1. Mass Spectrometry Resource, Department of Biochemistry, Boston University School of Medicine, 670 Albany Street, Boston, Massachusetts 02118, Agilent Laboratories, Agilent Technologies, 5301 Stevens Creek Blvd., MS 3 L-WA, Santa Clara, California 95051, and Agilent Technologies, Hewlett-Packard-Street 8, 76337 Waldbronn, Germany