Linear dichroism and linearly polarised luminescence spectra of oriented samples collected on a new integrated instrument

Linearly polarised luminescence (LPL) has a wide range of potential applications giving optical and geometric parameters for oriented lumiphores. In this work we present the first wavelength scanned LPL spectra. Analytes are either oriented on stretched polyethylene films or in flow. Applications of the wavelength-dependence of g-factors are illustrated.

Inactivation efficacy and mechanisms of wavelength-specific UV sources for various strains of Legionella pneumophila serogroup 1

Infections of Legionnaires' disease in the United States caused by Legionella have increased ninefold between the years 2000-2018. Legionella harbored in biofilms or inside amoeba within premise plumbing can be more resistant to disinfectants, thus causing treatment challenges. Ultraviolet-light emitting diodes (UV-LEDs) are an emerging water disinfection technology with several advantages over conventional UV lamps. In this study, we evaluated the effects of UV-LEDs (255, 265, and 285 nm), a low-pressure (LP) mercury UV lamp (254 nm), and a bandpass filtered medium-pressure (MP) mercury UV lamp (220 nm) on properties and inactivation of three strains of L. pneumophila serogroup 1. The UV-LEDs emitting at 255 and 265 nm showed greater inactivation performance against all the strains compared to the UV-LED at 285 nm and the LP UV lamp at 254 nm. Our results showed that strains of the same serogroup exhibited different UV sensitivities. Analyses of DNA and protein damage revealed that UV exposure using 254, 255, and 265 nm predominantly causes DNA damage, while protein damage is predominant at 220 nm. Both DNA and protein damage were observed at 285 nm, but the extent of DNA damage was relatively less significant compared to the other wavelengths. Electric energy consumption analysis showed that water treatment using UV-LEDs is currently unsatisfactory compared to conventional LP UV lamps due to the mediocre wall plug efficiency (WPE) of UV-LEDs. However, recent studies indicate that the WPE of UV-LEDs is continuously improving. Overall, our study highlights that UV-LEDs are a promising technology for inactivating waterborne pathogens and have the potential to replace existing UV mercury lamps for water disinfection applications.

Extracellular matrix type 0: From ancient collagen lineage to a versatile product pipeline - JellaGel™

Extracellular matrix type 0 is reported. The matrix is developed from a jellyfish collagen predating mammalian forms by over 0.5 billion years. With its ancient lineage, compositional simplicity, and resemblance to multiple collagen types, the matrix is referred to as the extracellular matrix type 0. Here we validate the matrix describing its physicochemical and biological properties and present it as a versatile, minimalist biomaterial underpinning a pipeline of commercialised products under the collective name of JellaGelTM. We describe an extensive body of evidence for folding and assembly of the matrix in comparison to mammalian matrices, such as bovine collagen, and its use to support cell growth and development in comparison to known tissue-derived products, such as Matrigel™. We apply the matrix to co-culture human astrocytes and cortical neurons derived from induced pluripotent stem cells and visualise neuron firing synchronicity with correlations indicative of a homogenous extracellular material in contrast to the performance of heterogenous commercial matrices. We prove the ability of the matrix to induce spheroid formation and support the 3D culture of human immortalised, primary, and mesenchymal stem cells. We conclude that the matrix offers an optimal solution for systemic evaluations of cell-matrix biology. It effectively combines the exploitable properties of mammalian tissue extracts or top-down matrices, such as biocompatibility, with the advantages of synthetic or bottom-up matrices, such as compositional control, while avoiding the drawbacks of the two types, such as biological and design heterogeneity, thereby providing a unique bridging capability of a stem extracellular matrix.

α-Tocopherol phosphate as a photosensitizer in the reaction of nucleosides with UV light: formation of 5,6-dihydrothymidine

Introduction

α-Tocopherol phosphate, a natural water-soluble α-tocopherol analog, exists in biological tissues and fluids. Synthesized α-tocopherol phosphate is used as an ingredient of cosmetics.

Findings

When a neutral mixed solution of 2′-deoxycytidine, 2′-deoxyguanosine, thymidine, and 2′-deoxyadenosine was irradiated with UV light at wavelengths longer than 300 nm in the presence of α-tocopherol phosphate, thymidine was markedly consumed in an α-tocopherol phosphate dose-dependent manner, whereas other nucleosides only slightly decreased. Two major product peaks were detected in an HPLC chromatogram. The products were identified as diastereomers of 5,6-dihydrothymidine. The addition of radical scavengers had almost no effects on the generation of 5,6-dihydrothymidine, whereas the reactions of nucleosides other than thymidine were suppressed. Trolox, another water-soluble α-tocopherol analog, did not generate 5,6-dihydrothymidine, although all nucleosides were slightly consumed. When UV irradiation of thymidine with α-tocopherol phosphate was conducted in D₂O, two deuterium atoms were added to 5 and 6 positions of thymidine with both syn and anti configurations. The ratio of syn and anti configurations alternated depending on pD of the solution.

