Immobilization Reduces the Activity of Surface-Bound Cationic Antimicrobial Peptides with No Influence upon the Activity Spectrum

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Abstract
Early studies of immobilized peptides mainly focused upon the relationship between structural properties and the activity of soluble and surface-tethered sequences. The intention of this study was to analyze the influence of immobilization parameters upon the activity profile of peptides. Resin beads (TentaGel S NH 2 , HypoGel 400 NH 2 , and HypoGel 200 NH 2 ) with polyethylene glycol spacers of different lengths were rendered antimicrobial by linkage of an amphipathic model KLAL peptide and magainin-derived MK5E. Standard solid-phase peptide synthesis, thioalkylation, and ligation strategies were used to immobilize the peptides at the C and N termini and via different side-chain positions. Depending upon the resin capacity and the coupling strategies, peptide loading ranged between 0.1 and 0.25 μmol/mg for C-terminally and around 0.03 μmol/mg for N-terminally and side-chain-immobilized peptides. Tethering conserved the activity spectra of the soluble peptides at reduced concentrations. The resin-bound peptides were antimicrobial toward Escherichia coli and Bacillus subtilis in the millimolar range compared to the results seen with micromolar concentrations of the free peptides. B. subtilis was more susceptible than E. coli . The antimicrobial activity distinctly decreased with reduction of the spacer length. Slight differences in the antimicrobial effect of KLAL and MK5E bound at different chain positions on TentaGel S NH 2 suggest that the activity is less dependent upon the position of immobilization. Soluble KLAL was active toward red blood cells, whereas MK5E was nonhemolytic at up to about 400 μM. Resin-induced hemolysis hampered the determination of the hemolytic effect of the immobilized peptides. TentaGel S NH 2 -bound peptides enhanced the permeability of the POPC (1-palmitoyl-2-oleoyl- sn -glycero-3-phospho-choline) and mixed POPC/1-palmitoyl-2-oleoyl- sn -glycero-3-[phospho-rac-(1-glycerol)] (POPC/POPG) bilayers used to model the charge properties of the biological targets. The results suggest that surface immobilization of the cationic amphipathic antimicrobial peptides does not influence the membrane-permeabilizing mode of action. Peptide insertion into the target membrane and likely the exchange of membrane-stabilizing bivalent cations contribute to the antimicrobial effect. In conclusion, reasonable antimicrobial activity of surface-bound peptides requires the optimization of the coupling parameters, with the length of the spacer and the amount of target-accessible peptide being the most important factors.