Use of the 3-nitro-2-pyridine sulfenyl protecting group to introduce N epsilon-branching at lysine during solid-phase peptide synthesis. I. Application to the synthesis of a peptide template containing two addressable sites.

TASPs (template-assembled synthetic peptides) are generated by the covalent attachment of linear peptides to a common peptide backbone, thus generating larger synthetic peptides/proteins with prefolded structure. In this work we present a strategy for the synthesis of a heterotemplate-assembled synthetic peptide containing two addressable sites. This orthogonal protection strategy would allow the selective introduction of different peptide chains via the epsilon-amino functions of template lysines being protected by either fluorenylmethoxycarbonyl (Fmoc) or 3-nitro-2-pyridine sulfenyl (Npys) groups. The N alpha-Boc-N epsilon-Npys-L-lysine required for solid-phase peptide synthesis (SPPS) is not readily available at a reasonable cost. To facilitate the more widespread use of this reagent we have compared the two published procedures for synthesizing this protected amino acid and evaluated the suitability of the products for SPPS. Two resin-bound peptides, a tripeptide Ac-G-K-Npys)-G-resin and an octapeptide template Ac-P1-K2-K3-L4-K5-K6-P7-G8-resin, were synthesized by SPPS. The epsilon-amino functions of lysines K2 & K6 and K3 & K5 of the octapeptide were protected by Fmoc and Npys groups, respectively. Secondly, these peptides were used to evaluate various reagents and reaction conditions for the deprotection of the epsilon-amino function of lysines bearing the N epsilon-Npys protecting group. A procedure for the optimized selective and quantitative deprotection of the Npys group from the epsilon-amino function of lysine in a resin-bound peptide using 2-mercaptopyridine-N-oxide is described.