Conclusions

The results indicate that α-tocopherol phosphate is a photosensitizer of nucleosides, especially thymidine, and that it introduces two hydrogen atoms to thymidine from H₂O, generating 5,6-dihydrothymidine.

Linear Dichroism Activity of Chiral Poly(p-Aryltriazole) Foldamers

Controllable higher-order assembly is a central aim of macromolecular chemistry. An essential challenge to developing these molecules is improving our understanding of the structures they adopt under different conditions. Here, we demonstrate how flow linear dichroism (LD) spectroscopy is used to provide insights into the solution structure of a chiral, self-assembled fibrillar foldamer. Poly(para-aryltriazole)s fold into different structures depending on the monomer geometry and variables such as solvent and ionic strength. LD spectroscopy provides a simple route to determine chromophore alignment in solution and is generally used on natural molecules or molecular assemblies such as DNA and M13 bacteriophage. In this contribution, we show that LD spectroscopy is a powerful tool in the observation of self-assembly processes of synthetic foldamers when complemented by circular dichroism, absorbance spectroscopy, and microscopy. To that end, poly(para-aryltriazole)s were aligned in a flow field under different solvent conditions. The extended aromatic structures in the foldamer give rise to a strong LD signal that changes in sign and in intensity with varying solvent conditions. A key advantage of LD is that it only detects the large assemblies, thus removing background due to monomers and small oligomers.

Uric Acid as a Photosensitizer in the Reaction of Deoxyribonucleosides with UV Light of Wavelength Longer than 300 nm: Identification of Products from 2'-Deoxycytidine

DNA reacts directly with UV light with a wavelength shorter than 300 nm. Although ground surface sunlight includes little of this short-wavelength UV light due to its almost complete absorption by the atmosphere, sunlight is the primary cause of skin cancer. Photosensitization by endogenous substances must therefore be involved in skin cancer development mechanisms. Uric acid is the final metabolic product of purines in humans, and is present at relatively high concentrations in cells and fluids. When a neutral mixed solution of 2'-deoxycytidine, 2'-deoxyguanosine, thymidine, and 2'-deoxyadenosine was irradiated with UV light with a wavelength longer than 300 nm in the presence of uric acid, all the nucleosides were consumed in a uric acid dose-dependent manner. These reactions were inhibited by the addition of radical scavengers, ethanol and sodium azide. Two products from 2'-deoxycytidine were isolated and identified as N⁴-hydroxy-2'-deoxycytidine and N⁴,5-cyclic amide-2'-deoxycytidine, formed by cycloaddition of an amide group from uric acid. A ¹⁵N-labeled uric acid, uric acid-1,3-¹⁵N₂, having two ¹⁴N and two ¹⁵N atoms per molecule, produced N⁴,5-cyclic amide-2'-deoxycytidine containing both ¹⁴N and ¹⁵N atoms from uric acid-1,3-¹⁵N₂. Singlet oxygen, hydroxyl radical, peroxynitrous acid, hypochlorous acid, and hypobromous acid generated neither N⁴-hydroxy-2'-deoxycytidine nor N⁴,5-cyclic amide-2'-deoxycytidine in the presence of uric acid. These results indicate that uric acid is a photosensitizer for the reaction of nucleosides by UV light with a wavelength longer than 300 nm, and that an unidentified radical derived from uric acid with a delocalized unpaired electron may be generated.

Tunable Circularly Polarized Luminescence from Single Crystal and Powder of the Simplest Tetraphenylethylene Helicate

Tetraphenylethylene and its derivatives are a class of aggregation-induced emission (AIE) compounds that are most extensively and successfully studied. It has been found that the simplest TPE is easy to crystallize into homochiral M crystals or P crystals. However, no research on circularly polarized luminescence (CPL) of TPE solid is documented. In this paper, we report that TPE can grow into big and nonefflorescent single crystals in single helical conformation and has large birefringence that is comparative with commercially available products. The TPE single crystals can emit strong CPL with a very high g_lum value up to 0.53. Moreover, the sense and magnitude of CPL signals can be willfully tuned by simple rotation of the single crystal due to anisotropy of the crystals. This simple and promising CPL photonic material integrates emission, chirality, and birefringence together in one single crystal without needing an additional chiral dopant or conjugate polymer that can produce linearly polarized light. After being ground into fine powder and pressed as KBr pellets, the obtained nanocrystals of TPE also emit strong CPL light. Exceptionally, by mixing other achiral luminescent dyes together with TPE powder in KBr pellets, induced CPL signals were obtained, which could give full-color CPL emission. Although there were no interactions between TPE and the dyes in the pellets, induced CPL signals were realized through radiative energy transfer, providing a very simple method for the tuning of CPL emission.

