The Solution of Large Small-Molecule Structures and Small Macromolecules

As pointed out in a recent editorial inScience (Moffat, 1992), the phase problem is probably most difficult to solve for ‘mid-sized’ molecules of about 200-500 non-hydrogen atoms. Direct methods rarely fail on structures of less than I 00 atoms and are often successful up to about 200 atoms; the largest unknown structure to be solved purely by direct methods is probably gramicidin-A, with 334 unique atoms (Langs, 1988). Classically, most proteins have been solved by the preparation of heavy-atom derivatives, but the introduction of such atoms distorts smaller molecules severely, leading to lack of isomorphism, an essential requirement of the MIR method. Molecular replacement - using a fragment of a related structure as a search probe, usually in Patterson space - is applicable to both small molecules and macromolecules, but many mid-sized structures of biological interest possess more conformational flexibility than this approach can tolerate.