The sarcosine effect on protein stability: A case of nonadditivity?

We have used differential scanning calorimetry to determine the effect of l ow concentrations (C = 0-2 M) of the osmolyte sarcosine on the Gibbs energy changes (Delta G) for the unfolding of hen-egg-white lysozyme, ribonucleas e A. and ubiquitin, under the same buffer and pH conditions. We have also c omputed this effect on the basis of the additivity assumption and using pub lished values of the transfer Gibbs energies for the amino acid side chains and the peptide backbone unit. The values thus predicted for the slope a D elta G/aC agree with the experimental ones, but only if the unfolded state is assumed to be compact (that is, if the, accessibility to solvent of the unfolded state is modeled using segments excised from native structures). T he additivity-based calculations predict similar a Delta G/aC values for th e three proteins studied. We point out that, to the extent chat this approx imate constancy of a Delta G/aC holds, osmolyte-induced increases in denatu ration temperature will be larger far proteins with low unfolding enthalpy (small proteins that bury a large proportion of apolar surface), The experi mental results reported here are consistent with this hypothesis.