Rism-based pressure-dependent computational spectroscopy

Abstract

Spectroscopic measurements are an indispensable tool in chemical analysis; even under extreme conditions such as high hydrostatic pressures, they can provide valuable insights. Theoretical methods that can reliably reproduce observables in solution can be used to validate the obtained results. A common theoretical model is the Reference Interaction Site Model (RISM), which was used in this work. In the first part, a previously developed method for calculating IR frequencies with the embedded cluster(EC)-RISM under equilibrium conditions was extended to non-equilibrium thermodynamics for IR spectroscopy. The pressure-dependent IR frequency shifts of TMAO and the cyanide anion were investigated as model systems. Furthermore, EC-RISM was used here for the first time to calculate EPR observables at ambient conditions. First, experiments with the geometrically optimized structure showed that EC-RISM gives significantly better results than a standard continuum calculation despite a large deviation from the experiment. A significant improvement in the direction of the experimental values was achieved by using a large number of snapshots from an ab initio molecular dynamics simulation (AIMD) instead of a single geometry. In general, in the context of the theoretical description of high-pressure effects on proteins, the critical question can be raised whether using force fields parameterized for ambient conditions is appropriate for high-pressure conditions. To answer this question, the pressure dependence of the peptide backbone was investigated in the third part, and the small molecules N-methyl acetamide (NMA) and Ac-Gly/Ala-NHMe were used as model systems. In this work, it was shown that EC-RISM is a suitable method of choice for the calculation of spectroscopic observables in solution. Especially when non-ambient conditions are to be examined, EC-RISM shows its strength since it is relatively easily extensible, e.g., high-pressure environments.