Competitive binding of peptides containing basic amino acids to disrupt or prevent the Tat-TAR interaction could result in diminished transcription as well as translation and hence constitutes an alternative way of controlling HN replication. Therefore, we synthesized guanidinium and amino containing amino acids, based on a proline or an alanine scaffold. The introduction of the guanidinium moiety was best accomplished using 1H-pyrazole-1-carboxamidine hydrochloride, with Pmc used for its protection. The absence of racemization, maintained throughout the whole synthesis, was confirmed by chiral purity determination. These building blocks were smoothly incorporated into oligopeptides, which proved their suitability for use in a combinatorial approach for selecting TAR binding ligands. (C) 2000 Elsevier Science Ltd. All rights reserved.