A globular protein with slower amide proton exchange from an alpha helix than from antiparallel beta sheets

In proteinase inhibitor IIA from bull seminal plasma, which is a small globular protein with 57 amino acid residues, measurements of individual amide proton exchange rates by two-dimensional correlated 1H NMR spectroscopy (COSY) showed that the exchange was slowest for some hydrogen bonded amide groups in an alpha-helix. This contrasts with all other proteins which were so far studied in detail, where the slowest exchange rates were observed for hydrogen bonded amide protons in antiparallel beta-sheets.