Membrane Association Modes of Natural Anticancer Peptides: Mechanistic Details on Helicity, Orientation, and Surface Coverage

Anticancer peptides (ACPs) could potentially offer many advantages over other cancer therapies. ACPs often target cell membranes, where their surface mechanism is coupled to a conformational change into helical structures. However, details on their binding are still unclear, which would be crucial to reach progress in connecting structural aspects to ACP action and to therapeutic developments. Here we investigated natural helical ACPs, Lasioglossin LL-III, Macropin 1, Temporin-La, FK-16, and LL-37, on model liposomes, and also on extracellular vesicles (EVs), with an outer leaflet composition similar to cancer cells. The combined simulations and experiments identified three distinct binding modes to the membranes. Firstly, a highly helical structure, lying mainly on the membrane surface; secondly, a similar, yet only partially helical structure with disordered regions; and thirdly, a helical monomeric form with a non-inserted perpendicular orientation relative to the membrane surface. The latter allows large swings of the helix while the N-terminal is anchored to the headgroup region. These results indicate that subtle differences in sequence and charge can result in altered binding modes. The first two modes could be part of the well-known carpet model mechanism, whereas the newly identified third mode could be an intermediate state, existing prior to membrane insertion.

Plasmonic and label-free real-time quantitative PCR for point-of-care diagnostics

In response to the world's medical community's need for accurate and immediate infectious pathogen detection, many researchers have focused on adapting the standard molecular diagnostic method of polymerase chain reaction (PCR) for point-of-care (POC) applications. PCR technology is not without its shortcomings; current platforms can be bulky, slow, and power-intensive. Although there have been some advances in microfluidic PCR devices, a simple-to-operate and fabricate PCR device is still lacking. In the first part of this paper, we introduce a compact plasmonic PCR thermocycler in which fast DNA amplification is derived from efficient photothermal heating of a colloidal reaction mixture containing gold nanorods (AuNRs) using a small-scale vertical-cavity surface-emitting laser (VCSEL). Using this method, we demonstrate 30 cycle-assay time of sub-ten minutes for successful Chlamydia trachomatis DNA amplification in 20 μL total PCR sample volume. In the second part, we report an ultrasensitive real-time amplicon detection strategy which is based on cycle-by-cycle monitoring of 260 nm absorption of the PCR sample. This was accomplished by irradiating the PCR sample using a UV LED and collecting the transmitted optical power with a photodetector. The UV absorption dependency on the nucleotides' structural degree of freedom gives rise to distinctive features in the shape of UV amplification curves for the determination of PCR results, thus circumventing the need for the complicated design of target-specific probes or intercalating fluorophores. This amplicon quantification method has a high detection sensitivity of one DNA copy. This is the first demonstration of a compact plasmonic thermocycler combined with a real-time fluorophore-free quantitative amplicon detection system. The small footprint of our PCR device stems from hardware miniaturization, while abundant sample volume facilitates highly sensitive detection and fluid handling required for in-field sample analysis, thereby making it an excellent candidate for POC molecular diagnostics.

Fluorescence detected circular dichroism (FDCD) for supramolecular host-guest complexes

Fluorescence-detected circular dichroism (FDCD) spectroscopy is applied for the first time to supramolecular host-guest and host-protein systems and compared to the more known electronic circular dichroism (ECD). We find that FDCD can be an excellent choice for common supramolecular applications, e.g. for the detection and chirality sensing of chiral organic analytes, as well as for reaction monitoring. Our comprehensive investigations demonstrate that FDCD can be conducted in favorable circumstances at much lower concentrations than ECD measurements, even in chromophoric and auto-emissive biofluids such as blood serum, overcoming the sensitivity limitation of absorbance-based chiroptical spectroscopy. Besides, the combined use of FDCD and ECD can provide additional valuable information about the system, e.g. the chemical identity of an analyte or hidden aggregation phenomena. We believe that simultaneous FDCD- and ECD-based chiroptical characterization of emissive supramolecular systems will be of general benefit for characterizing fluorescent, chiral supramolecular systems due to the higher information content obtained by their combined use.

