Devising Self-Assembled-Monolayers for Surface-Enhanced Infrared Spectroscopy of pH-Driven Poly-l-lysine Conformational Changes

Cation Dependence of Enniatin B/Membrane-Interactions Assessed Using Surface-Enhanced Infrared Absorption (SEIRA) Spectroscopy

Enniatins are mycotoxins with well-known antibacterial, antifungal, antihelmintic and antiviral activity, which have recently come to attention as potential mitochondriotoxic anticancer agents. The cytotoxicity of enniatins is traced back to ionophoric properties, in which the cyclodepsipeptidic structure results in enniatin:cation-complexes of various stoichiometries proposed as membrane-active species. In this work, we employed a combination of surface-enhanced infrared absorption (SEIRA) spectroscopy, tethered bilayer lipid membranes (tBLMs) and density functional theory (DFT)-based computational spectroscopy to monitor the cation-dependence (Mz+=Na+, K+, Cs+, Li+, Mg2+, Ca2+) on the mechanism of enniatin B (EB) incorporation into membranes and identify the functionally relevant EBn : Mz+ complexes formed. We find that Na+ promotes a cooperative incorporation, modelled via an autocatalytic mechanism and mediated by a distorted 2 : 1-EB2 : Na+ complex. K+ (and Cs+) leads to a direct but less efficient insertion into membranes due to the adoption of "ideal" EB2 : K+ sandwich complexes. In contrast, the presence of Li+, Mg2+, and Ca2+ causes a (partial) extraction of EB from the membrane via the formation of "belted" 1 : 1-EB : Mz+ complexes, which screen the cationic charge less efficiently. Our results point to a relevance of the cation dependence for the transport into the malignant cells where the mitochondriotoxic anticancer activity is exerted.

In Situ Spectroscopic Detection of Large-Scale Reorientations of Transmembrane Helices During Influenza A M2 Channel Opening

Viroporins are small ion channels in membranes of enveloped viruses that play key roles during viral life cycles. To use viroporins as drug targets against viral infection requires in-depth mechanistic understanding and, with that, methods that enable investigations under in situ conditions. Here, we apply surface-enhanced infrared absorption (SEIRA) spectroscopy to Influenza A M2 reconstituted within a solid-supported membrane, to shed light on the mechanics of its viroporin function. M2 is a paradigm of pH-activated proton channels and controls the proton flux into the viral interior during viral infection. We use SEIRA to track the large-scale reorientation of M2's transmembrane α-helices in situ during pH-activated channel opening. We quantify this event as a helical tilt from 26° to 40° by correlating the experimental results with solid-state nuclear magnetic resonance-informed computational spectroscopy. This mechanical motion is impeded upon addition of the inhibitor rimantadine, giving a direct spectroscopic marker to test antiviral activity. The presented approach provides a spectroscopic tool to quantify large-scale structural changes and to track the function and inhibition of the growing number of viroporins from pathogenic viruses in future studies.

On-nanoparticle monolayers as a solute-specific, solvent-like phase

In addition to modifying surface properties, self-assembled monolayers, SAMs, on nanoparticles can selectively incorporate small molecules from the surrounding solution. This selectivity has been used in the design of substrate-specific catalytic systems but its degree has not been quantified. This work uses catalytic centers embedded in on-nanoparticle hydrophobic SAMs to monitor and quantify the partitioning of molecules between the bulk solvent and these monolayers. A combination of experiments and theory allows us to relate the logarithm of the incorporation-into-SAM constant to the "bulk" log P values, characterizing the incoming substrates. These results are in line with classic, semi-empirical linear free energy relationships between partitioning solvent systems; in this way, they substantiate the view of nanoscopic on-particle SAMs acting akin to a bulk solvent phase.

