Molecular bases of cyclodextrin adapter interactions with engineered protein nanopores

Stochastic Sensing of Chloride Anions Using an α-Hemolysin Pore with a semiaza-Bambusuril Adapter

Pores containing molecular adapters provide internal selective binding sites, thereby allowing the stochastic sensing of analytes. Herein, we demonstrate that semiaza-bambusuril (BU) acts as a non-covalent molecular adapter when lodged within the lumen of the wild-type α-hemolysin (WT-αHL) protein pore. Because the bambusurils are recognized as anion receptors, the anion binding site within the adapter-nanopore complex allows the detection of chloride anions, thus converting a non-selective pore into an anion sensor.

Progress in cyclodextrins as important molecules regulating catalytic processes of glycoside hydrolases

Cyclodextrins (CDs) are important starch derivatives and commonly comprise α-, β-, and γ-CDs. Their hydrophilic surface and hydrophobic inner cavity enable regulation of enzyme catalysis through direct or indirect interactions. Clarifying interactions between CDs and enzyme is of great value for enzyme screening, mechanism exploration, regulation of catalysis, and applications. We summarize the interactions between CDs and glycoside hydrolases (GHs) according to two aspects: 1) CD as products, substrates, inhibitors and activators of enzymes, directly affecting the reaction process; 2) CDs indirectly affecting the enzymatic reaction by solubilizing substrates, relieving substrate/product inhibition, increasing recombinant enzyme production and storage stability, isolating and purifying enzymes, and serving as ligands in crystal structure to identify functional amino acid residues. Additionally, CD enzyme mimetics are developed and used as catalysts in traditional artificial enzymes as well as nanozymes, making the application of CDs no longer limited to GHs. This review concerns the regulation of GHs catalysis by CDs, and gives insights into research on interactions between enzymes and ligands.

Sensing PEGylated Peptide Conformations Using a Protein Nanopore

Membrane pores are exploited for the stochastic sensing of various analytes, and here, we use electrical recordings to explore the interaction of PEGylated peptides of different sizes with a protein pore, CymA. This wide-diameter natural pore comprises densely filled charged residues, facilitating electrophoretic binding of polyethylene glycol (PEG) tagged with a nonaarginine peptide. The small PEG 200 peptide conjugates produced monodisperse blockages and exhibited voltage-dependent translocation across the pores. Notably, the larger PEG 1000 and 2000 peptide conjugates yielded heterogeneous blockages, indicating a multitude of PEG conformations hindering their translocation through the pore. Furthermore, a much larger PEG 5000 peptide occludes the pore entrance, resulting in complete closure. The competitive binding of different PEGylated peptides with the same pore produced specific blockage signals reflecting their identity, size, and conformation. Our proposed model of sensing distinct polypeptide conformations corresponds to disordered protein unfolding, suggesting that this pore can find applications in proteomics.

Recent advances in nanopore-based analysis for carbohydrates and glycoconjugates

Saccharides are not only the basic constituents and nutrients of living organisms, but also participate in various life activities, and play important roles in cell recognition, immune regulation, development, cancer, etc. The analysis of carbohydrates and glycoconjugates is a necessary means to study their transformations and physiological roles in living organisms. Existing detection techniques can hardly meet the requirements for the analysis of carbohydrates and glycoconjugates in complex matrices as they are expensive, involve complex derivatization, and are time-consuming. Nanopore sensing technology, which is amplification-free and label-free, and is a high-throughput process, provides a new solution for the identification and sequencing of carbohydrates and glycoconjugates. This review highlights recent advances in novel nanopore-based single-molecule sensing technologies for the detection of carbohydrates and glycoconjugates and discusses the advantages and challenges of nanopore sensing technologies. Finally, current issues and future perspectives are discussed with the aim of improving the performance of nanopores in complex media diagnostic applications, as well as providing a new direction for the quantification of glycan chains and the study of glycan chain properties and functions.

Mapping the Acetylamino and Carboxyl Groups on Glycans by Engineered α-Hemolysin Nanopores

Glycan is a crucial class of biological macromolecules with important biological functions. Functional groups determine the chemical properties of glycans, which further affect their biological activities. However, the structural complexity of glycans has set a technical hurdle for their direct identification. Nanopores have emerged as highly sensitive biosensors that are capable of detecting and characterizing various analytes. Here, we identified the functional groups on glycans with a designed α-hemolysin nanopore containing arginine mutations (M113R), which is specifically sensitive to glycans with acetamido and carboxyl groups. Molecular dynamics simulations indicated that the acetamido and carboxyl groups of the glycans produce unique electrical signatures by forming polar and electrostatic interactions with the M113R nanopores. Using these electrical features as the fingerprints, we mapped the length of the glycans containing acetamido and carboxyl groups at the monosaccharide, disaccharide, and trisaccharide levels. This proof-of-concept study provides a promising foundation for developing single-molecule glycan fingerprinting libraries and demonstrates the capability of biological nanopores in glycan sequencing.

Cyclodextrins: Only Pharmaceutical Excipients or Full-Fledged Drug Candidates?