Controlling Peptide Function by Directed Assembly Formation: Mechanistic Insights Using Multiscale Modeling on an Antimicrobial Peptide-Drug-Membrane System

Owing to their potential applicability against multidrug-resistant bacteria, antimicrobial peptides (AMPs) or host defense peptides (HDPs) gain increased attention. Besides diverse immunomodulatory roles, their classical mechanism of action mostly involves membrane disruption of microbes. Notably, their unbalanced overexpression has also been associated with host cell cytotoxicity in various diseases. Relatedly, AMPs can be subject to aggregate formation, either via self-assembly or together with other compounds, which has demonstrated a modulation effect on their biological functions, thus highly relevant both for drug targeting projects and understanding their in vivo actions. However, the molecular aspects of the related assembly formation are not understood. Here, we focused in detail on an experimentally studied AMP-drug system, i.e., CM15-suramin, and performed all-atom and coarse-grain (CG) simulations. Results obtained for all systems were in close line with experimental observations and indicate that the CM15-suramin aggregation is an energetically favorable and dynamic process. In the presence of bilayers, the peptide-drug assembly formation was highly dependent on lipid composition, and peptide aggregates themselves were also capable of binding to the membranes. Interestingly, longer CG simulations with zwitterionic membranes indicated an intermediate state in the presence of both AMP-drug assemblies and monomeric peptides located on the membrane surface. In sharp contrast, larger AMP-drug aggregates could not be detected with a negatively charged membrane, rather the AMPs penetrated its surface in a monomeric form, in line with previous in vitro observations. Considering experimental and theoretical results, it is promoted that in biological systems, cationic AMPs may often form associates with anionic compounds in a reversible manner, resulting in lower bioactivity. This is only mildly affected by zwitterionic membranes; however, membranes with a negative charge strongly alter the energetic preference of AMP assemblies, resulting in the dissolution of the complexes into the membrane. The phenomenon observed here at a molecular level can be followed in several experimental systems studied recently, where peptides interact with food colors, drug molecules, or endogenous compounds, which strongly indicates that reversible associate formation is a general phenomenon for these complexes. These results are hoped to be exploited in novel therapeutic strategies aiming to use peptides as drug targets and control AMP bioactivity by directed assembly formation.

Towards New Chiroptical Transitions Based on Thought Experiments and Hypothesis

We studied supramolecular chirality induced by circularly polarized light. Photoresponsive azopolymers form a helical intermolecular network. Furthermore, studies on photochemical materials using optical vortex light will also attract attention in the future. In contrast to circularly polarized light carrying spin angular momentum, an optical vortex with a spiral wave front and carrying orbital angular momentum may impart torque upon irradiated materials. In this review, we summarize a few examples, and then theoretically and computationally deduce the differences in spin angular momentum and orbital angular momentum depending on molecular orientation not on, but in, polymer films. UV-vis absorption and circular dichroism (CD) spectra are consequences of electric dipole transition and magnetic dipole transition, respectively. However, the basic effect of vortex light is postulated to originate from quadrupole transition. Therefore, we explored the simulated CD spectra of azo dyes with the aid of conventional density functional theory (DFT) calculations and preliminary theoretical discussions of the transition of CD. Either linearly or circularly polarized UV light causes the trans–cis photoisomerization of azo dyes, leading to anisotropic and/or helically organized methyl orange, respectively, which may be detectable by CD spectroscopy after some technical treatments. Our preliminary theoretical results may be useful for future experiments on the irradiation of UV light under vortex.

Pillared cobalt metal-organic frameworks act as chromatic polarizers

The ease with which molecular building blocks can be ordered in metal-organic frameworks is an invaluable asset for many potential applications. In this work, we exploit this inherent order to produce chromatic polarizers based on visible-light linear dichroism via cobalt paddlewheel chromophores.

Superchiral near fields detect virus structure

Optical spectroscopy can be used to quickly characterise the structural properties of individual molecules. However, it cannot be applied to biological assemblies because light is generally blind to the spatial distribution of the component molecules. This insensitivity arises from the mismatch in length scales between the assemblies (a few tens of nm) and the wavelength of light required to excite chromophores (≥150 nm). Consequently, with conventional spectroscopy, ordered assemblies, such as the icosahedral capsids of viruses, appear to be indistinguishable isotropic spherical objects. This limits potential routes to rapid high-throughput portable detection appropriate for point-of-care diagnostics. Here, we demonstrate that chiral electromagnetic (EM) near fields, which have both enhanced chiral asymmetry (referred to as superchirality) and subwavelength spatial localisation (∼10 nm), can detect the icosahedral structure of virus capsids. Thus, they can detect both the presence and relative orientation of a bound virus capsid. To illustrate the potential uses of the exquisite structural sensitivity of subwavelength superchiral fields, we have used them to successfully detect virus particles in the complex milieu of blood serum.

