H-Bond Network in Amino Acid Cocrystals with H2O or H2O2. The DFT Study of Serine-H2O and Serine-H2O2

The structure, IR spectrum, and H-bond network in the serine-H2O and serine-H2O2 crystals were studied using DFT computations with periodic boundary conditions. Two different basis sets were used: the all-electron Gaussian-type orbital basis set and the plane wave basis set. Computed frequencies of the IR-active vibrations of the titled crystals are quite different in the range of 10-100 cm(-1). Harmonic approximation fails to reproduce IR active bands in the 2500-2800 frequency region of serine-H2O and serine-H2O2. The bands around 2500 and 2700 cm(-1) do exist in the anharmonic IR spectra and are caused by the first overtone of the OH bending vibrations of H2O and a combination vibration of the symmetric and asymmetric bendings of H2O2. The quantum-topological analysis of the crystalline electron density enables us to describe quantitatively the H-bond network. It is much more complex in the title crystals than in a serine crystal. Appearance of water leads to an increase of the energy of the amino acid-amino acid interactions, up to similar to 50 kJ/mol. The energy of the amino acid-water H-bonds is similar to 30 kJ/mol. The H2O/H2O2 substitution does not change the H-bond network; however, the energy of the amino acid-H2O2 contacts increases up to 60 kJ/mol. This is caused by the fact that H2O2 is a much better proton donor than H2O in the title crystals.