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

The quantum-chemical semiempiric PM3 method is used to calculate the thermodynamic parameters of clusterization for the racemic α-amino acids C n H2n+1CHNH2COOH with n = 5−15 at 278 and 298 K. Possible relative orientations of the monomers in the heterochiral clusters are considered. It is shown that, for the racemic mixtures of α-amino acids, the formation of heterochiral 2D films is most energetically preferable with the alternating (rather than “checkered”) packing of the enantiomers with opposite specific rotation. The two enantiomeric forms of α-amino acids in the heterochiral 2D clusters are tilted with respect to the normal to the q direction at angles of φ1 = 20° and φ2 = 33°, whereas the single enantiomeric forms are oriented at an angle of δ = 9° with respect to the normal to the p direction. It is shown that the heterochiral 2D film based on the α-amino acid structures oriented at the angle φ2 = 33° with respect to the normal to the q direction possesses a rectangular unit cell with the geometric parameters a = 4.62 Å and b = 10.70 Å and the tilt angle of the alkyl chain of the molecule with respect to the interface t 2 = 35°, which is in good agreement with the X-ray structural data a = 4.80 Å, b = 9.67 Å, and t 2 = 37°. The parameters of the lattice structure of monolayers formed by amphiphilic amino acids are shown to be determined by the “a” type of the intermolecular H−H interactions, whereas the tilt angle of the molecules with respect to the interface depends on the volume and the structure of the functional groups involved in the hydrophilic part of the molecule. Spontaneous clusterization of the racemic form of α-amino acids at the air/water interface at 278 K takes place if the alkyl chain length is equal or higher than 12−13 carbon atoms, whereas for 298 K this clusterization threshold corresponds to 14 carbon atoms in the hydrocarbon chain. These values agree with the experimental data.