During solid-phase peptide synthesis (SPPS) the three nitrogen atoms of the guanidine group of Arg (Figure 9.1), being strongly nucleophilic, are prone to alkylation and subsequent Orn (ornithine) formation upon base-mediated decomposition [1], and therefore need to be protected. However, in common practice, most protecting groups block only the v-nitrogen. In addition, free unprotected Arg residues tend to cyclize upon activation of the a-carboxylic group to form d-lactams.
In the fluorenylmethoxycarbonyl (Fmoc)/tert-butyl (tBu) strategy the most commonly used protecting groups of Arg are the arylsulfonyl-based derived from the tosyl group (Tos) [2] such as the 4-methoxy-2,3,6-trimethylbenzenesulfonyl (Mtr) group 1, now superseded by the two cyclic ether derivates 2,2,5,7,8-pentamethylchroman-6sulfonyl (Pmc) 2 and 2,2,4,6,7-pentamethyldihydrobenzofuran-5-sulfonyl (Pbf) 3. Mtr removal requires several hours of trifluoroacetic acid (TFA) treatment and often causes sulfonation of Trp residues [3], which can be avoided using 1 M bromotrimethylsilane (TMSBr) in TFA [4]. Moreover, long TFA treatment can cause Osulfonation of Ser and Thr, which can be suppressed by adding thiocresol to the cleavage cocktail [5]. The Pmc group, being much more acid sensitive than Mtr, can be removed faster and the Trp/Tyr modifications are less pronounced [6], and can be overcome if the Trp indole ring is Boc protected [7]. The Pbf group [8], the dihydrofuran analog of the Pmc group, is at present the most widely used Argprotecting group. It has proved to be more acid labile than Pmc (its removal is 1-2 times faster) and generates less alkylation than the other arylsulfonyl-protecting groups.