1. School of Biological and Chemical Sciences, Birkbeck College, University of London, Gordon House, 29 Gordon Square, London WC1H 0PP, U.K., Institute of Biology, University of Southern Denmark, 5230 Odense, Denmark, and Michael Barber Centre for Mass Spectrometry, Department of Chemistry, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester, M60 1QD, U.K.