Why Hofmeister effects of many salts favor protein folding but not DNA helix formation

The majority (∼70%) of surface buried in protein folding is hydrocarbon, whereas in DNA helix formation, the majority (∼65%) of surface buried is relatively polar nitrogen and oxygen. Our previous quantification of salt exclusion from hydrocarbon (C) accessible surface area (ASA) and accumulation at amide nitrogen (N) and oxygen (O) ASA leads to a prediction of very different Hofmeister effects on processes that bury mostly polar (N, O) surface compared to the range of effects commonly observed for processes that bury mainly nonpolar (C) surface, e.g., micelle formation and protein folding. Here we quantify the effects of salts on folding of the monomeric DNA binding domain (DBD) of lac repressor (lac DBD) and on formation of an oligomeric DNA duplex. In accord with this prediction, no salt investigated has a stabilizing Hofmeister effect on DNA helix formation. Our ASA-based analyses of model compound data and estimates of the surface area buried in protein folding and DNA helix formation allow us to predict Hofmeister effects on these processes. We observe semiquantitative to quantitative agreement between these predictions and the experimental values, obtained from a novel separation of coulombic and Hofmeister effects. Possible explanations of deviations, including salt-dependent unfolded ensembles and interactions with other types of surface, are discussed.