1. Contribution from the Department of Chemistry, Biophysical Chemistry, Umeå University, S-901 87 Umeå, Sweden, Department of Medical Biosciences, Medical Biochemistry, Umeå University, S-901 87 Umeå, Sweden, and Physikalische Chemie, Universität Kaiserslautern, Erwin-Schrödinger-Strasse, D-67663 Kaiserslautern, Germany