1. Laboratory of Comparative Carcinogenesis, National Cancer Institute, FCRDC, Frederick, Maryland 21702, Peptide Analysis and Synthesis Unit, Research Technology Branch, National Institute of Allergy and Infectious Diseases, Rockville, Maryland 20852, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and Faculty of Chemistry, University of Wroclaw, 50-383 Wroclaw, Poland