Different Interfacial Behaviors of Peptides Chemically Immobilized on Surfaces with Different Linker Lengths and via Different Termini

Conformation and Orientation of Antimicrobial Peptides MSI-594 and MSI-594A in a Lipid Membrane

There is significant interest in the development of antimicrobial compounds to overcome the increasing bacterial resistance to conventional antibiotics. Studies have shown that naturally occurring and de novo-designed antimicrobial peptides could be promising candidates. MSI-594 is a synthetic linear, cationic peptide that has been reported to exhibit a broad spectrum of antimicrobial activities. Investigation into how MSI-594 disrupts the cell membrane is important for better understanding the details of this antimicrobial peptide (AMP)'s action against bacterial cells. In this study, we used two different synthetic lipid bilayers: zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and anionic 7:3 POPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho(1'-rac-glycerol) (POPG). Sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) were used to determine the orientations of MSI-594 and its analogue MSI-594A associated with zwitterionic POPC and anionic 7:3 POPC/POPG lipid bilayers. The simulated ATR-FTIR and SFG spectra using nuclear magnetic resonance (NMR)-determined structures were compared with experimental spectra to optimize the bent angle between the N- (1-11) and C- (12-24) termini helices and the membrane orientations of the helices; since the NMR structure of the peptide was determined from lipopolysaccharide (LPS) micelles, the optimization was needed to find the most suitable conformation and orientation in lipid bilayers. The reported experimental results indicate that the optimized MSI-594 helical hairpin structure adopts a complete lipid bilayer surface-bound orientation (denoted "face-on") in both POPC and 7:3 POPC/POPG lipid bilayers. The analogue peptide, MSI-584A, on the other hand, exhibited a larger bent angle between the N- (1-11) and C- (12-24) termini helices with the hydrophobic C-terminal helix inserted into the hydrophobic region of the bilayer (denoted "membrane-inserted") when interacting with both POPC and 7:3 POPC/POPG lipid bilayers. These experimental findings on the membrane orientations suggest that both peptides are likely to disrupt the cell membrane through the carpet mechanism.

Polysiloxane-Based Liquid-like Layers for Reducing Polymer and Wax Fouling

Surface fouling occurs when undesired matter adheres and accumulates on a surface, resulting in a decrease or loss of functionality. Polymer and wax fouling can cause costly blockages to crude oil pipelines, clog jet fuel injectors, foul chemical reaction vessels, and significantly decrease the efficiency of heat exchangers. Fouling occurs in many forms but can be segmented based on adherent size, modulus, and chemical functionality. Depending on the foulant, surface design strategies can vary greatly. Few strategies exist to prevent the buildup of wax and polymers on surfaces. In this report, we investigate the potential of highly disordered, siloxane liquid-like layers as a strategy for reducing wax and polymer deposition. In our tests, it was found that the liquid-like layers developed here were able to reduce postadsorption roughness for polymer and wax by as much as 35- and 47-fold, respectively, when compared to the control. SFG was utilized to investigate the molecular-level interfacial properties for each of the modified surfaces to help understand the antifouling mechanism. The data showed that the likely higher grafting density and a large degree of random conformational freedom at the liquid-surface interface make the developed siloxane-covered surfaces energetically unfavorable for polymer and wax accretion.

Sum frequency spectroscopy studies on cell membrane fusion induced by divalent cations

Cell membrane fusion is a fundamental biological process involved in a number of cellular living functions. Regarding this, divalent cations can induce fusion of the lipid bilayers through binding and bridging of divalent cations to the charged lipids, thus leading to the cell membrane fusion. How-ever, the elaborate mechanism of cell membrane fusion induced by divalent cations is still needed to be elucidated. Here, surface/interface sensitive sum frequency generation vibrational spectroscopy (SFG-VS) and dynamic light scattering (DLS) were applied in this research to study the responses of phospholipid monolayer to the exposure of divalent metal ions i.e. Ca2+ and Mg2+. According to the particle size distribution results measured by DLS experiments, it was found that Ca2+ could induce inter-vesicular fusion while Mg2+ could not. An octadecyltrichlorosilane self-assembled monolayer (OTS SAM)-lipid monolayer system was designed to model the cell membrane for the SFG-VS experiment. Ca2+ could interact with the lipid PO2− head groups more strongly, resulting in cell membrane fusion more easily, in comparison with Mg2+. No specific interaction between the two metal cations and the C=O groups was observed. However, the C=O orientations changed more after Ca2+-PO2− binding than Mg2+ mediation on lipid monolayer. Meanwhile, Ca2+ could induce dehydration of the lipids (which should be related to the strong Ca2+-PO2− interaction), leading to the reduced hindrance for cell membrane fusion.