Cyclodextrins, representing a versatile family of cyclic oligosaccharides, have extensive pharmaceutical applications due to their unique truncated cone-shaped structure with a hydrophilic outer surface and a hydrophobic cavity, which enables them to form non-covalent host-guest inclusion complexes in pharmaceutical formulations to enhance the solubility, stability and bioavailability of numerous drug molecules. As a result, cyclodextrins are mostly considered as inert carriers during their medical application, while their ability to interact not only with small molecules but also with lipids and proteins is largely neglected. By forming inclusion complexes with cholesterol, cyclodextrins deplete cholesterol from cellular membranes and thereby influence protein function indirectly through alterations in biophysical properties and lateral heterogeneity of bilayers. In this review, we summarize the general chemical principles of direct cyclodextrin-protein interactions and highlight, through relevant examples, how these interactions can modify protein functions in vivo, which, despite their huge potential, have been completely unexploited in therapy so far. Finally, we give a brief overview of disorders such as Niemann-Pick type C disease, atherosclerosis, Alzheimer's and Parkinson's disease, in which cyclodextrins already have or could have the potential to be active therapeutic agents due to their cholesterol-complexing or direct protein-targeting properties.

Selective Translocation of Cyclic Sugars through Dynamic Bacterial Transporter

The selective translocation of molecules through membrane pores is an integral process in cells. We present a bacterial sugar transporter, CymA of unusual structural conformation due to a dynamic N terminus segment in the pore, reducing its diameter. We quantified the translocation kinetics of various cyclic sugars of different charge, size, and symmetry across native and truncated CymA devoid of the N terminus using single-channel recordings. The chemically divergent cyclic hexasaccharides bind to the native and truncated pore with high affinity and translocate effectively. Specifically, these sugars bind and translocate rapidly through truncated CymA compared to native CymA. In contrast, larger cyclic heptasaccharides and octasaccharides do not translocate but bind to native and truncated CymA with distinct binding kinetics highlighting the importance of molecular charge, size and symmetry in translocation consistent with liposome assays. Based on the sugar-binding kinetics, we suggest that the N terminus most likely resides inside the native CymA barrel, regulating the transport rate of cyclic sugars. Finally, we present native CymA as a large nanopore sensor for the simultaneous single-molecule detection of various sugars at high resolution, establishing its functional versatility. This natural pore is expected to have several applications in nanobiotechnology and will help further our understanding of the fundamental mechanism of molecular transport.

Functionalised nanopores: chemical and biological modifications

Nanopore technology has established itself as a powerful tool for single-molecule studies. By analysing changes in the ion current flowing through a single transmembrane channel, a wealth of molecular information can be elucidated. Early studies utilised nanopore technology for sensing applications, and subsequent developments have diversified its remit. Nanopores can be synthetic, solid-state, or biological in origin, but recent work has seen these boundaries blurred as hybrid functionalised pores emerge. The modification of existing pores and the construction of novel synthetic pores has been an enticing goal for creating systems with tailored properties and functionality. Here, we explore chemically functionalised biological pores and the bio-inspired functionalisation of solid-state pores, highlighting how the convergence of these domains provides enhanced functionality.

The convergence of chemistry, biology, and solid-state approaches enables the construction hybrid nanopores with enhanced single-molecule applications.

Epitranscriptomics of cardiovascular diseases (Review)

RNA modifications have recently become the focus of attention due to their extensive regulatory effects in a vast array of cellular networks and signaling pathways. Just as epigenetics is responsible for the imprinting of environmental conditions on a genetic level, epitranscriptomics follows the same principle at the RNA level, but in a more dynamic and sensitive manner. Nevertheless, its impact in the field of cardiovascular disease (CVD) remains largely unexplored. CVD and its associated pathologies remain the leading cause of death in Western populations due to the limited regenerative capacity of the heart. As such, maintenance of cardiac homeostasis is paramount for its physiological function and its capacity to respond to environmental stimuli. In this context, epitranscriptomic modifications offer a novel and promising therapeutic avenue, based on the fine‑tuning of regulatory cascades, necessary for cardiac function. This review aimed to provide an overview of the most recent findings of key epitranscriptomic modifications in both coding and non‑coding RNAs. Additionally, the methods used for their detection and important associations with genetic variations in the context of CVD were summarized. Current knowledge on cardiac epitranscriptomics, albeit limited still, indicates that the impact of epitranscriptomic editing in the heart, in both physiological and pathological conditions, holds untapped potential for the development of novel targeted therapeutic approaches in a dynamic manner.

Cyclodextrins Exert a Ligand-like Current Inhibitory Effect on the KV1.3 Ion Channel Independent of Membrane Cholesterol Extraction

Cyclodextrins (CDs) are cyclic oligosaccharides capable of forming water-soluble complexes with a variety of otherwise poorly soluble molecules including cholesterol and different drugs. Consistently, CDs are widely used in research and clinical practice to deplete cholesterol from cellular membranes or to increase solubility and bioavailability of different pharmaceuticals at local concentrations in the millimolar range. Effects of CDs exerted on cellular functions are generally thought to originate from reductions in cholesterol levels. Potential direct, ligand-like CD effects are largely neglected in spite of several recent studies reporting direct interaction between CDs and proteins including AMP-activated protein kinase, β-amyloid peptides, and α-synuclein. In this study, by using patch-clamp technique, time-resolved quantitation of cholesterol levels and biophysical parameters and applying cholesterol-extracting and non-cholesterol-extracting CDs at 1 and 5 mM concentrations, we provide evidence for a previously unexplored ligand-like, cholesterol-independent current inhibitory effect of CDs on K_V1.3, a prototypical voltage-gated potassium channel with pathophysiological relevance in various autoimmune and neurodegenerative disorders. Our findings propose that potential direct CD effects on K_V channels should be taken into consideration when interpreting functional consequences of CD treatments in both research and clinical practice. Furthermore, current-blocking effects of CDs on K_V channels at therapeutically relevant concentrations might contribute to additional beneficial or adverse effects during their therapeutic applications.

