N.m.r. and computer‐simulated conformational analyses of a nonapeptide found in a human salivary proline‐rich glycoprotein

The amino acid sequence G(1)‐P(2)‐P(3)‐P(4)‐H(5)‐P(6)‐G(7)‐K(8)‐P(9) occurs twice in the proline‐rich glycoprotein (PRG) found in human parotid saliva. As part of our efforts to elucidate the structure‐function relationships of PRG, this nonapeptide sequence (PRG9) was synthesized for the purpose of conformational analyses by high‐resolution proton n.m.r. spectroscopy and computer‐modeling. The empirical n.m.r. spectrum differed from the simulated spectrum in that the overall chemical shift locations were displaced from their random coil positions and the five proline residues had non‐degenerate CαHα protons. Other n.m.r. data indicated that no intramolecular hydrogen‐bonding was present in the PRG. In conjunction with X‐ray crystallo‐graphic data on a triproline‐containing model compound (Kartha, g., Ashida, T. & Kakudo, M. (1974) Acta Cryst.B30, 1861‐1866), four energy‐minimized PRG9 structures were obtained. Two of the structures were energetically unfavorable, while the other two conformations were reasonable. The two most likely structures gave all prolines an S‐type ring pucker, the P(2)‐P(3)‐P(4) sequence as a poly‐L‐proline II helix, the H(5)ϕ=− 90.3°, P(6) and P(9) with trans peptide bond orientation, G(7) in an extended state, and the K(8)ϕ=− 93.2° or ‐ 146.8° for structures # 1 and #2. respectively.