Natural bond orbital (NBO); quantum theory of atoms in molecule (QTAIM); localized molecular orbital energy decomposition analysis (LMO-EDA); tryptophan; hydrogen bond.
The tryptophan-water (Trp-H2O) complexes formed by hydrogen bonding interactions were investigated at the 𝜔B97XD/6-311++G(d,p) level. Five Trp-H2O complexes possessing various types of hydrogen bonds (H-bonds) were characterized by geometries, energies, vibrational frequencies. The nature of the H-bonds were characterized by the natural bond orbital (NBO) and the quantum theory of atoms in molecule (QTAIM) analyses as well. The intramolecular H-bond formed between the amino and carboxyl oxygen atom of tryptophan was retained in most of the complexes, and the cooperativity between the intra and intermolecular H-bonds exist in some complexes. The intramolecular H-bond and some intermolecular H-bonds are strong and have partial covalent character. The H-bonds formed between carboxyl and oxygen/nitrogen atoms are stronger than other H-bonds. The H-bonds involving methylene of tryptophan as H-donor are weak H-bonds. For all complexes,𝛥𝐸ele and 𝛥𝐸ex makes major contributions to the total interaction energy (𝛥𝐸MP2), while 𝛥𝐸disp is the smallest component of the interaction energy. Both hydrogen bonding interaction and structural deformation play important roles in the relative stabilities of the complexes. Regardless of strong H-bonds, the stabilities of some complexes are weakened by the serious structural deformations.