1. Contribution from the Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003, Institute of Biophysics, Academy of Sciences of the Czech Republic, 61265 Brno, Czech Republic, and Center for Biomolecular Recognition, IMBG, Department of Biochemistry B, The Panum Institute, Blegdamsvej 3c, DK 2200 Copenhagen, Denmark