Department of Chemistry, Hunter College, City University of New York, 695 Park Avenue, NY 10065, USA; Program in Chemistry, CUNY Graduate Center, 365 5(th) Avenue, New York, NY 10016, USA; Program in Biochemistry, CUNY Graduate Center, 365 5(th) Avenue, New York, NY 10016, USA. Electronic address: whardi@hunter.cuny.edu.