1. German National Research Center for Information Technology, Institute for Algorithms and Scientific Computing (GMD-SCAI), Schloss Birlinghoven, D-53754 Sankt Augustin, Germany, C&C Research Laboratories NEC Europe Ltd., D-53757 Sankt Augustin, Germany, and Friedrich-Schiller-Universität Jena, Biologisch-Pharmazeutische-Fakultät, Institut für Biochemie & Biophysik, Philosophenweg 12, D-07743 Jena, Germany