1. Laboratory of Physical and Structural Biology, PPB, NICHD, NIH, Bethesda, Maryland 20892-0924; Department of Physiology, A. A. Dugoni School of Dentistry, University of the Pacific, San Francisco, California 94115; Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana, and Theoretical Physics Department, J. Stefan Institute, SI-1000 Ljubljana, Slovenia; and Center for Biomembrane Physics, University of Southern Denmark, DK-5230 Odense, Denmark