Anisotropic acoustic phonon polariton-enhanced infrared spectroscopy for single molecule detection

Nanoscale Fourier transform infrared spectroscopy (nano-FTIR) based on scanning probe microscopy enables the identification of the chemical composition and structure of surface species with a high spatial resolution (∼10 nm), which is crucial for exploring catalytic reaction processes, cellular processes, virus detection, etc. However, the characterization of a single molecule with nano-FTIR is still challenging due to the weak coupling between the molecule and infrared light due to a large size mismatch. Here, we propose a novel structure (monolayer α-MoO₃/air nanogap/Au) to excite anisotropic acoustic phonon polaritons (APhPs) with ultra-high field confinement (mode volume, V_APhPs∼ 10^-11V₀) and electromagnetic energy enhancement (>10⁷), which largely enhance the interaction of single molecules with infrared light. In addition, the anisotropic APhP-assisted nano-FTIR can detect single molecular dipoles in directions both along and perpendicular to the probe axis, while pristine nano-FTIR mainly detects molecular dipoles along the probe axis. The proposed structure provides a way to detect a single molecule, which will impact the fields of biology, chemistry, energy, and environment through fundamental research and applications.

High-sensitivity nanophotonic sensors with passive trapping of analyte molecules in hot spots

Nanophotonic resonators can confine light to deep-subwavelength volumes with highly enhanced near-field intensity and therefore are widely used for surface-enhanced infrared absorption spectroscopy in various molecular sensing applications. The enhanced signal is mainly contributed by molecules in photonic hot spots, which are regions of a nanophotonic structure with high-field intensity. Therefore, delivery of the majority of, if not all, analyte molecules to hot spots is crucial for fully utilizing the sensing capability of an optical sensor. However, for most optical sensors, simple and straightforward methods of introducing an aqueous analyte to the device, such as applying droplets or spin-coating, cannot achieve targeted delivery of analyte molecules to hot spots. Instead, analyte molecules are usually distributed across the entire device surface, so the majority of the molecules do not experience enhanced field intensity. Here, we present a nanophotonic sensor design with passive molecule trapping functionality. When an analyte solution droplet is introduced to the sensor surface and gradually evaporates, the device structure can effectively trap most precipitated analyte molecules in its hot spots, significantly enhancing the sensor spectral response and sensitivity performance. Specifically, our sensors produce a reflection change of a few percentage points in response to trace amounts of the amino-acid proline or glucose precipitate with a picogram-level mass, which is significantly less than the mass of a molecular monolayer covering the same measurement area. The demonstrated strategy for designing optical sensor structures may also be applied to sensing nano-particles such as exosomes, viruses, and quantum dots.

Following the Chemical Immobilization of Membrane Proteins on Plasmonic Nanoantennas Using Infrared Spectroscopy

Plasmonic nanoantennas are promising sensing platforms for detecting chemical and biological molecules in the infrared region. However, integrating fragile biological molecules such as proteins on plasmonic nanoantennas is an essential requirement in the detection procedure. It is crucial to preserve the structural integrity and functionality of proteins while attaching them. In this study, we attached lactose permease, a large membrane protein, onto plasmonic nanoantennas by means of the nickel-nitrile triacetic acid immobilization technique. We followed the individual steps of the immobilization procedure for different lengths of the nanoantennas. The impact of varying the length of the nanoantennas on the shape of the vibrational signal of the chemical layers and on the protein spectrum was studied. We showed that these large proteins are successfully attached onto the nanoantennas, while the chemical spectra of the immobilization monolayers show a shape deformation which is an effect of the coupling between the vibrational mode and the plasmonic resonance.

