1. Department of Neuro-Oncology, The University of Texas M. D. Anderson Cancer Center, Box 316, 1515 Holcombe Boulevard, Houston, Texas 77030, Department of Chemistry, Rice University, 6100 Main, Houston, Texas 77005, and Department of Chemistry, The University of Houston, 4800 Calhoun, Houston, Texas 77204 jsmcmur@mdanderson.org