1. Contribution from the Physical Chemistry Division, Arrhenius Laboratory, Stockholm University, S-10691 Sweden, Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, NL 2300 RA Leiden, The Netherlands, and Department of Biochemistry, Institute of Cellular Signalling, University of Nijmegen, P.O. Box 9101, NL 6500 HB Nijmegen, The Netherlands