Tuning the Internal Compartmentation of Single-Chain Nanoparticles as Fluorescent Contrast Agents.

Controlling the internal structures of single-chain nanoparticles (SCNPs) is an important factor for their targeted chemical design and synthesis, especially in view of nanosized compartments presenting different local environments as a main feature to control functionality. We here design SCNPs bearing near-infrared fluorescent dyes embedded in hydrophobic compartments for use as contrast agents in pump-probe photoacoustic (PA) imaging, displaying improved properties by the location of the dye in the hydrophobic particle core. Compartment formation is controlled via single-chain collapse and subsequent crosslinking of an amphiphilic polymer using external crosslinkers in reaction media of adjustable polarity. Different SCNPs with hydrodynamic diameters of 6-12 nm bearing adjustable label densities are synthesized. It is found that the specific conditions for single-chain collapse have a major impact on the formation of the desired core-shell structure, in turn adjusting the internal nanocompartments together with the formation of excitonic dye couples, which in turn increase their fluorescence lifetime and PA signal generation. SCNPs with the dye molecules accumulate at the core also show a nonlinear PA response as a function of pulse energy-a property that can be exploited as a contrast mechanism in molecular PA tomography.

Nanoscopic Dynamics Dictate the Phase Separation Behavior of Intrinsically Disordered Proteins.

Many intrinsically disordered proteins (IDPs) in nature may undergo liquid-liquid phase separation to assemble membraneless organelles with varied liquid-like properties and stability/dynamics. While solubility changes underlie these properties, little is known about hydration dynamics in phase-separating IDPs. Here, by studying IDP polymers of similar composition but distinct liquid-like dynamics and stability upon separation, namely, thermal hysteresis, we probe at a nanoscopic level hydration/dehydration dynamics in IDPs as they reversibly switch between phase separation states. Using continuous-wave electron paramagnetic resonance (CW EPR) spectroscopy, we observe distinct backbone and amino acid side-chain hydration dynamics in these IDPs. This nanoscopic view reveals that side-chain rehydration creates a dynamic water shield around the main-chain backbone that effectively and counterintuitively prevents water penetration and governs IDP solubility. We find that the strength of this superficial water shell is a sequence feature of IDPs that encodes for the stability of their phase-separated assemblies. Our findings expose and offer an initial understanding of how the complexity of nanoscopic water-IDP interactions dictate their rich phase separation behavior.

Nanoscale Model System for the Human Myelin Sheath.

Neurodegenerative disorders are among the most common diseases in modern society. However, the molecular bases of diseases such as multiple sclerosis or Charcot-Marie-Tooth disease remain far from being fully understood. Research in this field is limited by the complex nature of native myelin and by difficulties in obtaining good in vitro model systems of myelin. Here, we introduce an easy-to-use model system of the myelin sheath that can be used to study myelin proteins in a native-like yet well-controlled environment. To this end, we present myelin-mimicking nanodiscs prepared through one of the amphiphilic copolymers styrene/maleic acid (SMA), diisobutylene/maleic acid (DIBMA), and styrene/maleimide sulfobetaine (SMA-SB). These nanodiscs were tested for their lipid composition using chromatographic (HPLC) and mass spectrometric (MS) methods and, utilizing spin probes within the nanodisc, their comparability with liposomes was studied. In addition, their binding behavior with bovine myelin basic protein (MBP) was scrutinized to ensure that the nanodiscs represent a suitable model system of myelin. Our results suggest that both SMA and SMA-SB are able to solubilize the myelin-like (cytoplasmic) liposomes without preferences for specific lipid headgroups or fatty acyl chains. In nanodiscs of both SMA and SMA-SB (called SMA(-SB)-lipid particles, short SMALPs or SMA-SBLPs, respectively), the polymers restrict the lipids' motion in the hydrophobic center of the bilayer. The headgroups of the lipids, however, are sterically less hindered in nanodiscs when compared with liposomes. Myelin-like SMALPs are able to bind bovine MBP, which can stack the lipid bilayers like in native myelin, showing the usability of these simple, well-controlled systems in further studies of protein-lipid interactions of native myelin.

The Effect of Ethanol on Gelation, Nanoscopic, and Macroscopic Properties of Serum Albumin Hydrogels.

Serum albumin has shown great potential in the development of new biomaterials for drug delivery systems. Different methods have been proposed to synthesis hydrogels out of serum albumin. It has been observed that ethanol can also act as a trigger for serum albumin denaturation and subsequent gelation. In this study, we focus on basic mechanisms of the albumin gelation process at 37 °C when using the chemical denaturant ethanol. The temperature of 37 °C was chosen to resemble human body temperature, and as under physiological conditions, albumin is in a non-denatured N conformation. As established in our previous publication for the triggers of pH and temperature (and time), we here explore the conformational and physical properties space of albumin hydrogels when they are ethanol-induced and show that the use of ethanol can be advisable for certain gel properties on the nanoscopic and macroscopic scale. To this end, we combine spectroscopic and mechanically (rheology) based data for characterizing the gels. We also study the gels' binding capacities for fatty acids with electron paramagnetic resonance (EPR) spectroscopy, which implies observing the effects of bound stearic acids on gelation. Ethanol reduces the fraction of the strongly bound FAs in bovine serum albumin (BSA) hydrogels up to 52% and induces BSA hydrogels with a maximum storage modulus of 5000 Pa. The loosely bound FAs in ethanol-based hydrogels, besides their relatively weak mechanical properties, introduce interesting new materials for fast drug delivery systems and beyond.

