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REFRAMING OSMOPHOBIC AND OSMOPHILIC EFFECTS**

Abstract

The osmophobic effect has been an accessible phrase to introduce biochemists to ideas of how osmolytes effect biochemical processes. It was built by analogy with the hydrophobic effect, central to protein folding, and the observation that urea primarily interacts with the backbone of proteins, and less significantly with side chains. We have revisited experiments underpinning the original formulation of the osmophobic effect and found that there is at least one crucial error in the data used. This error, in combination with other experimental findings, indicates a significant role for side chain-osmolyte interactions. We compare the predictive model based on the solubilities of protein backbone and side chains (group transfer free energy model) with a model based on molecular surface types (solute partitioning model). We show that our new solubility data is consistent with vapor pressure osmometry data used to construct the solute partitioning model. We conclude that available evidence supports using models for osmolyte effects on biochemical processes that rely on exposed atomic surface types rather than on molecular functional groups. But instead of discarding useful terminology that helps newcomers more quickly grasp the importance of the topic, we propose reframing the osmophobic effect from the idea that osmolytes primarily fear the protein backbone to more broadly applicable osmophobic and osmophilic effects where osmolytes are attracted to and excluded from different atomic surface types.

Acknowledgements

Middle Georgia State University Department of Natural Sciences and Faculty Develepment Funds

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