[1] Hope Coll, Dept Chem, Holland, MI 49423 USA
[2] Univ Warwick, Dept Chem, Coventry CV4 7AL, W Midlands, England
[3] Univ Texas, Inst Theoret Chem, Dept Chem, Austin, TX 78712 USA
[4] Univ Texas, Inst Theoret Chem, Dept Biochem, Austin, TX 78712 USA