1. Institute of Biophysics of the Bulgarian Academy of Science, Str. Acad. G. Bonchev 1, Block 21, 1113 Sofia, Bulgaria, Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia, Department of Chemistry, Imperial College, Str. Kensington, London SW7 2AY, U.K., and Philips Research, Str. Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands