1. Research Group of Peptide Chemistry, Hungarian Academy of Sciences, Eötvös L. University, Budapest 112, P.O. Box 32, H-1518, Hungary, Department of Organic Chemistry, University of Barcelona, Barcelona, Spain, Department of Chemistry, University of Ioannina, Ioannina, Greece, and Department of Medicinal Microbiology, University of Groningen, Groningen, The Netherlands