1. Natuur-
en Sterrenkunde and LaserLaB, Vrije Universiteit, Amsterdam 1081 HV, The Netherlands
2. Laboratory
of Organic Chemistry, ETH Zürich, Zurich 8093, Switzerland
3. Department
of Biochemistry, University of Washington, Seattle, Washington 98195-7350, United States
4. Moleculaire
Biofysica, Zernike instituut, Rijksuniversiteit Groningen, Groningen 9747 AG, The Netherlands