Experimental and computational study on the enantioseparation of four chiral fluoroquinolones by capillary electrophoresis with sulfated-β-cyclodextrin as chiral selector

In this work, enantioseparation of four chiral fluoroquinolones (FQs), namely, ofloxacin, gemifloxacin, lomefloxacin, and gatifloxacin, was achieved by capillary electrophoresis with sulfated-β-cyclodextrin (S-β-CD) as chiral selector. Factors affecting the enantiomeric resolution, such as the concentrations of S-β-CD, BGE pH conditions, and the buffer types and concentrations, were optimized and discussed. A BGE consisting of 30 g/L S-β-CD and 30-mM phosphate at pH 4.0 was found fit for enantiomeric resolution of ofloxacin and gemifloxacin, while the same BGE at pH 3.0 was suitable for enantioseparation of lomefloxacin and gatifloxacin. The pH-dependent experiments showed that separation resolutions of four FQs enantiomers were significantly affected by BGE pH, which was thought to be related with the varying electrostatic attraction between the enantiomers and chiral selector. To verify this speculation, molecular docking studies were used for further investigation of the enantiomeric recognition mechanism of S-β-CD. Molecular model indicated that hydrophobic effect and hydrogen bond were involved in host-guest inclusion, but the electrostatic attraction enhanced the chiral discrimination by increasing the difference in binding energy between individual enantiomers and S-β-CD. This work provided a further insight into the chiral recognition mechanisms of CD derivatives.

Structural Basis of the Pore-Forming Toxin/Membrane Interaction

With the rapid growth of antibiotic-resistant bacteria, it is urgent to develop alternative therapeutic strategies. Pore-forming toxins (PFTs) belong to the largest family of virulence factors of many pathogenic bacteria and constitute the most characterized classes of pore-forming proteins (PFPs). Recent studies revealed the structural basis of several PFTs, both as soluble monomers, and transmembrane oligomers. Upon interacting with host cells, the soluble monomer of bacterial PFTs assembles into transmembrane oligomeric complexes that insert into membranes and affect target cell-membrane permeability, leading to diverse cellular responses and outcomes. Herein we have reviewed the structural basis of pore formation and interaction of PFTs with the host cell membrane, which could add valuable contributions in comprehensive understanding of PFTs and searching for novel therapeutic strategies targeting PFTs and interaction with host receptors in the fight of bacterial antibiotic-resistance.

Biological Nanopores: Engineering on Demand

Nanopore-based sensing is a powerful technique for the detection of diverse organic and inorganic molecules, long-read sequencing of nucleic acids, and single-molecule analyses of enzymatic reactions. Selected from natural sources, protein-based nanopores enable rapid, label-free detection of analytes. Furthermore, these proteins are easy to produce, form pores with defined sizes, and can be easily manipulated with standard molecular biology techniques. The range of possible analytes can be extended by using externally added adapter molecules. Here, we provide an overview of current nanopore applications with a focus on engineering strategies and solutions.

New Twists in Detecting mRNA Modification Dynamics

Modified nucleotides in mRNA are an essential addition to the standard genetic code of four nucleotides in animals, plants, and their viruses. The emerging field of epitranscriptomics examines nucleotide modifications in mRNA and their impact on gene expression. The low abundance of nucleotide modifications and technical limitations, however, have hampered systematic analysis of their occurrence and functions. Selective chemical and immunological identification of modified nucleotides has revealed global candidate topology maps for many modifications in mRNA, but further technical advances to increase confidence will be necessary. Single-molecule sequencing introduced by Oxford Nanopore now promises to overcome such limitations, and we summarize current progress with a particular focus on the bioinformatic challenges of this novel sequencing technology.

Analysis with biological nanopore: On-pore, off-pore strategies and application in biological fluids

Inspired from ion channels in biology, nanopores have been developed as promising analytical tools. In principle, nanopores provide crucial information from the observation and analysis of ionic current modulations caused by the interaction between target analytes and fluidic pores. In this respect, the biological, chemical and physical parameters of the nanopore regime need to be well-understood and regulated for intermolecular interaction. Because of well-defined molecular structures, biological nanopores consequently are of a focal point, allowing precise interaction analysis at single-molecule level. In this overview, two analytical strategies are summarized and discussed accordingly, upon the challenges arising in case-dependent analysis using biological nanopores. One kind of strategies relies on modification, functionalization and engineering on nanopore confined interface to improve molecular recognition sites (on-pore strategies); The other kind of highlighted strategies concerns to measurement of various chemistry/biochemistry based interactions triggered by employed molecular agents or probes (off-pore strategies). In particularly, a few recent paradigms using these strategies for practical application of accurate analysis of biomarkers in biological fluids are illustrated. To end, the challenging and future outlook of using analytical tools by means of biological nanopores are depicted.

