We use both combinatorial and site-directed mutagenesis
to explore the consequences of surface hydrophobic substitutions
for the folding of two small single domain proteins, the
src SH3 domain, and the IgG binding domain of Peptostreptococcal
protein L. We find that in almost every case, destabilizing
surface hydrophobic substitutions have much larger effects
on the rate of unfolding than on the rate of folding, suggesting
that nonnative hydrophobic interactions do not significantly
interfere with the rate of core assembly.