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Chelmical synthesis: Peptides were synthesized on a Rink amide resin, 0.45 mmol/g [Fmoc-Cys(Trityl)-Wang; Novabiochem, San Diego, Calif.] usinig N-(9-fluorenyl)methoxycarboxyl chemistry and standard side chain protection except on cysteine residues. Cysteine residues were protected in pairs with either S-trityl on the first and third cysteines or S-acetamidomethyl on the second and fourth cysteines. Amino acid derivatives were from Advanced Chemtech (Louisville, Ky.). The peptides were removed from the resin and precipitated, and a two-step oxidation protocol was used to selectively fold the peptides as described previously (Luo et al., 1999). Briefly, the first disulfide bridge was closed by dripping the peptide into an equal volume of 20 mM potassium feliicyanide and 0.1 M Tris, pH 7.5. The solution was allowed to react for 30 min, and the monocyclic peptide was purified by reverse-phase HPLC. Simultaneous removal of the S-acetamidomethyl groups and closure of the second disulfide bridge was carried out by iodine oxidation. The monocyclic peptide and HPLC eluent was dripped into an equal volume of iodine (10 mM) in H20/trifluoroacetic acid/acetonitrile (78:2:20 by volume) and allowed to react for 10 min. The reaction was terminated by the addition of ascorbic acid diluted 20-fold with 0.1percent trifluoroacetic acid and the bicyclic product purified by HPLC. Mass Spectrometry: Measurements were performed at the Salk Institute for Biological Studies (San Diego, Calif.) under the direction of Jean Rivier. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry and liquid secondary ionization mass spectrometry were used.
All peptides were synthesized on a 0.2 mmol scale using manual Fmoc-SPPS chemistry under flow using a 3 minute cycle for each amino acid. Specifically, all reagents and solvents are delivered to a stainless steel reactor containing resins at a constant flow rate using HPLC pump; temperature of the reactor was maintained at 60 °C during the synthesis using water bath. Procedure for each amino acid coupling cycle included a 30 second coupling with 1 mmol Fmoc-protected amino acid, 1 mmol HBTU, and 500 of diisopropyl ethyl amine (DIEA) in 2.5 mL of DMF at a flow rate of 6 mL/min (note that for coupling of cysteine and tryptophan, 190 of DIEA was used to prevent racemization); 1 minute wash with DMF at a flow rate of 20 mL/min; 20 second deprotection with 50percent (v/v) piperidine in DMF at a flow rate of 20 mL/min; and 1 minute wash with DMF at a flow rate was 20 mL/min. After completion of the stepwise SPPS, the resin was washed thoroughly with DCM and dried under vacuum. The peptide is simultaneously cleaved from the resin and side-chain deprotected by treatment with 2.5percent (v/v) water, 2.5percent (v/v) 1 ,2- ethanedithiol (EDT), and 1percent (v/v) triisoproprylsilane in neat trifluoroacetic acid (TFA) for 2 hours at room temperature. The resulting solution containing peptide was evaporated by blowing a stream of nitrogen gas over its surface for 15 minutes, then triturated and washed with cold diethyl ether three times. The obtained gummy-like solid was dissolved in 50percent H20: 50percent acetonitrile containing 0.1percent TFA and lyophilized. These same solvent compositions were used in majority of experiments and will be referred to as A: 0.1percent TFA in H20 and B: 0.1percent TFA in acetonitrile. c. Peptide Purification The crude peptide was dissolved in 95percent A: 5percent B with 6 M guanidinium hydrochloride and purified by semi-preparative RP-HPLC (Agilent Zorbax SB C18 column: 21.2 x 250 mm, 7 mutaueta, linear gradient: 5-50percent B over 90 min, flow rate: 5 mL/min). 1 of each HPLC fraction was mixed with 1 mu^ of alpha-cyano-4-hydroxycinnamic acid (CHCA) matrix in 75percent A: 25percent B, spotted with MALDI, and checked for fractions with desired molecular mass. The purity of fractions was confirmed by analytical RP-HPLC (Agilent Zorbax SB C3 column: 2.1 x 150 mm, 5 muiotaeta, gradient: 0-2 minutes 5percent B, 2-11 minutes 5- 65percent B, 11-12 minutes 65percent B, flow rate: 0.8 mL/min). HPLC fractions containing only product materials were confirmed by LC-MS analysis, combined, and then lyophilized. Peptides synthesized using fast flow-based SPPS and purified by RP-HPLC are listed in Table SI .
Single-Coupling Procedure To the reaction vessel containing resin from the previous step was added piperidine:DMF (20:80 v/v, 2.0mL). The mixture was periodically agitated for 3 minutes and then the solution was drained through the frit.To the reaction vessel was added piperidine:DMF (20:80 v/v, 2.0 mL). The mixture was periodically agitatedfor 3 minutes and then the solution was drained through the frit. The resin washed successively six times asfollows: for each wash, DMF (2.0 mL) was added to top of the vessel (not through the bottom frit) and theresulting mixture was periodically agitated for 30 seconds before the solution was drained through the frit. Tothe reaction vessel was added the amino acid (0.2M in DMF, 1.0 mL, 2 eq), then HATU (0.2M in DMF, 1.0mL, 2 eq), and finally DIPEA (0.4M in DMF, 1.0 mL, 4 eq). The mixture was periodically agitated for 15minutes, then the reaction solution was drained through the frit. The resin washed successively four times asfollows: for each wash, DMF (2.0 mL) was added to top of the vessel (not through the bottom frit) and theresulting mixture was periodically agitated for 30 seconds before the solution was drained through the frit. Tothe reaction vessel was added acetic anhydride (2.0 mL). The mixture was periodically agitated for 10minutes, then the solution was drained through the frit. The resin washed successively four times as follows:for each wash, DMF (2.0 mL) was added to top of the vessel (not through the bottom frit) and the resultingmixture was periodically agitated for 90 seconds before the solution was drained through the frit. Theresulting resin was used directly in the next step.
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