Real-Time Label-Free Kinetics Monitoring of Trypsin-Catalyzed Ester Hydrolysis by a Nanopore Sensor

Trypsin is an important proteolytic enzyme in the digestive system and its activity is a major indicator for evaluating diseases such as chronic pancreatitis. Here, we present a novel label-free method to detect trypsin kinetics using a nanopore technique. A mutant α-hemolysin (M113R)₇ protein nanopore equipped with a polyamine decorated β-cyclodextrin (am₇β-CD) was employed as a sensing platform for the real-time monitoring of the process of trypsin enzymatic cleavage of a substrate Nα-benzoyl-l-arginine ethyl ester (BAEE) at the single molecule level. Significantly, this sensor can exclusively respond to the current modulation caused by the product and prevent interference from the substrate, thus improving detection sensitivity, and it provides a new scheme to detect enzyme activity for cleaving small molecules.

Ion Mobility-Mass Spectrometry Reveals That α-Hemolysin from Staphylococcus aureus Simultaneously Forms Hexameric and Heptameric Complexes in Detergent Micelle Solutions

Many soluble and membrane proteins form symmetrical homooligomeric complexes. However, determining the oligomeric state of protein complexes can be difficult. Alpha-hemolysin (αHL) from Staphylococcus aureus is a symmetrical homooligomeric protein toxin that forms transmembrane β-barrel pores in host cell membranes. The stable pore structure of αHL has also been exploited in vitro as a nanopore tool. Early structural experiments suggested αHL forms a hexameric pore, while more recent X-ray crystal structure and solution studies have identified a heptameric pore structure. Here, using native ion mobility-mass spectrometry (IM-MS) we find that αHL simultaneously forms hexameric and heptameric oligomers in both tetraethylene glycol monooctyl ether (C₈E₄) and tetradecylphosphocholine (FOS-14) detergent solutions. We also analyze intact detergent micelle-embedded αHL porelike complexes by native IM-MS without the need to fully strip the detergent micelle, which can cause significant gas-phase unfolding. The highly congested native mass spectra are deconvolved using Fourier- and Gábor-transform (FT and GT) methods to determine charge states and detergent stoichiometry distributions. The intact αHL micelle complexes are found to contain oligomeric state-proportional numbers of detergent molecules. This evidence, combined with IM data and results from vacuum molecular dynamics simulations, is consistent with both the hexamer and the heptamer forming porelike complexes. The ability of αHL to form both oligomeric states simultaneously has implications for its use as a nanopore tool and its pore formation mechanism in vivo. This study also demonstrates more generally the power of FT and GT to deconvolve the charge state and stoichiometry distributions of polydisperse ions.

Nanopore-Based Confined Spaces for Single-Molecular Analysis

The field of nanopore sensing at the single-molecular level is in a "boom" period. Such nanopores, which are either composed of biological materials or are fabricated from solid-state substrates, offer a unique confined space that is compatible with the single-molecular scale. Under the influence of an electrical field, such single-biomolecular interfaces can read single-molecular information and, if appropriately fine-tuned, each molecule plays its individual ionic rhythm to compose a "molecular symphony". Over the past few decades, many research groups have worked on nanopore-based single-molecular sensors for a range of thrilling chemical and clinical applications. Furthermore, for the past decade, we have also focused on nanopore-based sensors. In this Minireview, we summarize the recent developments in fundamental research and applications in this area, along with data algorithms and advances in hardware, which act as infrastructure for the electrochemical analysis.

Salt-Mediated Nanopore Detection of ADAM-17

ADAM-17 (a disintegrin and metalloproteinase 17) plays an important role in various physiological and pathophysiological processes. Overexpression/underexpression of ADAM-17 could lead to various diseases. In this work, by taking advantage of ionic strength and salt gradient, and monitoring the cleavage of a substrate peptide by ADAM-17 in a nanopore, we developed a label-free sensor for the rapid detection of ADAM-17. The sensor was highly sensitive and selective: picomolar concentrations of ADAM-17 could be detected within minutes, while structure similar proteases such as ADAM-9 and MMP-9 did not interfere with its detection. Our developed nanopore sensing strategy should find useful applications in the development of nanopore sensors for other proteases of biological, pharmaceutical, and medical importance.

Polynucleotide transport through lipid membrane in the presence of starburst cyclodextrin-based poly(ethylene glycol)s

Symmetrical cyclodextrin-based 14-arm star polymers with poly(ethylene glycol) PEG branches were synthesized and characterized. Interactions of the star polymers with lipid bilayers were studied by the "black lipid membrane" technique in order to demonstrate the formation of monomolecular artificial channels. The conditions for the insertion are mainly based on dimensions and amphiphilic properties of the star polymers, in particular the molar mass of the water-soluble polymer branches. Translocation of single-strand DNA (ssDNA) through those synthetic nanopores was investigated, and the close dimension between the cross-section of ssDNA and the cyclodextrin cavity led to an energy barrier that slowed down the translocation process.

