[1] Texas A&M Univ, Dept Biochem & Biophys, College Stn, TX 77843 USA
[2] Texas A&M Univ, Ctr Macromol Design, College Stn, TX 77843 USA
[3] Bristol Myers Squibb Pharmaceut Res Inst, Dept Macromol Struct, Princeton, NJ 08543 USA
[4] Harvard Univ, Sch Med, Dept Microbiol & Mol Genet, Boston, MA 02115 USA