The high molecular weight (HMW) proteins from wheat contain a repetitive domain that forms 60‐80% of their sequence. The consensus peptides PGQGQQ and GYYPTSPQQ form more than 90% of the domain; both are predicted to adopt /3‐turn structure. This paper describes the structural characterization of these consensus peptides and forms the basis for the structural characterization of the repetitive HMW domain, described in the companion paper. The cyclic peptides cyclo‐[PGQGQQPGQGQQ] (peptide 1), cyclo‐[GYYPTSPQQGA] (peptide 2), and cyclo‐[PGQGQQGYYPTSPQQ] (peptide 3) were prepared using a novel synthesis route. In addition, the linear peptides (PGQGQQ) (n = 1, 3, 5) were prepared. CD, FTIR, and NMR data demonstrated a type II /3‐turn structure at QPGQ in the cyclic peptide 1 that was also observed in the linear peptides (PGQGQQ). A type I /3‐turn was observed at YPTS and SPQQ in peptides 2 and 3, with additional /3‐turns of either type I or II at GAGY (peptide 2) and QQGY (peptide 3). The proline in YPTS showed considerable cis/trans isomerization, with up to 50% of the population in the cis‐conformation; the other prolines were more than 90% in the trans conformation. The conversion from trans to cis destroys the type I /3‐turn at YPTS, but leads to an increase in turn character at SPQQ and GAGY (peptide 2) or QQGY (peptide 3).

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.

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