Molecular-Level Understanding of Ground- and Excited-State O-H⋅⋅⋅O Hydrogen Bonding Involving the Tyrosine Side Chain: A Combined High-Resolution Laser Spectroscopy and Quantum Chemistry Study

Abstract

The present study combines both laser spectroscopy and abinitio calculations to investigate the intermolecular O H···O hy-drogen bonding of complexes of the tyrosine side chainmodel chromophore compounds phenol (PH) andpara-cresol(pCR) with H2O, MeOH, PH andpCR in the ground (S0) state aswell as in the electronic excited (S1) state. All the experimentaland computational findings suggest that the H-bond strengthincreases in the S1state and irrespective of the hydrogen bondacceptor used, the dispersion energy contribution to the totalinteraction energy is about 10–15% higher in the S1state com-pared to that in the S0state. The alkyl-substituted (methyl;+Ieffect) H-bond acceptor forms a significantly stronger H bondboth in the S0and the S1state compared to H2O, whereas thearyl-substituted (phenyl; R effect) H-bond donor showsa minute change in energy compared to H2O. The theoreticalstudy emphasizes the significant role of the dispersive interac-tions in the case of thepCR and PH dimers, in particular theC H···O and the C H···pinteractions between the donor andacceptor subunits in controlling the structure and the energet-ics of the aromatic dimers. The aromatic dimers do not followthe acid–base formalism, which states that the stronger thebase, the more red-shifted is the X H stretching frequency,and consequently the stronger is the H-bond strength. This isdue to the significant contribution of the dispersion interactionto the total binding energy of these compounds