Hydration free energies of polypeptides from popular implicit solvent models versus all-atom simulation results based on molecular quasichemical theory

TL;DR

This study compares popular implicit solvent models with all-atom molecular quasi-chemical theory results to evaluate their accuracy in predicting hydration free energies of polypeptides, highlighting strengths and limitations of each approach.

Contribution

It provides a rigorous comparison of implicit solvent models against all-atom QCT calculations, revealing their relative accuracy and underlying assumptions.

Findings

GB/SA captures broad hydration trends better than EEF1 and ABSINTH.

EEF1 and ABSINTH underestimate hydration cooperativity between solute groups.

Validation of implicit models' physical components is crucial for accurate protein thermodynamics.

Abstract

The hydration free energy of a macromolecule is the central property of interest for understanding its distribution over conformations and its state of aggregation. Calculating the hydration free energy of a macromolecule in all-atom simulations has long remained a challenge, necessitating the use of models wherein the effect of the solvent is captured without explicit account of solvent degrees of freedom. This situation has changed with developments in the molecular quasi-chemical theory (QCT), an approach that enables calculation of the hydration free energy of macromolecules within all-atom simulations at the same resolution as is possible for small molecule solutes. The theory also provides a rigorous and physically transparent framework to conceptualize and model interactions in molecular solutions, and thus provides a convenient framework to investigate the assumptions in implicit-solvent models. In this study, we compare the results using molecular QCT versus predictions from EEF1, ABSINTH, and GB/SA implicit-solvent models for poly-glycine and poly-alanine solutes covering a range of chain lengths and conformations. Among the three models, GB/SA does best in capturing the broad trends in hydration free energy. We trace the deficiencies of the group-additive EEF1 and ABSINTH models to their under-appreciation of the cooperativity of hydration between solute groups; seen in this light, the better performance of GB/SA can be attributed to its treatment of the collective properties of hydration, albeit within a continuum dielectric framework. We highlight the importance of validating the individual physical components that enter implicit solvent models for protein solution thermodynamics.