1. Department of Medical Biochemistry and Genetics, Texas A&M University, College Station, Texas 77843-1114
2. Department of Chemistry, Texas A&M University, College Station, Texas 77843-1114
3. Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843-1114
4. Department of Biological Sciences, Stanford University, Stanford, California 94305-5020