To What Extent Does Surface Hydrophobicity Dictate Peptide Folding and Stability near Surfaces?

Protein-surface interactions are ubiquitous in both the cellular setting and in modern bioengineering devices, but how such interactions impact protein stability is not well understood. We investigate the folding of the GB1 hairpin peptide in the presence of self-assembled monolayers and graphite like surfaces using replica exchange molecular dynamics simulations. By varying surface hydrophobicity, and decoupling direct protein-surface interactions from water-mediated interactions, we show that surface wettability plays a surprisingly minor role in dictating protein stability. For both the ß-hairpin GB1 and the helical miniprotein TrpCage, adsorption and stability is largely dictated by the nature of the direct chemical interactions between the protein and the surface. Independent of the surface hydrophobicity profile, strong protein-surface interactions destabilize the folded structure while weak interactions stabilize it.

Reaction coordinates, one-dimensional Smoluchowski equations, and a test for dynamical self-consistency.

We propose a method for identifying accurate reaction coordinates among a set of trial coordinates. The method applies to special cases where motion along the reaction coordinate follows a one-dimensional Smoluchowski equation. In these cases the reaction coordinate can predict its own short-time dynamical evolution, i.e., the dynamics projected from multiple dimensions onto the reaction coordinate depend only on the reaction coordinate itself. To test whether this property holds, we project an ensemble of short trajectory swarms onto trial coordinates and compare projections of individual swarms to projections of the ensemble of swarms. The comparison, quantified by the Kullback-Leibler divergence, is numerically performed for each isosurface of each trial coordinate. The ensemble of short dynamical trajectories is generated only once by sampling along an initial order parameter. The initial order parameter should separate the reactants and products with a free energy barrier, and distributions on isosurfaces of the initial parameter should be unimodal. The method is illustrated for three model free energy landscapes with anisotropic diffusion. Where exact coordinates can be obtained from Kramers-Langer-Berezhkovskii-Szabo theory, results from the new method agree with the exact results. We also examine characteristics of systems where the proposed method fails. We show how dynamical self-consistency is related (through the Chapman-Kolmogorov equation) to the earlier isocommittor criterion, which is based on longer paths.