1. Department of Bioengineering, Rice University, P.O. Box 1892, MS 142, Houston, Texas 77251-1892, Department of Neurological Surgery and the Research Institute, University Hospitals of Cleveland and the Department of Neurological Surgery, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106-5042, and Department of Chemistry, Rice University, P.O. Box 1892, MS 60, Houston, Texas 77251-1892