Understanding the surface hydration of nonfouling materials such as zwitterionic polymers and poly(ethylene glycol) (PEG) aids in the design of new and effective nonfouling materials. Sum frequency generation (SFG) vibrational spectroscopy is a powerful technique used to probe water structures at solid/liquid interfaces. However, SFG signals of H 2 O consisting of symmetric and asymmetric stretches and Fermi resonance overlap heavily, which complicates the interpretation of the spectra and leads to controversy. In this work, isotopically diluted water was used instead of H 2 O to study the surface hydration of three zwitterionic polymers and a PEG coating. Because the water signal contains only an O−H stretch, strongly and weakly hydrogen-bonded water structures were easily distinguished. SFG results showed a majority of strongly hydrogen-bonded water molecules at the nonfouling polymer surfaces. For comparison, the water spectra at the surfaces of poly(methyl methacrylate) and poly(ethylene terephthalate) suggested significant amounts of weakly hydrogen-bonded water. The effects of pH on the surface hydration of the nonfouling polymers were also investigated. The materials respond to pH differently because of their different structural formulas. The flip of the water molecules at a carboxybetaine polymer surface in response to pH was also observed.
Surfaces immobilized with biological molecules such as peptides and proteins are widely used in many important applications including biosensors, medical devices, and food packaging. It was found that the structures of surface-immobilized peptides control their surface properties. In this study, we investigated interfacial behaviors of antimicrobial peptide cecropin P1 (CP1) immobilized onto a maleimide-terminated self-assembled monolayer (Mal SAM) and a mixed SAM (Mal-OH SAM, a mixture of maleimide-terminated SAM and hydroxyl-terminated SAM) surface via C-terminus modified cysteine (CP1c). The surface coverage, secondary structure, orientation, and antimicrobial activity of immobilized CP1c were investigated using surface plasmon resonance (SPR), circular dichroism (CD), sum frequency generation (SFG) vibrational spectroscopy, dynamic contact antimicrobial assay, and coarse grained molecular dynamics (MD) simulations. It was found that the surface coverages of CP1c on the Mal SAM and the mixed Mal-OH SAM are similar. CP1c on Mal SAM possessed a dominant helical structure with a single orientation of ∼32° versus surface normal. CP1c on Mal-OH mixed SAM surface also possessed a dominant helical structure but with multiple orientations. MD simulation results can be correlated to the experimental data: the simulation results indicate a narrow distribution of orientations of CP1c immobilized on Mal SAM, but multiple orientations are sampled on the more hydrophilic Mal-OH SAM. Even though the surface orientations of CP1c immobilized on the two SAM surfaces are different, activity against both Gram-negative and Gram-positive bacteria (Escherichia coli and Staphylococcus aureus) exhibited similar results for CP1c immobilized on both SAM surfaces. We believe that this is because the antimicrobial activity of the surface-immobilized peptides is mainly affected by the electrostatic interactions between the strong basic N-terminal residues and the negatively charged bacteria cell wall/cell membrane.
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