Metal-organic complex-functionalized protein nanopore sensor for aromatic amino acids chiral recognition

Chiral recognition at single-molecule level for small active molecules is important, as exhibited by many nanostructures and molecular assemblies in biological systems, but it presents a significant challenge. We report a simple and rapid sensing strategy to discriminate all enantiomers of natural aromatic amino acids (AAA) using a metal-organic complex-functionalized protein nanopore, in which a chiral recognition element and a chiral recognition valve were equipped. A trifunctional molecule, heptakis-(6-deoxy-6-amino)-β-cyclodextrin (am₇βCD), was non-covalently lodged within the nanopore of an α-hemolysin (αHL) mutant, (M113R)₇-αHL. Copper(ii) ion reversibly bonds to the amino group of am₇βCD to form an am₇βCD-Cu^II complex, which allowed chiral recognition for each enantiomer in the mixture of AAA by distinct current signals. The Cu^II plugging valve plays a crucial rule that holds chiral molecules in the nanocavity for a sufficient registering time. Importantly, six enantiomers of all nature AAA could be simultaneously recognized at one time. Enantiomeric excess (ee) could also be accurately detected by this approach. It should be possible to generalize this approach for sensing of other chiral molecules.

Experimental and Computational Comparative Study of the Supercritical Fluid Technology (SFT) and Kneading Method in Preparing β-Cyclodextrin Complexes with Two Essential Oils (Linalool and Carvacrol)

Supercritical fluid technology (SFT) offers many advantages as a potential complexation method compared to the conventional kneading technique. Its applicability to processess in which solvents are not required is a significant benefit. The main aim of this study was to evaluate, experimentally and computationally, the applicability of SFT in the preparation of β-cyclodextrin complexes with two selected essential oils, namely, carvacrol and linalool. Preparation of the complexes was performed using kneading and SFT method. Several methods were used in the solid-state characterization. These include thermal analysis, powder X-ray diffraction, Fourier transform infrared spectroscopy, and solid-state nuclear magnetic resonance. Besides, molecular dynamics simulations of all studied systems were conducted in order to have a deeper and a detailed insight, at the atomic level, of the nature of the two used techniques. Despite all the advantages of SFT, better results of guest molecule entrapment inside β-cyclodextrin were obtained with the kneading method. The percentages of oil content for linalool samples were 70 ± 14 and 84 ± 9% for SFT and kneading method, respectively, while the drug content values for carvacrol samples were 67 ± 15 and 81 ± 13% for SFT and kneading method, respectively. Interestingly, simulation results were in perfect agreement with the experimental ones and, moreover, they provided a plausible explanation for the obtained results. In conclusion, our results showed that the SFT was unsuccessful in enhancing the stability of the studied complexes contrary to that of the conventational kneading method, and in both cases, molecular dynamics simulations correctly predicted the expected outcomes.

Determining the Physical Properties of Molecules with Nanometer-Scale Pores

Nanometer-scale pores have been developed for the detection, characterization, and quantification of a wide range of analytes (e.g., ions, polymers, proteins, anthrax toxins, neurotransmitters, and synthetic nanoparticles) and for DNA sequencing. We describe the key requirements that made this method possible and how the technique evolved. Finally, we show that, despite sound theoretical work, which advanced both the conceptual framework and quantitative capability of the method, there are still unresolved questions that need to be addressed to further improve the technique.

In Situ Synthetic Functionalization of a Transmembrane Protein Nanopore

Monitoring current flow through a single nanopore has proved to be a powerful technique for the in situ detection of molecular structure, binding, and reactivity. Transmembrane proteins, such as α-hemolysin, provide particularly attractive platforms for nanopore sensing applications due to their atomically precise structures. However, many nanopore applications require the introduction of functional groups to tune selectivity. To date, such modifications have required genetic modification of the protein prior to functionalization. Here we demonstrate the in situ synthetic modification of a wild-type α-hemolysin nanopore embedded in a membrane. We show that reversible dynamic covalent iminoboronate formation and the resulting changes in the ion current flowing through an individual nanopore can be used to map the reactive behavior of lysine residues within the nanopore channel. Crucially, the modification of lysine residues located outside the nanopore channel was found not to affect the stability or utility of the nanopore. Finally, knowledge of the reactivity patterns enabled the irreversible functionalization of a single, assignable lysine residue within the nanopore channel. The approach constitutes a simple, generic tool for the rapid, in situ synthetic modification of protein nanopores that circumvents the need for prior genetic modification.

Opening eyes to nanomedicine: Where we are, challenges and expectations on nanotherapy for diabetic retinopathy

People affected with ocular diseases will significantly increase over the next decades, and, consequently, a substantial increase in health costs is expected. Diabetic retinopathy is the most common chronic complication of diabetes. The treatment of eye diseases affecting the posterior segment, such as diabetic retinopathy, is quite challenging due to the anatomy, physiology and biochemistry of the eye. Therefore, the development of new therapeutics for posterior eye diseases has been a major focus of pharmaceutical research in the area of vision sciences. Several nanosystems already offer efficient solutions for ophthalmological conditions, targeting internal eye tissues, as the retina, and many novel products are expected to appear hereafter. This review provides an insight on nanoparticle-based solutions for therapies directed to posterior segment of the eye diseases, particularly diabetic retinopathy, the present scenario, and the demands and expectations for the future.