Intrinsically distinct hole and electron transport in conjugated polymers controlled by intra and intermolecular interactions

It is still a matter of controversy whether the relative difference in hole and electron transport in solution-processed organic semiconductors is either due to intrinsic properties linked to chemical and solid-state structure or to extrinsic factors, as device architecture. We here isolate the intrinsic factors affecting either electron or hole transport within the same film microstructure of a model copolymer semiconductor. Relatively, holes predominantly bleach inter-chain interactions with H-type electronic coupling character, while electrons' relaxation more strongly involves intra-chain interactions with J-type character. Holes and electrons mobility correlates with the presence of a charge transfer state, while their ratio is a function of the relative content of intra- and inter-molecular interactions. Such fundamental observation, revealing the specific role of the ground-state intra- and inter-molecular coupling in selectively assisting charge transport, allows predicting a more favorable hole or electron transport already from screening the polymer film ground state optical properties.

Orientation of Chiral Schiff Base Metal Complexes Involving Azo-Groups for Induced CD on Gold Nanoparticles by Polarized UV Light Irradiation

In this study, we report the synthesis, characterization, and chiroptical properties of azo-group-containing chiral salen type Schiff base Ni(II), Cu(II), and Zn(II) complexes absorbed on gold nanoparticles (AuNPs) of 10 nm diameters. Induced circular dichroism (CD) around the plasmon region from the chiral species weakly adsorbed on the surface of AuNP were observed when there were appropriate dipole–dipole interactions at the initial states. Spectral changes were also observed by not only cis-trans photoisomerization of azo-groups but also changes of orientation due to Weigert effect of azo-dyes after linearly polarized UV light irradiation. Spatial features were discussed based on dipole-dipole interactions mainly within an exciton framework.

Flow Alignment of Extracellular Vesicles: Structure and Orientation of Membrane-Associated Bio-macromolecules Studied with Polarized Light

Extracellular vesicles (EVs) are currently in scientific focus, as they have great potential to revolutionize the diagnosis and therapy of various diseases. However, numerous aspects of these species are still poorly understood, and thus, additional insight into their molecular-level properties, membrane-protein interactions, and membrane rigidity is still needed. We here demonstrate the use of red-blood-cell-derived EVs (REVs) that polarized light spectroscopy techniques, linear and circular dichroism, can provide molecular-level structural information on these systems. Flow-linear dichroism (flow-LD) measurements show that EVs can be oriented by shear force and indicate that hemoglobin molecules are associated to the lipid bilayer in freshly released REVs. During storage, this interaction ceases; this is coupled to major protein conformational changes relative to the initial state. Further on, the degree of orientation gives insight into vesicle rigidity, which decreases in time parallel to changes in protein conformation. Overall, we propose that both linear dichroism and circular dichroism spectroscopy can provide simple, rapid, yet efficient ways to track changes in the membrane-protein interactions of EV components at the molecular level, which may also give insight into processes occurring during vesiculation.

Nanosheets of an Organic Molecular Assembly from Aqueous Medium Exhibit High Solid-State Emission and Anisotropic Charge-Carrier Mobility

A π-conjugated amphiphilic diketopyrrolopyrrole (PDPP-Amphi) forms crystalline 2D supramolecular nanosheets in water when compared to that from methyl cyclohexane. These nanosheets exhibit high fluorescence quantum yield in the solid-state with anisotropic charge-carrier mobility of 0.33 cm² V^-1 s^-1 .

Engineering monolayer poration for rapid exfoliation of microbial membranes

A novel mechanism of monolayer poration leading to the rapid exfoliation and lysis of microbial membranes is reported.

The spread of bacterial resistance to traditional antibiotics continues to stimulate the search for alternative antimicrobial strategies. All forms of life, from bacteria to humans, are postulated to rely on a fundamental host defense mechanism, which exploits the formation of open pores in microbial phospholipid bilayers. Here we predict that transmembrane poration is not necessary for antimicrobial activity and reveal a distinct poration mechanism that targets the outer leaflet of phospholipid bilayers. Using a combination of molecular-scale and real-time imaging, spectroscopy and spectrometry approaches, we introduce a structural motif with a universal insertion mode in reconstituted membranes and live bacteria. We demonstrate that this motif rapidly assembles into monolayer pits that coalesce during progressive membrane exfoliation, leading to bacterial cell death within minutes. The findings offer a new physical basis for designing effective antibiotics.