Probing Orientations and Conformations of Peptides and Proteins at Buried Interfaces

Molecular structures of peptides/proteins at interfaces determine their interfacial properties, which play important roles in many applications. It is difficult to probe interfacial peptide/protein structures because of the lack of appropriate tools. Sum frequency generation (SFG) vibrational spectroscopy has been developed into a powerful technique to elucidate molecular structures of peptides/proteins at buried solid/liquid and liquid/liquid interfaces. SFG has been successfully applied to study molecular interactions between model cell membranes and antimicrobial peptides/membrane proteins, surface-immobilized peptides/enzymes, and physically adsorbed peptides/proteins on polymers and 2D materials. A variety of other analytical techniques and computational simulations provide supporting information to SFG studies, leading to more complete understanding of structure-function relationships of interfacial peptides/proteins. With the advance of SFG techniques and data analysis methods, along with newly developed supplemental tools and simulation methodology, SFG research on interfacial peptides/proteins will further impact research in fields like chemistry, biology, biophysics, engineering, and beyond.

Molecular Structure of the Surface-Immobilized Super Uranyl Binding Protein

Recently, a super uranyl binding protein (SUP) was developed, which exhibits excellent sensitivity/selectivity to bind uranyl ions. It can be immobilized onto a surface in sensing devices to detect uranyl ions. Here, sum frequency generation (SFG) vibrational spectroscopy was applied to probe the interfacial structures of surface-immobilized SUP. The collected SFG spectra were compared to the calculated orientation-dependent SUP SFG spectra using a one-excitonic Hamiltonian approach based on the SUP crystal structures to deduce the most likely surface-immobilized SUP orientation(s). Furthermore, discrete molecular dynamics (DMD) simulation was applied to refine the surface-immobilized SUP conformations and orientations. The immobilized SUP structures calculated from DMD simulations confirmed the SUP orientations obtained from SFG data analyzed based on the crystal structures and were then used for a new round of SFG orientation analysis to more accurately determine the interfacial orientations and conformations of immobilized SUP before and after uranyl ion binding, providing an in-depth understanding of molecular interactions between SUP and the surface and the effect of uranyl ion binding on the SUP interfacial structures. We believe that the developed method of combining SFG measurements, DMD simulation, and Hamiltonian data analysis approach is widely applicable to study biomolecules at solid/liquid interfaces.

Probing Molecular Interactions between Surface-Immobilized Antimicrobial Peptides and Lipopolysaccharides In Situ

Lipopolysaccharide (LPS) is a component of the outer membrane of Gram-negative bacteria. Recently, a label-free immobilized antimicrobial peptide (AMP) surface plasmon resonance platform was developed to successfully distinguish LPS from multiple bacterial strains. Among the tested AMPs, SMAP29 exhibited excellent affinity with LPS and has two independent LPS-binding sites located at two termini of the peptide. In this study, sum frequency generation vibrational spectroscopy was applied to investigate molecular interactions between three LPS samples and surface-immobilized SMAP29 via the N-terminus, the C-terminus, and a middle site at the solid/liquid interface in situ in real-time, supplemented by circular dichroism spectroscopy. It was found that the conformations and orientations of surface-immobilized SMAP29 via different sites are different when interacting with the same LPS, with different interaction kinetics. The same SMAP29 sample also has different structures and interaction kinetics while interacting with different LPS samples with different charge densities and hydrophobicities. The observed results on molecular interactions between surface-immobilized peptides and LPS can well interpret the different adsorption amounts of various LPSs on different surface-immobilized peptides.

Probing Biological Molecule Orientation and Polymer Surface Structure at the Polymer/Solution Interface In Situ

Polymers are widely used for many applications ranging from biomedical materials, marine antifouling coatings, membranes for biomolecule separation, to substrates for enzyme molecules for biosensing. For such applications, it is important to understand molecular interactions between biological molecules and polymer materials in situ in real time. Such understanding provides vital knowledge to manipulate biological molecule-polymer interactions and to optimize polymer surface structures to improve polymer performance. In this research, sum frequency generation (SFG) vibrational spectroscopy was applied to study interactions between peptides (serving as models for biological molecules) and deuterated polystyrene (d8-PS, serving as a model for polymer materials). The peptide conformations/orientations and polymer surface phenyl orientation during the peptide-d8-PS interactions were determined using SFG. It was found that the π-π interaction between the aromatic amino acids on peptides and phenyl groups on d8-PS surface does not play a significant role. Instead, the peptide-d8-PS interactions are mediated by general hydrophobic interactions between the peptides and the polymer surface.

