Alginate is a biofilm exopolysaccharide secreted by the opportunistic pathogen Pseudomonas aeruginosa that
acts to prevent the diffusion of antibiotics toward the bacterial cell membrane. Cationic antimicrobial peptides (CAPs)
have been increasingly recognized as a viable alternative for prospective antimicrobial agents. The D-isomer chiral counterparts
of active L-isomer CAPs tend to show slightly greater antimicrobial activities because bacteria lack proteases to
hydrolyze the unnatural D-isomers. Using an enantiomeric pair of synthetic CAPs designed in our laboratory (L-4Leu in
the sequence KKKKKKALFALWLAFLA–NH2 and its D-analog D-4Leu), we studied the binding and interactions of Lvs.
D-isomers of CAPs with alginate using circular dichroism and Raman spectroscopic techniques. We found that the
peptide D-4Leu underwent a more rapid structural transition over time from an initial alginate-induced α-helical conformation
to a less soluble β-sheet conformation than L-4Leu, indicating that the D-isomer of this peptide has a relatively
greater affinity for alginate. Through Raman spectroscopy it was observed that Raman modes at 1297 cm-1 and 1453 cm-1
wavenumbers were found to differ between the spectra obtained from the insoluble complexes formed between L-4Leu vs.
D-4Leu and alginate. These modes were tentatively assigned to CH, and CH3 deformation modes, respectively. Our findings
reveal previously undetected subtleties in the binding of this diastereomeric pair of peptides in the microenvironment
of a biofilm exopolysaccharide, and provide guidelines for future development of antimicrobial peptides.
Keywords: Cationic antimicrobial peptides, alginate, biofilm, circular dichroism spectroscopy, Raman spectroscopy, peptide-exopolysaccharide
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