Resonant Plasmonic Nanoslits Enable in Vitro Observation of Single-Monolayer Collagen-Peptide Dynamics

Proteins perform a variety of essential functions in living cells and thus are of critical interest for drug delivery as well as disease biomarkers. The different functions are derived from a hugely diverse set of structures, fueling interest in their conformational states. Surface-enhanced infrared absorption spectroscopy has been utilized to detect and discriminate protein monomers. As an important step forward, we are investigating collagen peptides consisting of  a  triple helix. While they constitute the main structural building blocks in many complex proteins, they are also a perfect model system for the complex proteins relevant in biological systems. Their complex spectroscopic information as well as the overall small size present a significant challenge for their detection and discrimination. Using resonant plasmonic nanoslits, which are known to show larger specificity compared to nanoantennas, we overcome this challenge. We perform in vitro surface-enhanced absorption spectroscopy studies and track the conformational changes of these collagen peptides under two different external stimuli, which are temperature and chemical surroundings. Modeling the coupling between the amide I vibrational modes and the plasmonic resonance, we can extract the conformational state of the collages and thus monitor the folding and unfolding dynamics of even a single monolayer. This leads to new prospects in studies of single layers of proteins and their folding behavior in minute amounts in a living environment.

In Vitro Monitoring Conformational Changes of Polypeptide Monolayers Using Infrared Plasmonic Nanoantennas

Proteins and peptides play a predominant role in biochemical reactions of living cells. In these complex environments, not only the constitution of the molecules but also their three-dimensional configuration defines their functionality. This so-called secondary structure of proteins is crucial for understanding their function in living matter. Misfolding, for example, is suspected as the cause of neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Ultimately, it is necessary to study a single protein and its folding dynamics. Here, we report a first step in this direction, namely ultrasensitive detection and discrimination of in vitro polypeptide folding and unfolding processes using resonant plasmonic nanoantennas for surface-enhanced vibrational spectroscopy. We utilize poly-l-lysine as a model system which has been functionalized on the gold surface. By in vitro infrared spectroscopy of a single molecular monolayer at the amide I vibrations we directly monitor the reversible conformational changes between α-helix and β-sheet states induced by controlled external chemical stimuli. Our scheme in combination with advanced positioning of the peptides and proteins and more brilliant light sources is highly promising for ultrasensitive in vitro studies down to the single protein level.

Immobilization approaches can affect protein dynamics: a surface-enhanced infrared spectroscopic study on lipid–protein interactions

Near-field detection of SEIRA reveals that surface immobilization alters conformational properties of α-synuclein.

Chemical Routes to Surface Enhanced Infrared Absorption (SEIRA) Substrates

Bottom-up strategies for fabricating SEIRA substrates are presented. For this purpose, wet-chemically prepared gold nanoparticles are coated with a polystyrene shell and subsequently self-assembled into different nanostructures such as quasi-hexagonally ordered gold nanoparticle monolayers, double layers, and honeycomb structures. Furthermore elongated gold nanostructures are obtained by sintering of gold nanoparticle double layers. The optical properties of these different gold nanostructures are directly connected to their morphology and geometrical arrangement – leading to surface plasmon resonances from the visible to the infrared wavelength range. Finally, SEIRA enhancement factors are determined. Gold nanoparticle double layers show the best performance as SEIRA substrates.

Disc Antenna Enhanced Infrared Spectroscopy: From Self-Assembled Monolayers to Membrane Proteins

Plasmonic surfaces have emerged as a powerful platform for biomolecular sensing applications and can be designed to optimize the plasmonic resonance for probing molecular vibrations at utmost sensitivity. Here, we present a facile procedure to generate metallic microdisc antenna arrays that are employed in surface-enhanced infrared absorption (SEIRA) spectroscopy of biomolecules. Transmission electron microscopy (TEM) grids are used as shadow mask deployed during physical vapor deposition of gold. The resulting disc-shaped antennas exhibit enhancement factors of the vibrational bands of 4 × 10⁴ giving rise to a detection limit <1 femtomol (10^-15 mol) of molecules. Surface-bound monolayers of 4-mercaptobenzoic acid show polyelectrolyte behavior when titrated with cations in the aqueous medium. Conformational rigidity of the self-assembled monolayer is validated by density functional theory calculations. The membrane protein sensory rhodopsin II is tethered to the disc antenna arrays and is fully functional as inferred from the light-induced SEIRA difference spectra. As an advance to previous studies, the accessible frequency range is improved and extended into the fingerprint region.