Ligand-Binding Cooperativity Effects in Polymer-Protein Conjugation.

We present an electron paramagnetic resonance (EPR) spectroscopic characterization of structural and dynamic effects that stem from post-translational modifications of bovine serum albumin (BSA), an established model system for polymer-protein conjugation. Beyond the typical drug delivery and biocompatibility aspect of such systems, we illustrate the causes that alter internal dynamics and therefore functionality in terms of ligand-binding to the BSA protein core. Uptake of the paramagnetic fatty acid derivative 16-doxyl stearic acid by several BSA-based squaric acid macroinitiators and polymer-protein conjugates was studied by EPR spectroscopy, aided by dynamic light scattering (DLS) and zeta potential measurements. The conjugates were grafted from oligo(ethylene glycol) methyl ether methacrylate (OEGMA), forming an overall core-shell-like structure. It is found that ligand-binding and associated parameters such as binding affinity, cooperativity, and the number of binding sites of BSA change drastically with the extent of surface modification. In the course of processing BSA, the ligands also change their preference for individual binding sites, as observed from a comparative view of their spatial alignments in double electron electron resonance (DEER) experiments. The protein-attached polymers constitute a diffusion barrier that significantly hamper ligand uptake. Moreover, zeta potentials (ζ) decrease linearly with the degree of surface modification in protein macroinitiators and an effective dielectric constant can be estimated for the polymer layer in the conjugates. All this suggests that ligand uptake characteristics in BSA can be fine-tuned by the extent and nature of such post-translational modifications (PTMs). We show that EPR spectroscopy is suitable for quantifying these subtle PTM-based functional effects from self-assembly of substrate and ligand.

Modulation of amyloid ß peptide aggregation by hydrophilic polymers.

A substantial number of diseases leading to loss of neurologic functions such as Morbus Alzheimer, Morbus Parkinson, or Chorea Huntington are related to the fibrillation of particular amyloidogenic peptides. In vitro amyloid fibrillation strongly depends on admixture with other proteins and peptides, lipids, nanoparticles, surfactants and polymers. We investigated amyloid-beta 1-40 peptide (Aß1-40) fibrillation in mixture with thermoresponsive poly(oligo(ethylene glycol)macrylates), in which the polymer's hydrophobicity is tuned by variation of the number of ethylene glycol-units in the side chain (m = 1-9), the end groups (B = butoxy; C = carboxy; D = dodecyl; P = pyridyldisulfide) and the degree of polymerization (n) of the polymers. The polymers were prepared via RAFT-polymerization, obtaining a broad range of molecular masses (Mn = 700 to 14 600 g mol-1 kDa-1, polydispersity indices PDI = 1.10 to 1.25) and tunable cloud point temperatures (Tcp), ranging from 42.4 °C to 80 °C, respectively. Proper combination of hydrophobic end groups with hydrophilic side chains of the polymer allowed to alter the hydrophilicity/hydrophobicity of these polymers, which is shown to enhance Aß1-40 aggregation significantly in case of the endgroup D (with n = 16, 23, 56). We observed that the less hydrophilic polymers (m = 1-2) were able to both decrease and elongate the lag (tlag) and characteristic times (tchar) of Aß1-40 fibril formation in dependence of their end groups, molecular mass and hydrophilicity. On the other hand, highly hydrophilic polymers (m = 3, 5, 9) either decreased, or only marginally influenced the lag and characteristic times of Aß1-40 fibrillation, in all cases forming ß-sheet rich fibrils as observed by TEM and CD-spectroscopy. Our results support that balanced hydrophobic and hydrophilic interactions of a polymer with Aß1-40 is important for inhibiting amyloid-formation pathways.

Substitutions mimicking deimination and phosphorylation of 18.5-kDa myelin basic protein exert local structural effects that subtly influence its global folding.