AMP-Chitosan Coating with Bactericidal Activity in the Presence of Human Plasma Proteins

Antibiotic resistance is increasing and new strategies are needed to fight infection. Advanced materials are promising tools that can be combined with innovative alternatives to conventional antibiotics to allow more targeted and efficient treatment. In this work, we explored the activity against Staphylococcus epidermidis (S. epidermidis) of the α-helical antimicrobial peptide (AMP) MSI-78(4-20) (KFLKKAKKFGKAFVKIL) when covalently bound to a chitosan coating. The AMP MSI-78(4-20) (17 mer) is an improved version of its parent MSI-78 (22 mer; commercially known as Pexiganan), a cost-effective short AMP, which was demonstrated to be as effective as MSI-78 and less toxic to eukaryotic cells. An MSI-78(4-20)–chitosan coating could be applied in several infection scenarios, ranging from bone implants to wound dressings, as chitosan possesses osteoconductive and hemostatic properties. Cysteine-modified MSI-78(4-20) was covalently immobilized onto the chitosan coating through a succinimidyl-[(N-maleimidopropionamido)-octaethyleneglycol] ester (SM(PEG)₈), a heterobifuncional crosslinker, with N-hydroxysuccinimide (NHS) ester and maleimide groups, by its N- and C- termini. The MSI-78(4-20)–chitosan coating demonstrated bactericidal properties independently of the tethering site and an improved performance in the presence of plasma proteins, which mimics conditions that will be encountered in vivo. This AMP–chitosan coating has therefore great potential for applications in medical devices such as implants or even wound dressings.

Functionalization of hydrophobic surfaces with antimicrobial peptides immobilized on a bio-interfactant layer

The design of functionalized polymer surfaces using bioactive compounds has grown rapidly over the past decade within many industries including biomedical, textile, microelectronics, bioprocessing and food packaging sectors. Polymer surfaces such as polystyrene (PS) must be treated using surface activation processes prior to the attachment of bioactive compounds. In this study, a new peptide immobilization strategy onto hydrocarbonaceus polymer surfaces is presented. A bio-interfactant layer made up of a tailored combination of laccase from trametes versicolor enzyme and maltodextrin is applied to immobilize peptides. Using this strategy, immobilization of the bio-inspired peptide KLWWMIRRWG-bromophenylalanine-3,4-dihydroxyphenylalanine-G and KLWWMIRRWG-bromophenylalanine-G on polystyrene (PS) was achieved. The interacting laccase layers allows to immobilize antimicrobial peptides avoiding the chemical modification of the peptide with a spacer and providing some freedom that facilitates different orientations. These are not strongly dominated by the substrate as it is the case on hydrophobic surfaces; maintaining the antimicrobial activity. Films exhibited depletion efficiency with respect to the growth of Escherichia coli bacteria and did not show cytotoxicity for fibroblast L929. This environmentally friendly antimicrobial surface treatment is both simple and fast, and employs aqueous solutions. Furthermore, the method can be extended to three-dimensional scaffolds as well as rough and patterned substrates.

A bio-interfactant layer is applied on hydrophobic surfaces to immobilize antimicrobial peptides.

Combinatorial functionalization with bisurea-peptides and antifouling bisurea additives of a supramolecular elastomeric biomaterial

The bioactive additive toolbox to functionalize supramolecular elastomeric materials expands rapidly. Here we have set an explorative step toward screening of complex combinatorial functionalization with antifouling and three peptide-containing additives in a bisurea-based supramolecular system. Thorough investigation of surface properties of thin films with contact angle measurements, X-ray photoelectron spectroscopy and atomic force microscopy, was correlated to cell-adhesion of endothelial and smooth muscle cells to apprehend their respective predictive values for functional biomaterial development. Peptides were presented at the surface alone, and in combinatorial functionalization with the oligo(ethylene glycol)-based non-cell adhesive additive. The bisurea-RGD additive was cell-adhesive in all conditions, whereas the endothelial cell-specific bisurea-REDV showed limited bioactive properties in all chemical nano-environments. Also, aspecific functionality was observed for a bisurea-SDF1α peptide. These results emphasize that special care should be taken in changing the chemical nano-environment with peptide functionalization. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1725-1735.

Prevention of urinary catheter-associated infections by coating antimicrobial peptides from crowberry endophytes

Urinary catheters are extensively used in hospitals, being responsible for about 75% of hospital-acquired infections. In this work, a de novo designed antimicrobial peptide (AMP) Chain201D was studied in the context of urinary catheter-associated infections. Chain201D showed excellent antimicrobial activity against relevant ATCC strains and clinical isolates of bacteria and yeast and demonstrated high stability in a wide range of temperatures, pH and salt concentrations. Moreover, the bactericidal activity of Chain201D immobilized on a model surface was studied against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), some of the most prevalent strains found in urinary catheter-associated infections. Chain201D was successfully tethered to ((1-mercapto-11-undecyl)-(tetra(ethylene glycol) (EG4)) terminated self-assembled monolayers (SAMs), (EG4-SAMs), activated by 1,1'-Carbonyldiimidazole (CDI) at different concentrations. Chain201D surfaces can bind and kill by contact a high percentage of adherent bacteria. These achievements are obtained without any peptide modification (for chemoselective conjugation) and without the use of a spacer. Moreover, increased amounts of immobilized AMP lead to higher numbers of adhered/dead bacteria, revealing a concentration-dependent behaviour and demonstrating that Chain201D has excellent potential for developing antimicrobial urinary catheters.

