  We study the medium effects on the selection of sequences in protein folding
by taking account of the surface potential in HP-model. Our analysis on the
proportion of H and P monomers in the sequences gives a direct interpretation
that the lowly designable structures possess small average gap. The numerical
calculation by means of our model exhibits that the surface potential enhances
the average gap of highly designable structures. It also shows that a most
stable structure may be no longer the most stable one if the medium parameters
changed.
