1. Department of Molecular and Structural Biochemistry, North Carolina State University, 128 Polk Hall, Raleigh, North Carolina 27695-7622, Institute of Organic Chemistry, Technical University, Zeromskiego 116, 90-924 Lǒdü, Poland, and Integrated DNA Technologies BVBA, Provisorium 2, Minderbroedersstraat 17-19, B-3000 Leuven, Belgium