Why does β-cyclodextrin prefer to bind nucleotides with an adenine base rather than other 2'-deoxyribonucleoside 5'-monophosphates?

β-Cyclodextrin (β-CD), which resides in the α-hemolysin (αHL) protein pore, can act as a molecular adapter in single-molecule exonuclease DNA sequencing approaches, where the different nucleotide binding behavior of β-CD is crucial for base discrimination. In the present contribution, the inclusion modes of β-CD towards four 2'-deoxyribonucleoside 5'-monophosphates (dNMPs) were investigated using quantum mechanics (QM) calculations. The calculated binding energy suggests that the binding affinity of dAMP to β-CD are highest among all the dNMPs in solution, in agreement with experimental results. Geometry analysis shows that β-CD in the dAMP complex undergoes a small conformational change, and weak interaction analysis indicates that there are small steric repulsion regions in β-CD. These results suggest that β-CD has lower geometric deformation energy in complexation with dAMP. Furthermore, topological analysis and weak interaction analysis suggest that the number and strength of intermolecular hydrogen bonds and van der Waals interactions are critical to dAMP binding, and they both make favorable contributions to the lower interaction energy. This work reveals the reason why β-CD prefers to bind dAMP rather than other dNMPs, while opening exciting perspectives for the design of novel β-CD-based molecular adapters in the single-molecule exonuclease method of sequencing DNA. Graphical Abstract The binding affinity of β-cyclodextrin towards four 2'-deoxyribonucleoside 5'-monophosphates was investigated using quantum mechanics calculations.

Unpicking the determinants of amide NH⋯OC hydrogen bond strength with diphenylacetylene molecular balances

The stereoelectronic properties affecting hydrogen bond strength are investigated with a series of diphenylacetylene-based molecular balances.

A monodisperse transmembrane α-helical peptide barrel

The fabrication of monodisperse transmembrane barrels formed from short synthetic peptides has not been demonstrated previously. This is in part because of the complexity of the interactions between peptides and lipids within the hydrophobic environment of a membrane. Here we report the formation of a transmembrane pore through the self-assembly of 35 amino acid α-helical peptides. The design of the peptides is based on the C-terminal D4 domain of the Escherichia coli polysaccharide transporter Wza. By using single-channel current recording, we define discrete assembly intermediates and show that the pore is most probably a helix barrel that contains eight D4 peptides arranged in parallel. We also show that the peptide pore is functional and capable of conducting ions and binding blockers. Such α-helix barrels engineered from peptides could find applications in nanopore technologies such as single-molecule sensing and nucleic-acid sequencing.

Enhancement of levodopa stability when complexed with β-cyclodextrin in transdermal patches

Levodopa is a promising candidate for administration via the transdermal route because it exhibits a short plasma half-life and has a small window of absorption in the upper section of the small intestine. The aim of this study was to prepare stable levodopa transdermal patches. Both xanthan gum and Carbopol 971 polymers were selected with ethylcellulose constituting the backing layer of the prepared patches. The effect of adding β-cyclodextrin on the prepared patches was investigated. The uniformity in thickness, weight and content of the studied patches was acceptable. Physicochemical characterization revealed that there was no interaction between levodopa and the applied polymer. The results proved that levodopa precipitated as an amorphous form in carbopol patches. Controlled drug release was achieved for all the tested patches over a 6 h period. However, increased permeation was achieved for the carbopol patches. Although cyclodextrin did not enhance levodopa permeation, the stability study confirmed that levodopa stability was enhanced when complexed with β-cyclodextrin. The cumulative amount of drug released from carbopol patches is slightly higher than that of xanthan patches. The optimal stability was achieved in the carbopol/levodopa:β-cyclodextrin patch. The levodopa-β-cyclodextrin complex was successfully characterized using X-ray diffraction, NMR analysis and molecular dynamics simulations. In conclusion, carbopol/levodopa:β-cyclodextrin patches can be considered as a promising stable and effective transdermal drug-delivery system.

Semisynthetic Nanoreactor for Reversible Single-Molecule Covalent Chemistry

Protein engineering has been used to remodel pores for applications in biotechnology. For example, the heptameric α-hemolysin pore (αHL) has been engineered to form a nanoreactor to study covalent chemistry at the single-molecule level. Previous work has been confined largely to the chemistry of cysteine side chains or, in one instance, to an irreversible reaction of an unnatural amino acid side chain bearing a terminal alkyne. Here, we present four different αHL pores obtained by coupling either two or three fragments by native chemical ligation (NCL). The synthetic αHL monomers were folded and incorporated into heptameric pores. The functionality of the pores was validated by hemolysis assays and by single-channel current recording. By using NCL to introduce a ketone amino acid, the nanoreactor approach was extended to an investigation of reversible covalent chemistry on an unnatural side chain at the single-molecule level.

