeling their behavior during monolayer formation. Therefore, we developed an approach for the description of 2D clusterization of nonionic surfactants (aliphatic alcohols, 19,20 carboxylic acids, 21,22 amides, 23 α-substituted carboxylic acids, 24,25 and so on) at the air/water interface. The values obtained for the calculated alkyl chain length of the surfactants enabling monolayer formation at a given temperature are in agreement with the existing experiments and values of the structural parameters of the corresponding monolayers.…”

Amino Acids at Water−Vapor Interfaces: Surface Activity and Orientational Ordering

“…The aggregation behavior of up to 15 amino acids at the air-water interface was studied in a quantum chemical analysis of model amino acids with varying aliphatic side chain lengths. 5 Shaytan et al calculated free energy profiles along the vacuum-water interface for side chain analogues from molecular dynamics (MD) simulations. 6 However, to our knowledge there are no studies focused on the surface properties of single amino acids as the surface activity or the surface orientation.…”