Simultaneous IR-Spectroscopic Observation of α-Synuclein, Lipids, and Solvent Reveals an Alternative Membrane-Induced Oligomerization Pathway

The intrinsically disordered protein α-synuclein (αS), a known pathogenic factor for Parkinson's disease, can adopt defined secondary structures when interacting with membranes or during fibrillation. The αS-lipid interaction and the implications of this process for aggregation and damage to membranes are still poorly understood. Therefore, we established a label-free infrared (IR) spectroscopic approach to allow simultaneous monitoring of αS conformation and membrane integrity. IR showed its unique sensitivity for identifying distinct β-structured aggregates. A comparative study of wild-type αS and the naturally occurring splicing variant αS Δexon3 yielded new insights into the membrane's capability for altering aggregation pathways.

Homogeneously Mixed Monolayers: Emergence of Compositionally Conflicted Interfaces

The ability to manipulate interfaces at the nanoscale via a variety of thin-film technologies offers a plethora of avenues for advancing surface applications. These include surfaces with remarkable antibiofouling properties as well as those with tunable physical and electronic properties. Molecular self-assembly is one notably attractive method used to decorate and modify surfaces. Of particular interest to surface scientists has been the thiolate-gold system, which serves as a reliable method for generating model thin-film monolayers that transform the interfacial properties of gold surfaces. Despite widespread interest, efforts to tune the interfacial properties using mixed adsorbate systems have frequently led to phase-separated domains of molecules on the surface with random sizes and shapes depending on the structure and chemical composition of the adsorbates. This feature article highlights newly emerging methods for generating mixed thin-film interfaces, not only to enhance the aforementioned properties of organic thin films, but also to give rise to interfacial compositions never before observed in nature. An example would be the development of monolayers formed from bidentate adsorbates and other unique headgroup architectures that provide the surface bonding stability necessary to allow the assembly of interfaces that expose mixtures of chains that are fundamentally different in character (i.e., either phase-incompatible or structurally dissimilar), producing compositionally "conflicted" interfaces. By also exploring the prior efforts to produce such homogeneously blended interfaces, this feature article seeks to convey the relationships between the methods of film formation and the overall properties of the resulting interfaces.

Reactive Landing of Gramicidin S and Ubiquitin Ions onto Activated Self-Assembled Monolayer Surfaces

Using mass-selected ion deposition combined with in situ infrared reflection absorption spectroscopy (IRRAS), we examined the reactive landing of gramicidin S and ubiquitin ions onto activated self-assembled monolayer (SAM) surfaces terminated with N-hydroxysuccinimidyl ester (NHS-SAM) and acyl fluoride (COF-SAM) groups. Doubly protonated gramicidin S, [GS + 2H]²⁺, and two charge states of ubiquitin, [U + 5H]⁵⁺ and [U + 13H]¹³⁺, were used as model systems, allowing us to explore the effect of the number of free amino groups and the secondary structure on the efficiency of covalent bond formation between the projectile ion and the surface. For all projectile ions, ion deposition resulted in the depletion of IRRAS bands corresponding to the terminal groups on the SAM and the appearance of several new bands not associated with the deposited species. These new bands were assigned to the C=O stretching vibrations of COOH and COO^- groups formed on the surface as a result of ion deposition. The presence of these bands was attributed to an alternative reactive landing pathway that competes with covalent bond formation. This pathway with similar yields for both gramicidin S and ubiquitin ions is analogous to the hydrolysis of the NHS ester bond in solution. The covalent bond formation efficiency increased linearly with the number of free amino groups and was found to be lower for the more compact conformation of ubiquitin compared with the fully unfolded conformation. This observation was attributed to the limited availability of amino groups on the surface of the folded conformation. Our results have provided new insights on the efficiency and mechanism of reactive landing of peptides and proteins onto activated SAMs. Graphical Abstract ᅟ.