  A two amino acid (hydrophobic and polar) scheme is used to perform the design
on target conformations corresponding to the native states of twenty single
chain proteins. Strikingly, the percentage of successful identification of the
nature of the residues benchmarked against naturally occurring proteins and
their homologues is around 75 % independent of the complexity of the design
procedure. Typically, the lowest success rate occurs for residues such as
alanine that have a high secondary structure functionality. Using a simple
lattice model, we argue that one possible shortcoming of the model studied may
involve the coarse-graining of the twenty kinds of amino acids into just two
effective types.