Flexible Peptide Linkers Enhance the Antimicrobial Activity of Surface-Immobilized Bacteriolytic Enzymes

Chemical linkers are frequently used in enzyme immobilization to improve enzyme flexibility and activity, whereas peptide linkers, although ubiquitous in protein engineering, are much less explored in enzyme immobilization. Here, we report peptide-linker-assisted noncovalent immobilization of the bacteriolytic enzyme lysostaphin (Lst) to generate anti- Staphylococcus aureus surfaces. Lst was immobilized through affinity tags onto a silica surface (glass slides) and nickel nitrilotriacetic acid (NiNTA) agarose beads via silica-binding peptides (SiBPs) or a hexahistidine tag (His-tag) fused at the C-terminus of Lst, respectively. By inserting specific peptide linkers upstream of the SiBP or His-tag, the immobilized enzymes killed >99.5% of S. aureus ATCC 6538 cells (10⁸ CFU/mL) within 3 h in buffer and could be reused multiple times without significant loss of activity. In contrast, immobilized Lst without a peptide linker was less active/stable. Molecular modeling of Lst-linker-affinity tag constructs illustrated that the presence of the peptide linkers enhanced the molecular flexibility of the proximal Lst binding domain, which interacts with the bacterial substrate, and such increased flexibility correlated with increased antimicrobial activity. We further show that Lst immobilized onto NiNTA beads retained the ability to kill ∼99% of a 10⁸ CFU/mL microbial challenge even in the presence of 1% of a commercial anionic surfactant, C12-14 alcohol EO 3:1 sodium sulfate, when the Lst construct contained a decapeptide linker containing glycine, serine, and alanine residues. This linker-assisted immobilization strategy could be extended to an unrelated lytic enzyme, the endolysin PlyPH, to target Bacillus anthracis Sterne cells either in buffer or in the presence of anionic surfactants. Our approach, therefore, provides a facile route to the use of antimicrobial enzymes on surfaces.

Chemically Immobilized Antimicrobial Peptide on Polymer and Self-Assembled Monolayer Substrates

Surfaces with chemically immobilized antimicrobial peptides have been shown to have great potential in various applications such as biosensors and antimicrobial coatings. This research investigated the chemical immobilization of a cecropin-melittin hybrid antimicrobial peptide on two different surfaces, a polymer surface prepared by chemical vapor deposition (CVD) polymerization and a self-assembled monolayer surface. We probed the structure of immobilized peptides using spectroscopic methods and correlated such structural information to the measured antimicrobial activity. We found that the hybrid peptide adopts an α-helical structure after immobilization onto both surfaces. As we have shown previously for another α-helical peptide, MSI-78, immobilized on a SAM, we found that the α-helical hybrid peptide lies down when it contacts bacteria. This study shows that the antimicrobial activity of the surface-immobilized peptides on the two substrates can be well explained by the spectroscopically measured peptide structural data. In addition, it was found that the polymer-based antimicrobial peptide coating is more stable. This is likely due to the fact that the SAM prepared using silane may be degraded after several days whereas the polymer prepared by CVD polymerization is more stable than the SAM, leading to a more stable antimicrobial coating.

Design and production of a novel antimicrobial fusion protein in Escherichia coli

In recent years, antimicrobial peptides (AMPs) have attracted increasing attention. The microbial cells provide a simple, cost-effective platform to produce AMPs in industrial quantities. While AMP production as fusion proteins in microorganisms is commonly used, the recovery of AMPs necessitates the use of expensive proteases and extra purification steps. Here, we develop a novel fusion protein DAMP4-F-pexiganan comprising a carrier protein DAMP4 linked to the AMP, pexiganan, through a long, flexible linker. We show that this fusion protein can be purified using a non-chromatography approach and exhibits the same antimicrobial activity as the chemically synthesized pexiganan peptide without any cleavage step. Activity of the fusion protein is dependent on a long, flexible linker between the AMP and carrier domains, as well as on the expression conditions of the fusion protein, with low-temperature expression promoting better folding of the AMP domain. The production of DAMP4-F-pexiganan circumvents the time-consuming and costly steps of chromatography-based purification and enzymatic cleavages, therefore shows considerable advantages over traditional microbial production of AMPs. We expect this novel fusion protein, and the studies on the effect of linker and expression conditions on its antimicrobial activity, will broaden the rational design and production of antimicrobial products based on AMPs.

