In 2017, Leoni et al. reported myticalins as novel cationic linear antimicrobial peptides obtained from marine mussels. The authors focused on myticalin A6 (29 amino acids), which has a relatively short chain length among myticalins and contains a repeating X-proline(Pro)-arginine (Arg) sequence in its structure. We investigated the antimicrobial activity of myticalin A6 against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus (S. aureus). Fragment derivatives of myticalin A6 were synthesized, and the site required for expression of antimicrobial activity was examined. To investigate the structure-antimicrobial activity relationship of myticalin A6, short-chain derivatives and partially substituted derivatives were synthesized, and the antimicrobial activity was measured. Furthermore, some cyclized derivatives were synthesized and examined for antimicrobial activity. Circular dichroism (CD) spectroscopy of myticalin A6 and its derivatives was carried out to evaluate the secondary structure. Myticalin A6 exhibited an antimicrobial activity of 1.9 µM against S. aureus. Myticalin A6 (3-23)-OH (21 amino acids) exhibited an antimicrobial activity of 2.4 µM against S. aureus, suggesting that the X-Pro-Arg repeat sequence is important for antimicrobial activity. Derivatives with different CD measurement results from myticalin A6 (3-23)-OH exhibited decreased activity. The myticalin A6 (3-23)-OH derivative in which all Arg residues were replaced with lysine (Lys) residues exhibited reduced antimicrobial activity against S. aureus. We succeeded in synthesizing cyclic derivatives using 9-fluorenylmethoxycarbonyl (Fmoc)-aspartic acid (Asp)(Wang resin)-[2-phenylisopropyl ester (OPis)], but the yield of derivatives with 21 amino acids was decreased. The myticalin derivatives synthesized in this study did not exhibit any enhancement in antimicrobial activity due to cyclization.
Keywords: Staphylococcus aureus; active site; antimicrobial activity; circular dichroism spectroscopy; cyclic peptide; myticalin A6 derivative.