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A new and efficient procedure for the preparation of protected cyclized and protected symmetrical dimeric peptide disulfides is described. A thiol is immobilized onto a solid phase through coupling of the thiol function with a resin‐linked trityl group. Following conventional peptide assembly using the Fmoc‐strategy, detachment is performed by oxidation with iodine in a suitable organic solvent. When N,N‐dimethylformamide is used as the solvent, and the peptide chain contains an acetamidomethylthio function, located N‐terminally in a Nx‐(9‐fluorenylmethyloxycarbonyl), or Nx‐tert‐butyloxycarbonyl cysteinyl residue, or occurring in the chain, then the corresponding fully protected cyclic peptide disulfide will be obtained in high yield and purity. In other solvents (e.g. dioxane or chloroform‐methanol 1:1, v/v), the iodine‐mediated oxidation gave not only the cyclic product, but also substantial amounts of the parallel symmetrical dimeric peptide retaining Cys(Acm) at the two identical N‐termini.

Abstract. N-Acetyl-S-(alky1sulfanyl)cysteine benzyl esters were synthesized as models for Nacylated S-(alkylsulfanyl)cysteine residues linked via an ester bond to a solid phase. The S-protections involved were: the ethylsulfanyl, the well-established tert-butylsulfanyl and the newly developed tritylsulfanyl group. We investigated the chemical behaviour of the disulfides in reagents commonly applied in solid-phase peptide synthesis using the Fmoc strategy (Fmoc SPPS). It was found that the tritylsulfanyl group as a thiol protection is comparable with the tert-butylsulfanyl group in these respects. It is stable in trifluoroacetic acid and is rapidly reduced by thiols and phosphines. For all three cysteine esters rapid racemization was observed in piperidine (25%) in DMF, the amides being chirally stable. The demonstrated chiral instability of cysteine esters has consequences for the solid-phase synthesis of peptides using the Fmoc protocol.

The cyclic disulfide peptide CGVSRQGKPYC was prepared to obtain a constrained analogue of residues 17-27 of the DNA-binding loop of the gene-V-encoded ssDNA-binding protein of filamentous bacteriophage MI 3. Amino acid sequences very similar to that of this P-loop have been found in various phage-encoded ssDNA-binding proteins, and it has been proposed that such a loop may occur as a common motif in this class of proteins. The conformation, in aqueous solution, of the synthetic gene-Vprotein binding-loop analogue has been investigated by means of two-dimensional-' H-NMR techniques.Subsequent structure calculations show that the molecule forms a P-loop that includes a turn formed by three residues. This structure, very unusually for a cyclic disulfide peptide, is highly similar to that of the analogous part of the binding loop of the native protein.Comparison with experiments on other cyclic disulfide peptides indicates that the formation, of the /I-sheet (P-hairpin) secondary structure is essentially governed by the amino acid composition of the 11-residue sequence. The disulfide bridge in the 1 1 -residue sequence is essential for conformational stability, as indicated by the finding that the open peptide analogue that encompasses residues Serl7FSer27 does not adopt a detectable secondary structure in water. The bridge replaces the role of the loop formed by residues 49-58 in the protein, which act as a scaffold to hold the N-terminal and C-terminal ends of the DNA-binding loop together.

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