NMR studies of the antimicrobial salivary peptides histatin 3 and histatin 5 in aqueous and nonaqueous solutions

Conformational studies of the salivary peptides histatin 3 (H3) and histatin 5 (H5) were performed by NMR and circular dichroism (CD) in aqueous and nonaqueous solutions. Histatin 5 has no defined structure in H2O but adopts a more helical conformation in dimethyl sulfoxide and aqueous trifluoroethanol. This is in agreement with the CD analysis, which shows no secondary structure in H2O but increasing helical content in the presence of trifluoroethanol. CD analysis shows that H3 has less propensity to form a helical structure than H5 in similar conditions. The NMR analysis of H3 in H2O at pH 7.4 reveals that its conformational mobility is less than that of H5 as indicated by the observation of backbone cross peaks alpha N (i, i + 1) and NN (i, i + 1) and the slow exchanging amide protons in the C-terminus. However, H3 remains essentially unordered as suggested by the lack of longer range nuclear Overhauser effects (NOEs) in the NOESY spectrum. H3 becomes much more ordered in a mixture of 50:50 H2O - dimethyl sulfoxide as indicated by the numerous NOEs, including several side chain to side chain and side chain to backbone conectivities. Our data suggest that in these conditions H3 contains a turn in the region of K-13 to K-17 and possibly a 3(10) helix at the C-terminus. This study demonstrates that H3 and H5 are both conformationally mobile and that each adopt different types of conformations in aqueous and nonaqueous solutions.