Simple, effective protocols have been developed for manual and machine-assisted Boc-chemistry solid phase custom peptide synthesis on polystyrene resins. These use in situ neutralization [i.e. neutralization simultaneous with coupling], high concentrations (>0.2 m ) of Boc-amino acid-OBt esters plus base for rapid coupling, 100% TFA for rapid Boc group removal, and a single short (30 s) DMF flow wash between deprotection/coupling and between coupling/deprotection. Single 10 min coupling times were used throughout.
Overall cycle times were 15 min for manual and 19 min for machine-assisted synthesis (75 residues per day). No racemization was detected in the . base-catalyzed coupling step. Several side reactions were studied, and eliminated. These included: pyrrolidonecarboxylic acid formation from Gln in hot TFA-DMF; chain-termination by reaction with excess HBTU; and, chain termination by acetylation (from HOAc in commercial Boc-amino acids). The in situ neutralization protocols gave a significant increase in the efficiency of chain assembly, especially for “difficult” sequences arising from sequence-dependent peptide chain aggregation in standard (neutralization prior to coupling) Boc-chemistry SPPS protocols or in Fmoc-chemistry SPPS. Reported syntheses include HIV-1 protease(1–50,Cys.amide), HIV-1 protease(53–99), and the full length HIV-l protease(1–99).
Well-characterized resins of high purity are critical for effective solid phase custom peptide synthesis (SPPS). The quality of commercial (4-methyl)benzhydrylamine-resin (MBHA-resin), used for the synthesis of peptide amides, is not consistent and residual ketone functionalities are frequently present. Such ketone or aldehyde impurities lead to the formation of acylation-resistant deletion peptides... [Show full abstract]