Quantum-Chemical Analysis of Condensed Monolayer Phases of N-Alkanoyl-substituted Alanine at the Air/Water Interface

Abstract The thermodynamic and structural parameters of monolayers of N -alkanoyl-substituted alanine with 8–17 methylene fragments in the alkyl chain are calculated on the basis of the quantum chemical semiempirical PM3 method. Four optimized structures of monomers are obtained. The energetically most preferred conformer is used for construction of dimers with “sequential” and “parallel” orientation of the hydrophilic parts of the monomers and tetramers with square structure. The hydrophobic chains of N -alkanoyl-substituted alanine are found to be inclined in the dimers to the normal of the spread monolayer: δ = 10° and φ = 43°. Comparison of clusterization Gibbs’ energy for small clusters suggests that 2D film formation takes place by formation of linear associates comprised of dimers with “parallel” orientation of the hydrophilic groups. The temperature increase causes growth of linear 1D clusters leading to the dendritic structure of the monolayer. The parameters of the unit cell of 2D film are calculated: a = 4.65 Ǻ, b = 6.20 Ǻ, the angle between the sides of the cell θ = 105˚. The tilt angle of amphiphilic molecules to the normal of the air/water interface is t = 47°. The calculated parameters of the unit cell are in good agreement with the existing experimental data. The spontaneous clusterization threshold for N -alkanoyl-substituted alanine at the air/water interface is calculated to be 15 carbon atoms in the chain at 288 K. This is only 2 carbon atoms more than the experimental value and within the margin of error of the model used for the calculation.