Rate-temperature relationships in lambda-repressor fragment lambda 6-85 folding.

Two classes of lambda(6-85) mutants (those richer in alanine, and those richer in glycine) have very similar slopes in an Arrhenius plot of the unfolding rates but very different temperature dependencies of the folding rates. Temperature-dependent interactions (e.g., hydrophobicity) play a large role in the initial stages of folding but not in the initial stages of unfolding of lambda(6-85). Placement of the transition state in terms of its surface exposure and entropy shows that at least two reaction coordinates are required to describe folding of all mutants over the full temperature range. The unusual Arrhenius plots of the very fastest mutant provide an additional kinetic signature for downhill folding.