Simultaneous Observation of the Orientation and Activity of Surface-Immobilized Enzymes

Surface immobilized enzymes have been widely used in many applications such as biosensors, biochips, biofuel production, and biofuel cell construction. Many factors dictate how enzymes' structure, activity, and stability may change when immobilized, including surface functionalization, immobilization chemistry, nature of the solid support, and enzyme surface density. To better understand how immobilization affects enzyme structure and activity, we have developed a method to measure both surface-sensitive protein vibrational spectra and enzymatic activity simultaneously. To accomplish this, an optical/fluorescence microscope was incorporated into a sum frequency generation (SFG) spectrometer. Using β-glucosidase (β-Glu) as a model system, enzymes were covalently tethered to a self-assembled monolayer surface using cysteine-maleimide chemistry. Their orientations were determined by SFG spectroscopy, with a single native cysteine residue oriented toward the functionalized surface, and activity measured simultaneously using a fluorogenic substrate resorufin β-d-glucopyranoside, with a loss of activity of 53% as compared to comparable solution measurements. Measuring β-Glu activity and orientation simultaneously provides more accurate information for designing and further improving enzymatic activity of surface-bound enzymes.

Monitoring Antimicrobial Mechanisms of Surface-Immobilized Peptides in Situ

Antimicrobial peptides (AMPs) in free solution can kill bacteria by disrupting bacterial cell membranes. Their modes of action have been extensively studied, and various models ranging from pore formation to carpet-like mechanisms were proposed. Surface-immobilized AMPs have been used as coatings to kill bacteria and as sensors to capture bacteria, but the interaction mechanisms of surface-immobilized AMPs and bacteria are not fully understood. In this research, an analytical platform, sum frequency generation (SFG) microscope, which is composed of an SFG vibrational spectrometer and a fluorescence microscope, was used to probe molecular interactions between surface-immobilized AMPs and bacteria in situ in real time at the solid/liquid interface. SFG probed the molecular structure of surface-immobilized AMPs while interacting with bacteria, and fluorescence images of dead bacteria were monitored as a function of time during the peptide-bacteria interaction. It was believed that upon bacteria contact, the surface-immobilized peptides changed their orientation and killed bacteria. This research demonstrated that the SFG microscope platform can examine the structure and function (bacterial killing) at the same time in the same sample environment, providing in-depth understanding on the structure-activity relationships of surface-immobilized AMPs.

Electric fields control the orientation of peptides irreversibly immobilized on radical-functionalized surfaces

Surface functionalization of an implantable device with bioactive molecules can overcome adverse biological responses by promoting specific local tissue integration. Bioactive peptides have advantages over larger protein molecules due to their robustness and sterilizability. Their relatively small size presents opportunities to control the peptide orientation on approach to a surface to achieve favourable presentation of bioactive motifs. Here we demonstrate control of the orientation of surface-bound peptides by tuning electric fields at the surface during immobilization. Guided by computational simulations, a peptide with a linear conformation in solution is designed. Electric fields are used to control the peptide approach towards a radical-functionalized surface. Spontaneous, irreversible immobilization is achieved when the peptide makes contact with the surface. Our findings show that control of both peptide orientation and surface concentration is achieved simply by varying the solution pH or by applying an electric field as delivered by a small battery.

Interactions between Surface-Immobilized Antimicrobial Peptides and Model Bacterial Cell Membranes

Sum frequency generation (SFG) vibrational spectroscopy was used to study surface immobilization effects on the interactions between antimicrobial peptide cecropin P1 (CP1) and model cell membranes. While free CP1 in solution interacted with a model cell membrane composed of a phosphatidylglycerol (PG) bilayer, electrostatic interaction led to the attachment of CP1 molecules onto the PG surface and the hydrophobic domain in the lipid bilayer enabled the peptides to insert into the bilayer and form α-helices from random coil structures. While CP1 molecules immobilized on a self-assembled monolayer interacted with PG lipid vesicles, the intensity of the SFG peak for the peptide α-helix decreased as the PG vesicle concentration increased. It was believed that when surface-immobilized CP1 molecules interacted with lipid vesicles, they lay down on the surface or became random coils. When the immobilized CP1 interacted with a PG lipid monolayer on water, the strong interaction led to the lying-down orientation of all of the surface-immobilized peptides as well. Differently, no significant interactions between surface-immobilized CP1 with the mammalian cell membrane model 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer were observed. Our results suggest that, instead of membrane insertion, the electrostatic interactions between the surface cationic charges of CP1 and anionic bacterial membranes may play an important role in the antimicrobial activity of the surface-immobilized CP1 peptide.

Titanium surfaces immobilized with the major antimicrobial fragment FK-16 of human cathelicidin LL-37 are potent against multiple antibiotic-resistant bacteria

Infections on implanted medical devices are a challenging problem, especially when bacteria form difficult-to-treat biofilms. Antimicrobial peptides are considered to be a solution due to their potency against antibiotic-resistant superbugs. Previously, the authors' laboratory demonstrated the prevention of staphylococcal biofilm formation in an animal catheter model by injecting merecidin (formerly known as 17BIPHE2), a peptide engineered based on the only human cathelicidin. This study documents an alternative solution via covalent immobilization of FK-16, amino acid sequence FKRIVQRIKDFLRNLV-amide, which corresponds to the major antimicrobial region (residues 17-32) of LL-37. FK-16 is superior to the longer peptide LL-37 in terms of synthesis cost and the shorter peptide KR-12 in terms of activity spectrum. Indeed, the FK16-coated titanium surface showed a broad-spectrum activity against the ESKAPE pathogens, including Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species. It also demonstrated anti-adhesion and biofilm inhibition capabilities against both S. aureus and E. coli.

