phi-Values beyond the ribosomally encoded amino acids: kinetic and thermodynamic consequences of incorporating trifluoromethyl amino acids in a globular protein.

The consequences of the substitution of 4,4,4-trifluorovaline for valine on the folding kinetics and thermodynamics of a globular protein are presented. Variants of the N-terminal domain of L9, a small alpha-beta protein, were prepared in which V3 or V21 was replaced by trifluorovaline. CD and NMR demonstrate that the structure is not perturbed. Both are more stable, the V3 variant by 0.8 kcal mol-1 and the V21 variant by 1.4 kcal mol-1. The increase of stability is significantly larger than that observed in coiled-coils on a per trifluoromethyl group basis. Folding is two-state, and the variants both fold faster than the wild type. The Phi-values are 0.16 and 0.11, respectively.