1. E. Delamarche, H. Schmid, B. Michel, H. Biebuyck, IBM Research Division, Zurich Research Laboratory, CH-8803 Rüschlikon, Switzerland.
2. A. Bernard, IBM Research Division, Zurich Research Laboratory, CH-8803 Rüschlikon, Switzerland, and Biochemisches Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.