How the hydrophobic factor drives protein folding

Significance A study of how hydrophobicity (HY) drives protein folding reveals two kinds of HY: intrinsic (proportional to surface area) and extrinsic (augmented by hydration shells). The Nozaki–Tanford method of measuring HY has been modified to use gaseous solutes. Hydration shells on alkanes explain the unusual HY of alkanes. Transfer of alkanes from water to cyclohexane causes release of alkane hydration shells. Comparing the energetics of formation and release shows that hydration shells are clearly present on alkanes and supports Kauzmann's 1959 mechanism for how the hydrophobic factor drives protein folding. A space-filling model is given for the hydration shells on linear alkanes. How hydrophobicity (HY) drives protein folding is studied. The 1971 Nozaki–Tanford method of measuring HY is modified to use gases as solutes, not crystals, and this makes the method easy to use. Alkanes are found to be much more hydrophobic than rare gases, and the two different kinds of HY are termed intrinsic (rare gases) and extrinsic (alkanes). The HY values of rare gases are proportional to solvent-accessible surface area (ASA), whereas the HY values of alkanes depend on special hydration shells. Earlier work showed that hydration shells produce the hydration energetics of alkanes. Evidence is given here that the transfer energetics of alkanes to cyclohexane [Wolfenden R, Lewis CA, Jr, Yuan Y, Carter CW, Jr (2015) Proc Natl Acad Sci USA 112(24):7484–7488] measure the release of these shells. Alkane shells are stabilized importantly by van der Waals interactions between alkane carbon and water oxygen atoms. Thus, rare gases cannot form this type of shell. The very short (approximately picoseconds) lifetime of the van der Waals interaction probably explains why NMR efforts to detect alkane hydration shells have failed. The close similarity between the sizes of the opposing energetics for forming or releasing alkane shells confirms the presence of these shells on alkanes and supports Kauzmann's 1959 mechanism of protein folding. A space-filling model is given for the hydration shells on linear alkanes. The model reproduces the n values of Jorgensen et al. [Jorgensen WL, Gao J, Ravimohan C (1985) J Phys Chem 89:3470–3473] for the number of waters in alkane hydration shells.

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