Effect of Linker Length on Cell Capture by Poly(ethylene glycol)-Immobilized Antimicrobial Peptides

Development of antimicrobial peptide (AMP)-functionalized materials has renewed interest in using poly(ethylene glycol) (PEG)-mediated linking to minimize unwanted interactions while engendering the peptides with sufficient flexibility and freedom of movement to interact with the targeted cell types. While PEG-based linkers have been used in many AMP-based materials, the role of the tether length has been minimally explored. Here, we assess the impact of varying the length of PEG-based linkers on the binding of bacterial cells by surface-immobilized AMPs. While higher surface densities of immobilized AMPs were observed using shorter PEG linkers, the increased density was insufficient to fully account for the increased binding activity of peptides. Furthermore, effects were specific to both the peptide and cell type tested. These results suggest that simple alterations in linking strategies-such as changing tether length-may result in large differences in the surface properties of the immobilized AMPs that are not easily predictable.

Immobilized enzymes: understanding enzyme – surface interactions at the molecular level

Interactions between immobilized enzymes and supporting surfaces are complex and context-dependent and can significantly alter enzyme structure, stability and activity.

Engineering and Characterization of Peptides and Proteins at Surfaces and Interfaces: A Case Study in Surface-Sensitive Vibrational Spectroscopy

Understanding molecular structures of interfacial peptides and proteins impacts many research fields by guiding the advancement of biocompatible materials, new and improved marine antifouling coatings, ultrasensitive and highly specific biosensors and biochips, therapies for diseases related to protein amyloid formation, and knowledge on mechanisms for various membrane proteins and their interactions with ligands. Developing methods for measuring such unique systems, as well as elucidating the structure and function relationship of such biomolecules, has been the goal of our lab at the University of Michigan. We have made substantial progress to develop sum frequency generation (SFG) vibrational spectroscopy into a powerful technique to study interfacial peptides and proteins, which lays a foundation to obtain unique and valuable insights when using SFG to probe various biologically relevant systems at the solid/liquid interface in situ in real time. One highlighting feature of this Account is the demonstration of the power of combining SFG with other techniques and methods such as ATR-FTIR, surface engineering, MD simulation, liquid crystal sensing, and isotope labeling in order to study peptides and proteins at interfaces. It is necessary to emphasize that SFG plays a major role in these studies, while other techniques and methods are supplemental. The central role of SFG is to provide critical information on interfacial peptide and protein structure (e.g., conformation and orientation) in order to elucidate how surface engineering (e.g., to vary the structure) can ultimately affect surface function (e.g., to optimize the activity). This Account focuses on the most significant recent progress in research on interfacial peptides and proteins carried out by our group including (1) the development of SFG analysis methods to determine orientations of regular as well as disrupted secondary structures, and the successful demonstration and application of an isotope labeling method with SFG to probe the detailed local structure and microenvironment of peptides at buried interfaces, (2) systematic research on cell membrane associated peptides and proteins including antimicrobial peptides, cell penetrating peptides, G proteins, and other membrane proteins, discussing the factors that influence interfacial peptide and protein structures such as lipid charge, membrane fluidity, and biomolecule solution concentration, and (3) in-depth discussion on solid surface immobilized antimicrobial peptides and enzymes. The effects of immobilization method, substrate surface, immobilization site on the peptide or protein, and surrounding environment are presented. Several examples leading to high impact new research are also briefly introduced: The orientation change of alamethicin detected while varying the model cell membrane potential demonstrates the feasibility to apply SFG to study ion channel protein gating mechanisms. The elucidation of peptide secondary structures at liquid crystal interfaces shows promising results that liquid crystal can detect and recognize different peptides and proteins. The method of retaining the native structure of surface immobilized peptides or proteins in air demonstrates the feasibility to protect and preserve such structures via the use of hydromimetic functionalities when there is no bulk water. We hope that readers in many different disciplines will benefit from the research progress reported in this Account on SFG studies of interfacial structure-function relationships of peptides and proteins and apply this powerful technique to study interfacial biomolecules in the future.

Next-generation nanoantibacterial tools developed from peptides

Bacteria resistant against various antimicrobial compounds have emerged in many countries, and the age of resistance has just started. Among the more promising novel antimicrobial compounds on which current research is focusing are the antimicrobial peptides (AMPs). These are often less susceptible to bacterial resistance since multiple modifications in the cellular membranes, cell wall and metabolism are required to reduce their effectiveness. Most likely, the use of pure AMPs will be insufficient for controlling pathogenic bacteria, and innovative approaches are required to employ AMPs in new antibiotic treatments. Therefore, here we review novel bionanotechnological approaches, including nanofibers, nanoparticles and magnetic particles for effectively using AMPs in fighting infectious diseases.

