NMR studies of the antimicrobial salivary peptides histatin 3 and histatin5 in aqueous and nonaqueous solutions

Conformational studies of the salivary peptides histatin 3 (H3) and histati n 5 (H5) were performed by NMR and circular dichroism (CD) in aqueous and n onaqueous solutions. Histatin 5 has no defined structure in H2O but adopts a more helical conformation in dimethyl sulfoxide and aqueous trifluoroetha nol. This is in agreement with the CD analysis, which shows no secondary st ructure in H2O but increasing helical content in the presence of trifluoroe thanol. CD analysis shows that H3 has less propensity to form a helical str ucture 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 indi cated 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 m ore 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 chai n to backbone conectivities. Our data suggest that in these conditions H3 c ontains 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 conformatio nally mobile and that each adopt different types of conformations in aqueou s and nonaqueous solutions.