The center of a
protein that consists of the
hydrophobic groups (naturally)
packed into an almost
crystalline state. Since this is the framework of the structure it is most often
conserved by
evolution. In other words, proteins might vary on their
surfaces - where this will not affect the shape - but not inside.
Similar folds may therefore have similar core packing, although it is a sort of knapsack problem. Combinatorial arrangements of sidechains allow several different possible tilings (as it were). However, for a given set of residues there will be a few arrangements that fold swiftly and stably.