The association of defensin HNP-2 with negatively charged membranes: A combined fluorescence and linear dichroism study

The association of defensin HNP-2 with negatively charged membranes has been studied using a new approach that combines fluorescence and linear dichroism (LD) spectroscopies with simulated LD spectra in order to characterise the binding kinetics and bound configurations of the peptide. Binding to membranes composed of mixtures of diacylglycerophosphocholines (PC) with either diacylglycerophosphoglycerol (PG) or diacylglycerophosphoserine (PS) was conducted at lipid:peptide ratios that yielded binding, but not membrane fusion. HNP-2 association with membranes under these conditions was a 2 stage-process, with both stages exhibiting first order kinetics. The fast initial step, with a half-life of < 1 min, was followed by a slower step with a half-life of > 3 min. Conversion between the states was estimated to have an enthalpy of activation of approximately 10 kJ mol(-1) and an entropy of activation of -0.2 kJ K mol(-1). LD spectra corresponding to each of the membrane bound states were generated by non-linear regression using a standard kinetic model. These spectra are interpreted in comparison with spectra calculated using the program Dichrocalc and reveal that the peptide associates with membranes in a small number of stable configurations. All of these configurations have a significant proportion of β-sheet structure residing in the plane of the membrane. Two configurations support structures previously proposed for defensins in membranes.

Stray light correction in the optical spectroscopy of crystals

It has long been known in spectroscopy that light not passing through a sample, but reaching the detector (i.e., stray light), results in a distortion of the spectrum known as absorption flattening. In spectroscopy with crystals, one must either include such stray light or take steps to exclude it. In the former case, the derived spectra are not accurate. In the latter case, a significant amount of the crystal must be masked off and excluded. In this paper, we describe a method that allows use of the entire crystal by correcting the distorted spectrum.

A physical operation of hydrodynamic orientation of an azobenzene supramolecular assembly with light and sound

Photoisomerizations of a newly designed azobenzene derivative reversibly change its self-assembly in a solution to form twisted supramolecular nanofibers and amorphous aggregates, respectively. When irradiating the sample solution with audible sound, the former assembly exhibits a LD response due to its hydrodynamic orientation, but the latter one is LD silent, in the sound-induced fluid flows.

Photochromism in sound-induced alignment of a diarylethene supramolecular nanofibre

A photochromic supramolecular nanofibre, composed of a diarylethene derivative, exhibits hydrodynamic alignment upon exposure to the audible sound.

Supramolecular amphipathicity for probing antimicrobial propensity of host defence peptides

Supramolecular amphipathicity exposes antimicrobial propensity of host defence peptides.

High anisotropy of flow-aligned bicellar membrane systems

In recent years, multi-lipid bicellar systems have emerged as promising membrane models. The fast orientational diffusion and magnetic alignability made these systems very attractive for NMR investigations. However, their alignment was so far achieved with a strong magnetic field, which limited their use with other methods that require macroscopic orientation. Recently, it was shown that bicelles could be aligned also by shear flow in a Couette flow cell, making it applicable to structural and biophysical studies by polarized light spectroscopy. Considering the sensitivity of this lipid system to small variations in composition and physicochemical parameters, efficient use of such a flow-cell method with coupled techniques will critically depend on the detailed understanding of how the lipid systems behave under flow conditions. In the present study we have characterized the flow alignment behavior of the commonly used dimyristoyl phosphatidylcholine/dicaproyl phosphatidylcholine (DMPC/DHPC) bicelle system, for various temperatures, lipid compositions, and lipid concentrations. We conclude that at optimal flow conditions the selected bicellar systems can produce the most efficient flow alignment out of any lipid systems used so far. The highest degree of orientation of DMPC/DHPC samples is noticed in a narrow temperature interval, at a practical temperature around 25 °C, most likely in the phase transition region characterized by maximum sample viscosity. The change of macroscopic orientation factor as function of the above conditions is now described in detail. The increase in macroscopic alignment observed for bicelles will most likely allow recording of higher resolution spectra on membrane systems, which provide deeper structural insight and analysis into properties of biomolecules interacting with solution phase lipid membranes.

