Methods, setup and safe handling for anhydrous hydrogen fluoride cleavage in Boc solid-phase peptide synthesis

Solid-phase peptide synthesis (SPPS) using tert-butyloxycarbonyl (Boc)/benzyl (Bzl) chemistry is an indispensable technique in many laboratories around the globe, and it provides peptides to the pharmaceutical industry and to thousands of scientists working in basic research. The Boc/Bzl strategy has several advantages, including reliability in the synthesis of long and difficult polypeptides, alternative orthogonality regarding protecting groups and ease of producing C-terminal thioesters for native chemical ligation applications. In this process, anhydrous hydrogen fluoride (HF) is used to remove the side chain protecting groups of the assembled peptide and to release the peptide from the resin, a process typically described as 'HF cleavage'. This protocol describes the general methodology, apparatus setup and safe handling of HF, with the aim of providing comprehensive information on the safe use of this valuable, well-studied and validated cleavage technique. We explain the cleavage mechanism, the physicochemical properties and risks of HF, first aid measures and the correct use of the apparatus. In addition, we provide advice on scavenger selection, as well as a troubleshooting section and video material illustrating key steps of the procedure. The protocol comprises precleavage sample preparation (30 min-2.5 h), complete HF cleavage procedure (2 h) and reaction workup (30 min).

A cleavage cocktail for methionine-containing peptides

A new cocktail has been developed for cleavage and deprotection of methionine-containing peptides synthesized by 9-fluorenylmethoxycarbonyl (Fmoc)-based solid-phase peptide synthesis methodology. The cocktail (trifluoroacetic acid 81%, phenol 5%, thioanisole 5%, 1,2-ethanedithiol 2.5%, water 3%, dimethylsulphide 2%, ammonium iodide 1.5% w/w) was designed to minimize methionine side-chain oxidation. Application of the new cocktail (Reagent H) is demonstrated with the synthesis of a model pentadecapeptide from the active site of DsbC, a periplasmic protein involved in protein disulphide bond formation. The model peptide, which contains one methionine and two cysteine residues, was cleaved with several cleavage cocktails, including Reagent H. The crude peptides obtained with the widely used cocktails K, R and B were found to be 15% to 55% in the methionine sulphoxide form, whereas no methionine sulphoxide was detected in the crude peptide obtained by cleavage and deprotection with Reagent H. Also, no methionine sulphoxide was detected when 1.5% w/w NH4I was added to cocktails K, R and B; however, the yield of the desired peptide was less than with Reagent H. A second 28 amino acid model peptide of the active site of DsbC was also cleaved and deprotected with Reagent H. The reduced dithiol form of the peptide was found to be the major component (51% yield) of the crude peptide obtained by cleavage for 3 h. When the cleavage time was extended to 10 h, the peptide was converted to the intramolecular disulphide form (35% yield). A proposed mechanism for the in situ oxidation of cysteine with Reagent H is presented.

Cleavage and deprotection of peptides on MBHA-resin with hydrogen bromide

Dilute hydrogen bromide in trifluoroacetic acid containing pentamethylbenzene and thioanisole was used in the cleavage and deprotection of peptides on MBHA-resin. Particular attention was paid to potential applicability of the method to kilogram scale synthesis of thymosin alpha 1. In the HPLC purification of the peptides, acetonitrile was replaced by relatively nontoxic isopropanol. The change should be economically and environmentally very attractive.

A cleavage method which minimizes side reactions following Fmoc solid phase peptide synthesis

The success of solid phase peptide synthesis utilizing 9-fluorenylmethoxycarbonyl (Fmoc) amino acids is often limited by deleterious side reactions which occur during TFA peptide-resin cleavage and side-chain deprotection. The majority of these side reactions modify susceptible residues, such as Trp, Tyr, Met, and Cys, with TFA-liberated side-chain protecting groups and linkers. The purpose of this study was to assess the relative effectiveness of various scavengers in suppressing these side reactions. We found that the cleavage mixture 82.5% TFA : 5% phenol : 5% H2O : 5% thioanisole : 2.5% EDT (Reagent K) was maximally efficient in inhibiting a great variety of side reactions. Synthesis and cleavage of 10 peptides, each containing 20-50 residues, demonstrated the complementarity of Fmoc chemistry with Reagent K for efficient synthesis of complex peptides.

Solid phase peptide synthesis utilizing 9-fluorenylmethoxycarbonyl amino acids

9-Fluorenylmethoxycarbonyl (Fmoc) amino acids were first used for solid phase peptide synthesis a little more than a decade ago. Since that time, Fmoc solid phase peptide synthesis methodology has been greatly enhanced by the introduction of a variety of solid supports, linkages, and side chain protecting groups, as well as by increased understanding of solvation conditions. These advances have led to many impressive syntheses, such as those of biologically active and isotopically labeled peptides and small proteins. The great variety of conditions under which Fmoc solid phase peptide synthesis may be carried out represents a truly "orthogonal" scheme, and thus offers many unique opportunities for bioorganic chemistry.

Specificity of a cysteine proteinase of Entamoeba histolytica against various unblocked synthetic peptides

The activity of highly purified cysteine proteinase of Entamoeba histolytica against different peptides of the sequence X-Gly-Phe-Phe was compared. The synthetic peptide Arg-Gly-Phe-Phe of the insulin B-chain was readily hydrolyzed yielding Arg-Gly and Phe-Phe as split products. Lys-Gly-Phe-Phe and Tyr-Gly-Phe-Phe were cleaved at rates of 20 and 4%, respectively. Val-Gly-Phe-Phe, Gly-Gly-Phe-Phe, Glu-Gly-Phe-Phe, and Ser-Gly-Phe-Phe were hydrolyzed at rates far below 1%. Gly-Arg-Phe-Phe, Gly-Phe-Phe, and Gly-Phe were completely resistant to the enzyme. Another good substrate was found in Arg-Gly-Leu-Hyp, which represents a model compound of a scissile site in collagen type I. Furthermore, peptide Arg-Arg-Phe-Phe was attacked by the enzyme releasing Arg-Arg and Phe-Phe. Compared with Arg-Gly-Phe-Phe at substrate concentrations of 2 mM the rates of hydrolysis of Arg-Arg-Phe-Phe and Arg-Gly-Leu-Hyp were 37 and 127%. The enzyme exhibited dipeptidyl peptidase activity against the nonapeptide Arg-Gly-Phe-Phe-Tyr-Thr-Pro-Lys-Ala releasing Arg-Gly.

Purification and partial characterization of high and low activity carbonic anhydrase isoenzymes from Malaclemys terrapin centrata

1. High activity (CA C) and low activity (CA B) carbonic anhydrase isoenzymes have been purified from turtle erythrocytes. 2. The two isoenzymes differed in CO2 hydration specific activity by 36-fold. 3. The low activity isoenzyme contained one half-cystine residue, whereas the high activity isoenzyme contained four half-cystines and required a reducing environment to maintain activity. Both isoenzymes contained zinc. 4. Molecular weights of 28,500 and 30,400 daltons were established for the low and high activity isoenzymes respectively. 5. Both isoenzymes were inhibited by acetazolamide, but only the high activity isoenzyme was inhibited by parachloromercuribenzoate. 6. The low activity isoenzyme was present in the erythrocytes at about 8-10 times the concentration of the high activity isoenzyme. 7. The high activity isoenzyme cross-reacted with antibodies prepared against pure chicken carbonic anhydrase C.