Engineered transmembrane pores

Today, hundreds of researchers are working on nanopores, making an impact in both basic science and biotechnology. Proteins remain the most versatile sources of nanopores, based on our ability to engineer them with sub-nanometer precision. Recent work aimed at the construction and discovery of novel pores has included unnatural amino acid mutagenesis and the application of selection techniques. The diversity of structures has now been increased through the development of helix-based pores as well as the better-known β barrels. New developments also include truncated pores, which pierce bilayers through lipid rearrangement, and hybrid pores, which do away with bilayers altogether. Pore dimers, which span two lipid bilayers, have been constructed and pores based on DNA nanostructures are gaining in importance. While nanopore DNA sequencing has received enthusiastic attention, protein pores have a wider range of potential applications, requiring specifications that will require engineering efforts to continue for years to come.

Semisynthetic protein nanoreactor for single-molecule chemistry

The covalent chemistry of individual reactants bound within a protein pore can be monitored by observing the ionic current flow through the pore, which acts as a nanoreactor responding to bond-making and bond-breaking events. In the present work, we incorporated an unnatural amino acid into the α-hemolysin (αHL) pore by using solid-phase peptide synthesis to make the central segment of the polypeptide chain, which forms the transmembrane β-barrel of the assembled heptamer. The full-length αHL monomer was obtained by native chemical ligation of the central synthetic peptide to flanking recombinant polypeptides. αHL pores with one semisynthetic subunit were then used as nanoreactors for single-molecule chemistry. By introducing an amino acid with a terminal alkyne group, we were able to visualize click chemistry at the single-molecule level, which revealed a long-lived (4.5-s) reaction intermediate. Additional side chains might be introduced in a similar fashion, thereby greatly expanding the range of single-molecule covalent chemistry that can be investigated by the nanoreactor approach.

Nucleobase Recognition by Truncated α-Hemolysin Pores

The α-hemolysin (αHL) protein nanopore has been investigated previously as a base detector for the strand sequencing of DNA and RNA. Recent findings have suggested that shorter pores might provide improved base discrimination. New work has also shown that truncated-barrel mutants (TBM) of αHL form functional pores in lipid bilayers. Therefore, we tested TBM pores for the ability to recognize bases in DNA strands immobilized within them. In the case of TBMΔ6, in which the barrel is shortened by ∼16 Å, one of the three recognition sites found in the wild-type pore, R1, was almost eliminated. With further mutagenesis (Met113 → Gly), R1 was completely removed, demonstrating that TBM pores can mediate sharpened recognition. Remarkably, a second mutant of TBMΔ6 (Met113 → Phe) was able to bind the positively charged β-cyclodextrin, am7βCD, unusually tightly, permitting the continuous recognition of individual nucleoside monophosphates, which would be required for exonuclease sequencing mediated by nanopore base identification.

Amino acid discrimination in a nanopore and the feasibility of sequencing peptides with a tandem cell and exopeptidase

Peptide sequencing in an electrolytic cell with two nanopores in tandem and exopeptidase.

Cyclodextrin complexes of reduced bromonoscapine in guar gum microspheres enhance colonic drug delivery

Here, we report improved solubility and enhanced colonic delivery of reduced bromonoscapine (Red-Br-Nos), a cyclic ether brominated analogue of noscapine, upon encapsulation of its cyclodextrin (CD) complexes in bioresponsive guar gum microspheres (GGM). Phase–solubility analysis suggested that Red-Br-Nos complexed with β-CD and methyl-β-CD in a 1:1 stoichiometry, with a stability constant (K_c) of 2.29 × 10³ M^–1 and 4.27 × 10³ M^–1. Fourier transforms infrared spectroscopy indicated entrance of an O–CH₂ or OCH₃–C₆H₄–OCH₃ moiety of Red-Br-Nos in the β-CD or methyl-β-CD cavity. Furthermore, the cage complex of Red-Br-Nos with β-CD and methyl-β-CD was validated by several spectral techniques. Rotating frame Overhauser enhancement spectroscopy revealed that the H_a proton of the OCH₃–C₆H₄–OCH₃ moiety was closer to the H₅ proton of β-CD and the H₃ proton of the methyl-β-CD cavity. The solubility of Red-Br-Nos in phosphate buffer saline (PBS, pH ∼ 7.4) was improved by ∼10.7-fold and ∼21.2-fold when mixed with β-CD and methyl-β-CD, respectively. This increase in solubility led to a favorable decline in the IC₅₀ by ∼2-fold and ∼3-fold for Red-Br-Nos−β-CD-GGM and Red-Br-Nos–methyl-β-CD-GGM formulations respectively, compared to free Red-Br-Nos−β-CD and Red-Br-Nos–methyl-β-CD in human colon HT-29 cells. GGM-bearing drug complex formulations were found to be highly cytotoxic to the HT-29 cell line and further effective with simultaneous continuous release of Red-Br-Nos from microspheres. This is the first study to showing the preparation of drug-complex loaded GGMS for colon delivery of Red-Br-Nos that warrants preclinical assessment for the effective management of colon cancer.