Vortex-induced alignment of a water soluble supramolecular nanofiber composed of an amphiphilic dendrimer

We have synthesized a novel amphiphilic naphthalene imide bearing a cationic dendrimer wedge (NID). NID molecules in water self-assemble to form a two-dimensional ribbon, which further coils to give a linear supramolecular nanofiber. The sample solution showed linear dichroism (LD) upon stirring of the solution, where NID nanofibers dominantly align at the center of vortex by hydrodynamic interaction with the downward torsional flows.

Anti-antimicrobial peptides: folding-mediated host defense antagonists

Antimicrobial or host defense peptides are innate immune regulators found in all multicellular organisms. Many of them fold into membrane-bound α-helices and function by causing cell wall disruption in microorganisms. Herein we probe the possibility and functional implications of antimicrobial antagonism mediated by complementary coiled-coil interactions between antimicrobial peptides and de novo designed antagonists: anti-antimicrobial peptides. Using sequences from native helical families such as cathelicidins, cecropins, and magainins we demonstrate that designed antagonists can co-fold with antimicrobial peptides into functionally inert helical oligomers. The properties and function of the resulting assemblies were studied in solution, membrane environments, and in bacterial culture by a combination of chiroptical and solid-state NMR spectroscopies, microscopy, bioassays, and molecular dynamics simulations. The findings offer a molecular rationale for anti-antimicrobial responses with potential implications for antimicrobial resistance.

Nanoscale imaging reveals laterally expanding antimicrobial pores in lipid bilayers

Antimicrobial peptides are postulated to disrupt microbial phospholipid membranes. The prevailing molecular model is based on the formation of stable or transient pores although the direct observation of the fundamental processes is lacking. By combining rational peptide design with topographical (atomic force microscopy) and chemical (nanoscale secondary ion mass spectrometry) imaging on the same samples, we show that pores formed by antimicrobial peptides in supported lipid bilayers are not necessarily limited to a particular diameter, nor they are transient, but can expand laterally at the nano-to-micrometer scale to the point of complete membrane disintegration. The results offer a mechanistic basis for membrane poration as a generic physicochemical process of cooperative and continuous peptide recruitment in the available phospholipid matrix.

Exploring the sequence-structure relationship for amyloid peptides

Amyloid fibril formation is associated with misfolding diseases, as well as fulfilling a functional role. The cross-β molecular architecture has been reported in increasing numbers of amyloid-like fibrillar systems. The Waltz algorithm is able to predict ordered self-assembly of amyloidogenic peptides by taking into account the residue type and position. This algorithm has expanded the amyloid sequence space, and in the present study we characterize the structures of amyloid-like fibrils formed by three peptides identified by Waltz that form fibrils but not crystals. The structural challenge is met by combining electron microscopy, linear dichroism, CD and X-ray fibre diffraction. We propose structures that reveal a cross-β conformation with 'steric-zipper' features, giving insights into the role for side chains in peptide packing and stability within fibrils. The amenity of these peptides to structural characterization makes them compelling model systems to use for understanding the relationship between sequence, self-assembly, stability and structure of amyloid fibrils.

Interactions of a photochromic spiropyran with liposome model membranes

The interactions between anionic or zwitterionic liposomes and a water-soluble, DNA-binding photochromic spiropyran are studied using UV/vis absorption and linear dichroism (LD) spectroscopy. The spectral characteristics as well as the kinetics of the thermal isomerization process in the absence and presence of the two different liposome types provide information about the environment and whether or not the spiropyran resides in the liposome membrane. By measuring LD on liposomes deformed and aligned by shear flow, further insight is obtained about interaction and binding geometry of the spiropyran at the lipid membranes. We show that the membrane interactions differ between the two types of liposomes used as well as the isomeric forms of the spiropyran photoswitch.

Circular and linear dichroism spectroscopy for the study of protein-ligand interactions

Circular dichroism (CD) is the difference in absorption of left and right circularly polarized light, usually by a solution containing the molecules of interest. A non-zero signal for solutions is only measured for chiral molecules such as proteins whose mirror image is not superposable on the original molecule. A CD spectrum provides information about the bonds and structures responsible for the chirality. When a small molecule (or ligand) binds to a protein, it acquires an induced CD (ICD) spectrum through chiral perturbation to its structure or electron rearrangements (transitions). The wavelengths of this ICD are determined by the ligand's own absorption spectrum, and the intensity of the ICD spectrum is determined by the strength and geometry of its interaction with the protein. Thus, ICD can be used to probe the binding of ligands to proteins. This chapter contains an outline of how to perform protein CD and ICD experiments, together with some of the issues relating to experimental design and implementation. Addition of a quarter wave plate to a CD spectropolarimeter converts it to a linear dichroism (LD) spectrometer. When protein samples are aligned either in flow (as for fibers or membrane proteins in liposomes) or on surfaces the orientations of ligands with respect to the protein backbone or other subunits can be determined.

