[1] Rutgers State Univ, Waksman Inst, Piscataway, NJ 08854 USA
[2] Rutgers State Univ, Dept Chem, Piscataway, NJ 08854 USA
[3] UCL, Inst Struct & Mol Biol, Div Biosci, London WC1E 6BT, England
[4] Imperial Coll London, Dept Med, Sect Struct Biol, London SW7 2AZ, England