Environmental Effect on Surface Immobilized Biological Molecules

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.

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.

Coupled-cluster sum-frequency generation nonlinear susceptibilities of methyl (CH3) and methylene (CH2) groups

The first vibrational sum frequency generation (SFG) spectra based on molecular properties calculated at the coupled cluster singles and doubles (CCSD) level of approximation have been simulated for interfacial model alkyl chains, providing benchmark data for comparisons with approximate methods, including density functional theory (DFT). The approach proceeds in three steps. In the first two steps, the molecular spectral properties are determined: the vibrational normal modes and frequencies and then the derivatives of the dipole moment and of the polarizability with respect to the normal coordinates. These derivatives are evaluated with a numerical differentiation approach, of which the accuracy was monitored using Romberg's procedure. Then, in the last step, a three-layer model is employed to evaluate the macroscopic second-order nonlinear optical responses and thereby the simulated SFG spectra of the alkyl interface. Results emphasize the following facts: (i) the dipole and polarizability derivatives calculated at the DFT level with the B3LYP exchange-correlation functional can differ, with respect to CCSD, by as much as ±10 to 20% and ±20 to 50% for the CH₃ and CH₂ vibrations, respectively; (ii) these differences are enhanced when considering the SFG intensities as well as their variations as a function of the experimental configuration (ppp versus ssp) and as a function of the tilt and rotation angles, defining the orientation of the alkyl chain at the interface; (iii) these differences originate from both the vibrational normal coordinates and the Cartesian derivatives of the dipole moment and polarizability; (iv) freezing the successive fragments of the alkyl chain strongly modifies the SFG spectrum and enables highlighting the delocalization effects between the terminal CH₃ group and its neighboring CH₂ units; and finally (v) going from the free chain to the free methyl model, and further to C_3v constraints on leads to large variations of two ratios that are frequently used to probe the molecular orientation at the interface, the (r + r)/r⁺ ratio for both antisymmetric and symmetric CH₃ vibrations and the I_ppp/I_ssp ratio.

Transport and Organization of Cholesterol in a Planar Solid-Supported Lipid Bilayer Depend on the Phospholipid Flip-Flop Rate

Understanding the transport behavior of the cholesterol molecules within a cell membrane is a key challenge in cell biology at present. Here, we have applied sum frequency generation vibrational spectroscopy to characterize the transport and organization of cholesterol in different kinds of planar solid-supported lipid bilayers by combining achiral- and chiral-sensitive polarization measurements. This method allows us to distinguish the organization of cholesterol in tail-to-tail, head-to-tail, head-to-head, and side-by-side manners. It is found that the movement of cholesterol in the lipid bilayer largely depends on the flip-flop rate of the phospholipid. The flip-flop dynamics of the phospholipid and cholesterol are synchronous. In the solid-supported zwitterionic phosphocholine lipid bilayer, the cholesterol molecules flip quickly from the distal leaflet to the neutral proximal leaflet of the bilayer and form tail-to-tail organization on both leaflets. The phosphocholine lipid and cholesterol show the same flip-flop rate. However, when the proximal leaflet is prepared using negative glycerol phospholipids, cholesterol organizes itself by mainly forming an α-β structure on the distal leaflet. Because of the strong interaction between the glycerol phospholipid and the substrate, no or only partial cholesterol molecules flip from the distal leaflet to the negatively charged proximal leaflet. However, the cholesterol molecules undergo flip-flop in the presence of salt solution because the ions weaken the interaction between the negative phospholipid and the substrate.

Prediction and clarification of structures of (bio)molecules on surfaces

The design of future materials for biotechnological applications via deposition of molecules on surfaces will require not only exquisite control of the deposition procedure, but of equal importance will be our ability to predict the shapes and stability of individual molecules on various surfaces. Furthermore, one will need to be able to predict the structure patterns generated during the self-organization of whole layers of (bio)molecules on the surface. In this review, we present an overview over the current state of the art regarding the prediction and clarification of structures of biomolecules on surfaces using theoretical and computational methods.

Preparation and Characterization of Biofunctionalized Inorganic Substrates

Integrating the function of biological molecules into traditional inorganic materials and substrates couples biologically relevant function to synthetic devices and generates new materials and capabilities by combining biological and inorganic functions. At this so-called "bio/abio interface," basic biological functions such as ligand binding and catalysis can be co-opted to detect analytes with exceptional sensitivity or to generate useful molecules with chiral specificity under entirely benign reaction conditions. Proteins function in dynamic, complex, and crowded environments (the living cell) and are therefore appropriate for integrating into multistep, multiscale, multimaterial devices such as integrated circuits and heterogeneous catalysts. However, the goal of reproducing the highly specific activities of biomolecules in the perturbed chemical and electrostatic environment at an inorganic interface while maintaining their native conformations is challenging to achieve. Moreover, characterizing protein structure and function at a surface is often difficult, particularly if one wishes to compare the activity of the protein to that of the dilute, aqueous solution phase. Our laboratory has developed a general strategy to address this challenge by taking advantage of the structural and chemical properties of alkanethiol self-assembled monolayers (SAMs) on gold surfaces that are functionalized with covalently tethered peptides. These surface-bound peptides then act as the chemical recognition element for a target protein, generating a biomimetic surface in which protein orientation, structure, density, and function are controlled and variable. Herein we discuss current research and future directions related to generating a chemically tunable biofunctionalization strategy that has potential to successfully incorporate the highly specialized functions of proteins onto inorganic substrates.

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.

Fibrillar structures formed by covalently bound, short, β-stranded peptides on self-assembled monolayers

The ability to maintain or reproduce biomolecular structures on inorganic substrates has the potential to impact diverse fields such as sensing and molecular electronics, as well as the study of biological self-assembly and structure-function relationships. Because the structure and self-assembly of biomolecules are exquisitely sensitive to their local chemical and electrostatic environment, the goal of reproducing or mimicking biological function in an abiological environment, including at a surface, is challenging. However, simple and well-characterized chemical modifications of prepared surfaces can be used to tune surface chemistry, structure, electrostatics, and reactivity of inorganic materials to facilitate biofunctionalization and function. Here, we describe the covalent attachment of 13-residue β-stranded peptides containing alkyne groups to a flat gold surface functionalized with an azide-terminated self-assembled monolayer through a Huisgen cycloaddition, or "click", reaction. The chemical composition and structural morphology of these surfaces were characterized using X-ray photoelectron spectroscopy, grazing incidence angle reflection-absorption infrared spectroscopy, surface circular dichroism, and atomic force microscopy. The surface-bound β-strands self-assemble into antiparallel β-sheets to form fibrillar structures 24.9 ± 1.6 nm in diameter and 2.83 ± 0.74 nm in height on the reactive surface. The results herein provide a platform for studying and controlling the self-assembly process of biomolecules into larger supermolecular structures while allowing tunable control through chemical functionalization of the surface. Interest in the mechanisms of formation of fibrillar structures has most commonly been associated with neurodegenerative diseases, such as Alzheimer's and Parkinson's, but fibrils may actually represent the thermodynamic low-energy conformation of a much larger class of peptides and proteins. The protocol developed here is an important step toward uncovering not only the factors that dictate self-assembly but also the mechanisms by which this fibrillar class of superstructures forms.