Single-molecule investigation of G-quadruplex folds of the human telomere sequence in a protein nanocavity

Human telomeric DNA consists of tandem repeats of the sequence 5'-TTAGGG-3' that can fold into various G-quadruplexes, including the hybrid, basket, and propeller folds. In this report, we demonstrate use of the α-hemolysin ion channel to analyze these subtle topological changes at a nanometer scale by providing structure-dependent electrical signatures through DNA-protein interactions. Whereas the dimensions of hybrid and basket folds allowed them to enter the protein vestibule, the propeller fold exceeds the size of the latch region, producing only brief collisions. After attaching a 25-mer poly-2'-deoxyadenosine extension to these structures, unraveling kinetics also were evaluated. Both the locations where the unfolding processes occur and the molecular shapes of the G-quadruplexes play important roles in determining their unfolding profiles. These results provide insights into the application of α-hemolysin as a molecular sieve to differentiate nanostructures as well as the potential technical hurdles DNA secondary structures may present to nanopore technology.

Channel-forming bacterial toxins in biosensing and macromolecule delivery

To intoxicate cells, pore-forming bacterial toxins are evolved to allow for the transmembrane traffic of different substrates, ranging from small inorganic ions to cell-specific polypeptides. Recent developments in single-channel electrical recordings, X-ray crystallography, protein engineering, and computational methods have generated a large body of knowledge about the basic principles of channel-mediated molecular transport. These discoveries provide a robust framework for expansion of the described principles and methods toward use of biological nanopores in the growing field of nanobiotechnology. This article, written for a special volume on "Intracellular Traffic and Transport of Bacterial Protein Toxins", reviews the current state of applications of pore-forming bacterial toxins in small- and macromolecule-sensing, targeted cancer therapy, and drug delivery. We discuss the electrophysiological studies that explore molecular details of channel-facilitated protein and polymer transport across cellular membranes using both natural and foreign substrates. The review focuses on the structurally and functionally different bacterial toxins: gramicidin A of Bacillus brevis, α-hemolysin of Staphylococcus aureus, and binary toxin of Bacillus anthracis, which have found their "second life" in a variety of developing medical and technological applications.

Structure-based engineering of a minimal porin reveals loop-independent channel closure

Porins, like outer membrane protein G (OmpG) of Escherichia coli, are ideal templates among ion channels for protein and chemical engineering because of their robustness and simple architecture. OmpG shows fast transitions between open and closed states, which were attributed to loop 6 (L6). As flickering limits single-channel-based applications, we pruned L6 by either 8 or 12 amino acids. While the open probabilities of both L6 variants resemble that of native OmpG, their gating frequencies were reduced by 63 and 81%, respectively. Using the 3.2 Å structure of the shorter L6 variant in the open state, we engineered a minimal porin (220 amino acids), where all remaining extramembranous loops were truncated. Unexpectedly, this minimized porin still exhibited gating, but it was 5-fold less frequent than in OmpG. The residual gating of the minimal pore is hence independent of L6 rearrangements and involves narrowing of the ion conductance pathway most probably driven by global stretching-flexing deformations of the membrane-embedded β-barrel.

Cyclodextrin ion channels

Seventeen derivatives of α- and β-cyclodextrins were prepared from the cyclodextrin per-6-azide by "click" cyclization with terminal alkynes. Sixteen of these "half-channel" compounds showed significant activity as ion channels in planar bilayer members as assessed by the voltage-clamp technique. Activity ranged from persistent square-top openings to highly erratic conductance; mixed behaviours were evident in virtually all data recorded. Some of the erratic behaviours were shown to follow an apparent power-law distribution of open duration times. The activities observed for the suite were summarized using a model-free activity grid method which displays conductance, duration, and opening behaviour. The overall activity shows the clustering of conductance-duration indicating that activity arises from system properties rather that solely as a property of the compound. The activity grids also support an analysis of structure-activity relationships as they apply to the global behaviour of the compounds and reveal the complexity of a single structure change in controlling the distribution of concurrent conductance behaviours. Transient blockage of channel activity by the hydrophobic guest of the cyclodextrin (1-adamantyl carboxylate) is consistent with the formation of an end-to-end dimer channel among several other competing and interconverting structures.

Functional truncated membrane pores

Membrane proteins are generally divided into two classes. Integral proteins span the lipid bilayer, and peripheral proteins are located at the membrane surface. Here, we provide evidence for membrane proteins of a third class that stabilize lipid pores, most probably as toroidal structures. We examined mutants of the staphylococcal α-hemolysin pore so severely truncated that the protein cannot span a bilayer. Nonetheless, the doughnut-like structures elicited well-defined transmembrane ionic currents by inducing pore formation in the underlying lipids. The formation of lipid pores, produced here by a structurally defined protein, is supported by the lipid and voltage dependences of pore formation, and by molecular dynamics simulations. We discuss the role of stabilized lipid pores in amyloid disease, the action of antimicrobial peptides, and the assembly of the membrane-attack complexes of the immune system.

Nanopore-based identification of individual nucleotides for direct RNA sequencing

We describe a label-free ribobase identification method, which uses ionic current measurement to resolve ribonucleoside monophosphates or diphosphates in α-hemolysin protein nanopores containing amino-cyclodextrin adapters. The accuracy of base identification is further investigated through the use of a guanidino-modified adapter. On the basis of these findings, an exosequencing approach is envisioned in which a processive exoribonuclease (polynucleotide phosphorylase) presents sequentially cleaved ribonucleoside diphosphates to a nanopore.