Secondary Structure Determination of Peptides and Proteins After Immobilization

The presentation of immobilized peptides and other small biomolecules attached to surfaces can be greatly affected by the attachment chemistry and linking moieties, resulting in altered activity and specificity. For this reason, it is critical to understand how the various aspects of surface immobilization-underlying substrate properties, tether structure, and site of linkage-affect the secondary and quaternary structures of the immobilized species. Here, we present methods for attaching cysteine-containing peptides to quartz surfaces and determining the secondary structure of surface-immobilized peptides. We specifically show that, even when covalently immobilized, changes in peptide conformation can still occur, with measurement occurring in real time.

Application of circular dichroism for structural analysis of surface-immobilized cecropin A interacting with lipoteichoic acid

The development of biomaterials integrating antimicrobial peptides (AMPs) for improved pathogen detection or use as therapeutic agents requires an understanding of how a peptide may behave once immobilized. Here, we use a combination of circular dichroism and capture assays to assess the structure-function relationship of the cationic amphipathic AMP, cecropin A (cecA), upon interaction with Gram-positive lipoteichoic acids (LTAs). In solution, cecA peptides underwent a change from a largely unstructured conformation in water to structures with significant α-helical content in the presence of both Bacillus subtilis and Staphylococcus aureus LTAs. After surface immobilization, cecA peptides attached by either C- or N-terminus were able to capture both LTAs as well as to undergo conformational changes in the presence of SDS similar to those observed in solution. However, in spite of demonstrated LTA binding activity and the ability to undergo conformational changes (i.e., with SDS), no structural changes were observed when cecA immobilized by its N-terminus was treated with either LTA preparation. On the other hand, cecA immobilized by its C-terminus underwent a conformational change in the presence of S. aureus, but not B. subtilis, LTA. These results indicate that after immobilization recognition of different targets by cationic AMPs may occur by mechanisms quite different from those in solution and that selectivity of these mechanisms is further dependent on the orientation of the immobilized peptide.

Conjugation Approach To Produce a Staphylococcus aureus Synbody with Activity in Serum

Synbodies show promise as a new class of synthetic antibiotics. Here, we explore improvements in their activity and production through conjugation chemistry. Maleimide conjugation is a widely used conjugation strategy due to its high yield, selectivity, and low cost. We used this strategy to conjugate two antibacterial peptides to produce a bivalent antibacterial peptide, called a synbody that has bactericidal activity against methicillin resistant Staphylococcus aureus (MRSA). The synbody was prepared by conjugation of a partially d-amino acid substituted synthetic antibacterial peptide to a bis-maleimide scaffold. The synbody slowly degrades in serum, but also undergoes exchange reactions with other serum proteins, such as albumin. Therefore, we hydrolyzed the thiosuccinimide ring using a mild hydrolysis protocol to produce a new synbody with similar bactericidal activity. The synbody was now resistant to exchange reactions and maintained bactericidal activity in serum for 2 h. This work demonstrates that low-cost maleimide coupling can be used to produce antibacterial peptide conjugates with activity in serum.

Poly(L-lysine) Interfaces via Dual Click Reactions on Surface-Bound Custom-Designed Dithiol Adsorbates

Surfaces modified with poly(L-lysine) can be used to immobilize selected biomolecules electrostatically. This report describes the preparation of a set of self-assembled monolayers (SAMs) from three different azide-terminated adsorbates as platforms for performing controlled surface attachments and as a means of determining the parameters that afford stable poly(L-lysine)-modified SAM surfaces having controlled packing densities. A maleimide-terminated alkyne linker was "clicked" to the azide-terminated surfaces via a copper-catalyzed cycloaddition reaction to produce the attachment sites for the polypeptides. A thiol-Michael addition was then used to immobilize cysteine-terminated poly(L-lysine) moieties on the gold surface, avoiding adsorbate self-reactions with this two-step procedure. Each step in this process was analyzed by ellipsometry, X-ray photoelectron spectroscopy, polarization modulation infrared reflection-absorption spectroscopy, and contact angle goniometry to determine which adsorbate structure most effectively produced the targeted polypeptide interface. Additionally, a series of mixed SAMs using an azidoalkanethiol in combination with a normal alkanethiol having an equivalent alkyl chain were prepared to provide data to determine how dilution of the azide reactive site on the SAM surface influences the initial click reaction. Overall, the collected data demonstrate the advantages of an appropriately designed bidentate absorbate and its potential to form effective platforms for biomolecule surface attachment via click reactions.

