Protein structures that might exist but don't

A recurrent problem in analysis of protein strucl ures is the question of whether features commonly observed are necessary, or are archaeological artefacts. Protein design can help to probe certain rules by trying to synthesize exceptions. Two kinds of design goals will be described, one at lhe atomic level: a redesign of the globin structure to change the class of packing at a helix-helix interface, and one at the global level, based on work towards the systematic enumeration of possible protein folding patterns to search for absences from the set of known structures of certain lopological patterns.