STRUCTURE CHARACTERIZATION OF THE CENTRAL REPETITIVE DOMAIN OF HIGH-MOLECULAR-WEIGHT GLUTEN PROTEINS .1. MODEL STUDIES USING CYCLIC AND LINEAR PEPTIDES

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