Interpretation

As mentioned previously, this very simple equation for the m value [Eq. (26.42)] is valid for the case of dilute protein in osmolyte solution. The denominator of the solvation expression for the m value [Eq. (26.42)] is independent of the protein and just contains properties of the osmolyte. This can be seen from a comparison of Eq. (26.42) with a modified version of Eq. (26.28)

The numerator in Eq. (26.42), however, contains the balance between protein hydration GPW and protein-osmolyte solvation GPO, that is, preferential interaction.

Using Eq. (26.43), we can solve Eq. (26.42) for the change in solvation preference upon unfolding m I (@lnao\

The quantities on the right-hand side of this equation can be measured, and thus A(GPW — GPO) can be calculated. Moreover, general principles can be derived from this equation. The m value is established to be constant for urea (Courtenay et al., 2000; Ferreon and Bolen, 2004; Greene and Pace, 1974; Makhatadze, 1999; Santoro and Bolen, 1988; Timasheff and Xie, 2003), and there is some evidence that this also holds for protecting osmolytes (Felitsky and Record, 2004; Holthauzen and Bolen, 2007; Mello and Barrick, 2003). Equation (26.44) therefore indicates that the relative variation in A(GPW — GPO) is independent of the specific protein. Only the amplitude of this variation (given by the m value) depends on the protein type. The magnitude of the m value in turn is dominated by the interaction between osmolyte and protein backbone (Auton and Bolen, 2005; Bolen and Baskakov, 2001), the most numerous chemical group in proteins.

We discussed already in case 1 the simple self-solvation and hydration of osmolyte. The findings in the current case 2 add another example for the very predictable solvation behavior of osmolytes, which is most likely a necessity for a compound to be useful as an osmolyte. Osmolytes should reasonably be expected to have an effect on proteins that is as uniform as possible. Otherwise, if osmolytes impacted each protein differently, they would wreak havoc on the cells they are supposed to protect.

Examples for A(GPW — GPo) of ribonuclease T1 in the presence of urea and sorbitol are given in Fig. 26.4A (Rosgen et al., 2007). Positive and negative numbers in this plot correspond to stabilizing solvation and destabilizing solvation, respectively. The change in solvation preference upon

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