Delineation of an Active Fragment and Poly(l-proline) II Conformation for Candidacidal Activity of Bactenecin 5

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Abstract
Bactenecin 5 and its fragments [BN22 (1−22), BN16 (7−22), and BC24 (20−43)] were synthesized by solid-phase methods. Their antifungal activities on Candida albicans have been studied and compared with those of the native bactenecin 5. The conformational preferences of these peptides in aqueous and nonaqueous solutions and in lipid vesicles were examined by circular dichroism. The highly active N-terminal fragment (BN16) was examined in aqueous solution using 500 MHz two-dimensional NMR. Bactenecin 5 and its fragments are potent candidacidal agents against C. albicans. The N-terminal fragments (BN22 and BN16) of bactenecin 5 are relatively more active than the C-terminal fragment BC24, especially at lower concentrations. The N-terminal region (7−22) which retains the activity of the whole molecule appears to be the functional domain for candidacidal activity. The CD spectra of bactenecin 5 and its fragments are reminiscent of the CD spectrum of poly(l-proline) type II structure in aqueous and nonaqueous solutions and also in lipid vesicles. The temperature dependence of NH chemical shifts and 1H/2H exchange effect on amide resonances suggest the absence of intramolecularly hydrogen-bonded NH groups. The coupling constant (JNH-CαH) values, conformational restriction offered by the Pro residues (φ = −60° ± 15°), the set of medium- and short-range nuclear Overhauser effects observed for the active N-terminal fragment (BN16), and the restrained structure calculation using DIANA suggest that poly(l-proline) type II conformers of the peptide molecules could be significantly populated in aqueous solution. The ability of bactenecin peptides to induce disruption of lipid vesicles correlates well with their activity. Our results suggest that poly(l-proline) type II structure may, indeed, be the biologically active conformation for candidacidal activity of bactenecin peptides.