Simple estimation of Förster Resonance Energy Transfer (FRET) orientation factor distribution in membranes

Because of its acute sensitivity to distance in the nanometer scale, Förster resonance energy transfer (FRET) has found a large variety of applications in many fields of chemistry, physics, and biology. One important issue regarding the correct usage of FRET is its dependence on the donor-acceptor relative orientation, expressed as the orientation factor κ². Different donor/acceptor conformations can lead to κ² values in the 0 ≤ κ² ≤ 4 range. Because the characteristic distance for FRET, R₀, is proportional to (κ²)^1/6, uncertainties in the orientation factor are reflected in the quality of information that can be retrieved from a FRET experiment. In most cases, the average value of κ² corresponding to the dynamic isotropic limit (<κ²> = 2/3) is used for computation of R₀ and hence donor-acceptor distances and acceptor concentrations. However, this can lead to significant error in unfavorable cases. This issue is more critical in membrane systems, because of their intrinsically anisotropic nature and their reduced fluidity in comparison to most common solvents. Here, a simple numerical simulation method for estimation of the probability density function of κ² for membrane-embedded donor and acceptor fluorophores in the dynamic regime is presented. In the simplest form, the proposed procedure uses as input the most probable orientations of the donor and acceptor transition dipoles, obtained by experimental (including linear dichroism) or theoretical (such as molecular dynamics simulation) techniques. Optionally, information about the widths of the donor and/or acceptor angular distributions may be incorporated. The methodology is illustrated for special limiting cases and common membrane FRET pairs.

Single molecule experimentation in biological physics: exploring the living component of soft condensed matter one molecule at a time

The soft matter of biological systems consists of mesoscopic length scale building blocks, composed of a variety of different types of biological molecules. Most single biological molecules are so small that 1 billion would fit on the full-stop at the end of this sentence, but collectively they carry out the vital activities in living cells whose length scale is at least three orders of magnitude greater. Typically, the number of molecules involved in any given cellular process at any one time is relatively small, and so real physiological events may often be dominated by stochastics and fluctuation behaviour at levels comparable to thermal noise, and are generally heterogeneous in nature. This challenging combination of heterogeneity and stochasticity is best investigated experimentally at the level of single molecules, as opposed to more conventional bulk ensemble-average techniques. In recent years, the use of such molecular experimental approaches has become significantly more widespread in research laboratories around the world. In this review we discuss recent experimental approaches in biological physics which can be applied to investigate the living component of soft condensed matter to a precision of a single molecule.

Mechanical stress affects glucagon fibrillation kinetics and fibril structure

Mechanical stress can strongly influence the capability of a protein to aggregate and the kinetics of aggregation, but there is little insight into the underlying mechanism. Here we study the effect of different mechanical stress conditions on the fibrillation of the peptide hormone glucagon, which forms different fibrils depending on temperature, pH, ionic strength, and concentration. A combination of spectroscopic and microscopic data shows that fibrillar polymorphism can also be induced by mechanical stress. We observed two classes of fibrils: a low-stress and a high-stress class, which differ in their kinetic profiles, secondary structure as well as morphology and that are able to self-propagate in a template-dependent fashion. The bending rigidity of the low-stress fibrils is sensitive to the degree of mechanical perturbation. We propose a fibrillation model, where interfaces play a fundamental role in the switch between the two fibrillar classes. Our work also raises the cautionary note that mechanical perturbation is a potential source of variability in the study of fibrillation mechanisms and fibril structures.

Probing small molecule binding to amyloid fibrils

Much effort has focussed in recent years on probing the interactions of small molecules with amyloid fibrils and other protein aggregates. Understanding and control of such interactions are important for the development of diagnostic and therapeutic strategies in situations where protein aggregation is associated with disease. In this perspective article we give an overview over the toolbox of biophysical methods for the study of such amyloid-small molecule interactions. We discuss in detail two recently developed techniques within this framework: linear dichroism, a promising extension of the more traditional spectroscopic techniques, and biosensing methods, where surface-bound amyloid fibrils are exposed to solutions of small molecules. Both techniques rely on the measurement of physical properties that are very directly linked to the binding of small molecules to amyloid aggregates and therefore provide an attractive route to probe these important interactions.