Selective and Reversible Binding of Thiol-Functionalized Biomolecules on Polymers Prepared via Chemical Vapor Deposition Polymerization

We use chemical vapor deposition polymerization to prepare a novel dibromomaleimide-functionalized polymer for selective and reversible binding of thiol-containing biomolecules on a broad range of substrates. We report the synthesis and CVD polymerization of 4-(3,4-dibromomaleimide)[2.2]paracyclophane to yield nanometer thick polymer coatings. Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the chemical composition of the polymer coating. The reactivity of the polymer coating toward thiol-functionalized molecules was confirmed using fluorescent ligands. As a proof of concept, the binding and subsequent release of cysteine-modified peptides from the polymer coating were also demonstrated via sum frequency generation spectroscopy. This reactive polymer coating provides a flexible surface modification approach to selectively and reversibly bind biomolecules on a broad range of materials, which could open up new opportunities in many biomedical sensing and diagnostic applications where specific binding and release of target analytes are desired.

Isolation of Camelid Single-Domain Antibodies Against Native Proteins Using Recombinant Multivalent Peptide Ligands

Generation of antibodies against desired epitopes on folded proteins may be hampered by various characteristics of the target protein, including antigenic and immunogenic dominance of irrelevant epitopes and/or steric occlusion of the desired epitope. In such cases, peptides encompassing linear epitopes of the native protein represent attractive alternative reagents for immunization and screening. Peptide antigens are typically prepared by fusing or conjugating the peptide of interest to a carrier protein. The utility of such antigens depends on many factors including the peptide's amino acid sequence, display valency, display format (synthetic conjugate vs. recombinant fusion) and characteristics of the carrier. Here we provide detailed protocols for: (1) preparation of DNA constructs encoding peptides fused to verotoxin (VT) multimerization domain; (2) expression, purification, and characterization of the multivalent peptide-VT ligands; (3) concurrent panning of a non-immune phage-displayed camelid VHH library against the peptide-VT ligands and native protein; and (4) identification of VHHs enriched via panning using next-generation sequencing techniques. These methods are simple, rapid and can be easily adapted to yield custom peptide-VT ligands that appear to maintain the antigenic structures of the peptide. However, we caution that peptide sequences should be chosen with great care, taking into account structural, immunological, and biophysical information on the protein of interest.

Theoretical Sum Frequency Generation Spectroscopy of Peptides

Vibrational sum frequency generation (SFG) has become a very promising technique for the study of proteins at interfaces, and it has been applied to important systems such as anti-microbial peptides, ion channel proteins, and human islet amyloid polypeptide. Moreover, so-called "chiral" SFG techniques, which rely on polarization combinations that generate strong signals primarily for chiral molecules, have proven to be particularly discriminatory of protein secondary structure. In this work, we present a theoretical strategy for calculating protein amide I SFG spectra by combining line-shape theory with molecular dynamics simulations. We then apply this method to three model peptides, demonstrating the existence of a significant chiral SFG signal for peptides with chiral centers, and providing a framework for interpreting the results on the basis of the dependence of the SFG signal on the peptide orientation. We also examine the importance of dynamical and coupling effects. Finally, we suggest a simple method for determining a chromophore's orientation relative to the surface using ratios of experimental heterodyne-detected signals with different polarizations, and test this method using theoretical spectra.

Molecular interactions between amantadine and model cell membranes

Sum frequency generation (SFG) vibrational spectroscopy was applied to study molecular interactions between amantadine and substrate supported lipid bilayers serving as model cell membranes. Both isotopically asymmetric and symmetric lipid bilayers were used in the research. SFG results elucidated how the water-soluble drug, amantadine, influenced the packing state of each leaflet of a lipid bilayer and how the drugs affected the lipid flip-flop process. It is difficult to achieve such detailed molecular-level information using other analytical techniques. Especially, from the flip-flop rate change of isotopically asymmetric lipid bilayer induced by amantadine, important information on the drug-membrane interaction mechanism can be derived. The results show that amantadine can be associated with zwitterionic PC bilayers but has a negligible influence on the flip-flop behavior of PC molecules unless at high concentrations. Different effects of amantadine on the lipid bilayer were observed for the negatively charged DPPG bilayer; low concentration amantadine (e.g., 0.20 mM) in the subphase could immediately disturb the outer lipid leaflet and then the lipid associated amantadine molecules gradually reorganize to cause the outer leaflet to return to the original orderly packed state. Higher concentration amantadine (e.g., 5.0 mM) immediately disordered the packing state of the outer lipid leaflet. For both the high and low concentration cases, amantadine molecules only bind to the outer PG leaflet and cannot translocate to the inner layer. The presence of amantadine within the negatively charged lipid layers has certain implications for using liposomes as drug delivery carriers for amantadine. Besides, by using PC or PG bilayers with both leaflets deuterated, we were able to examine how amantadine is distributed and/or oriented within the lipid bilayer. The present work demonstrates that SFG results can provide an in-depth understanding of the molecular mechanisms of interactions between water-soluble drugs and model cell membranes.