1. Department of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS, UK; Dipartimento di Chimica Fisica, Universita' di Palermo, Viale delle Scienze, Parco d'Orleans II, I-90128 Palermo, Italy; and Institute for Biophysics and X-ray Structure Research, Austrian Academy of Science, Steyrerg. 17, 8010 Graz, Austria