[1] Univ Washington, Dept Chem, Seattle, WA 98195 USA
[2] Univ Washington, Dept Biochem, Seattle, WA 98195 USA
[3] Univ Illinois, Dept Chem MC 111, Chicago, IL 60607 USA
[4] Univ Southampton, Sch Biol Sci, Div Biochem & Mol Biol, Southampton SO16 7PX, Hants, England