Intrinsically-disordered proteins (IDPs) present a complex interplay of conformational variability and multifunctionality, modulated by environment and post-translational modifications. The 18.5-kDa myelin basic protein (MBP) is essential to the formation of the myelin sheath of the central nervous system and is exemplary in this regard. We have recently demonstrated that the unmodified MBP-C1 component undergoes co-operative global conformational changes in increasing concentrations of trifluoroethanol, emulating the decreasing dielectric environment that the protein encounters upon adsorption to the oligodendrocyte membrane [K.A. Vassall et al., Journal of Molecular Biology, 427, 1977-1992, 2015]. Here, we extended this study to the pseudo-deiminated MBP-C8 charge component, one found in greater proportion in developing myelin and in multiple sclerosis. A similar tri-conformational distribution as for MBP-C1 was observed with slight differences in Gibbs free energy. A more dramatic difference was observed by cathepsin D digestion of the protein in both aqueous and membrane environments, which showed significantly greater accessibility of the F42-F43 cut site of MBP-C8, indicative of a global conformational change. In contrast, this modification caused little change in the protein's density of packing on myelin-mimetic membranes as ascertained by double electron-electron resonance spectroscopy [D.R. Kattnig et al., Biochimica et Biophysica Acta (Biomembranes), 1818, 2636-2647, 2012], or in its affinity for Ca(2+)-CaM. Site-specific threonyl pseudo-phosphorylation at residues T92 and/or T95 did not appreciably affect any of the thermodynamic mechanisms of conformational transitions, susceptibility to cathepsin D, or affinity for Ca(2+)-CaM, despite previously having been shown to affect local structure and disposition on the membrane surface.

Load-collapse-release cascades of amphiphilic guest molecules in charged dendronized polymers through spatial separation of noncovalent forces.

The ability to pack guest molecules into charged dendronized polymers (denpols) and the possibility to release these guest molecules from subsequently densely aggregated denpols in a load-collapse-release cascade is described. Charged denpols, which constitute molecular objects with a persistent, well-defined envelope and interior, are capable of incorporating large amounts of amphiphilic guest molecules. Simultaneously, multivalent ions can coordinate to the surfaces of charged denpols, leading to counterion-induced aggregation of the already guest-loaded host structures. Thus, although the local guest concentration in denpol-based molecular transport might already be initially high due to the dense guest packing inside the dendritic denpol scaffolding, the "local" guest concentration can nonetheless be further increased by packing (through aggregation) of the host-guest complexes themselves. Subsequent release of guest compounds from densely aggregated dendronized polymers is then possible (e.g., through increasing the solution concentration of imidazolium-based ions). Augmented with this release possibility, the concept of twofold packing of guests, firstly through hosting itself and secondly through aggregation of the hosts, gives rise to a load-collapse-release cascade that strikingly displays the high potential of dendronized macromolecules for future molecular transport applications.

Nanoscale inhomogeneities in thermoresponsive polymers.

This article highlights the occurrence and nature of nanoscale inhomogeneities in thermoresponsive polymers and focuses on different experimental techniques for their observation and characterization. Such inhomogeneities can be regarded as nanoscopic domains of collapsed polymer segments (or of a small number of unimers), which provide a nonpolar, hydrophobic interior. Continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy on amphiphilic reporter molecules (spin probes) as an intrinsically local technique is particularly emphasized. In combination with different ensemble-averaging methods, it provides a holistic understanding of the often inhomogeneous nanoscale processes during the temperature-induced collapse of a thermoresponsive polymer.

Shaping membrane interfaces in lipid vesicles mimicking the cytoplasmic leaflet of myelin through variation of cholesterol and myelin basic protein contents.

Myelin basic protein (MBP) is an intrinsically disordered protein and in the central nervous system (CNS) mainly responsible for connecting the cytoplasmic surfaces of the multilamellar, compact myelin. Increased posttranslational modification of MBP is linked to both, the natural development (from adolescent to adult brains) of myelin, and features of multiple sclerosis. Here, we study how a combination of this intrinsically disordered myelin protein with varying the natural cholesterol content may alter the characteristics of myelin-like membranes and interactions between these membranes. Large unilamellar vesicles (LUVs) with a composition mimicking the cytoplasmic leaflet of myelin were chosen as the model system, in which different parameters contributing to the interactions between the lipid membrane and MBP were investigated. While we use cryo-transmission electron microscopy (TEM) for imaging, dynamic light scattering (DLS) and electrophoretic measurements through continuously-monitored phase-analysis light scattering (cmPALS) were used for a more global overview of particle size and charge, and electron paramagnetic resonance (EPR) spectroscopy was utilized for local behavior of lipids in the vesicles' membranes in aqueous solution. The cholesterol content was varied from 060 % in these LUVs and measurements were performed in the presence and absence of MBP. We find that the composition of the lipid layers is relevant to the interaction with MBP. Not only the size, the shape and the aggregation behavior of the vesicles depend on the cholesterol content, but also within each membrane, cholesterol's freedom of movement, its environmental polarity and its distribution were found to depend on the content using the EPR-active spin-labeled cholesterol (CSOSL). In addition, DLS and EPR measurements probing the transition temperatures of the lipid phases allow a correlation of specific behavior with the human body temperature of 37 °C. Overall, our results aid in understanding the importance of the native cholesterol content in the healthy myelin membrane, which serves as the basis for stable and optimum protein-bilayer interactions. Although studied in this specific myelin-like system, from a more general and materials science-oriented point of view, we could establish how membrane and vesicle properties depend on cholesterol and/or MBP content, which might be useful generally when specific membrane and vesicle characteristics are sought for.

EPR spectroscopy in polymer science.

Synthetic polymers belong to the vast realm of soft matter and are one of the key types of materials to address societal needs at the beginning of the twenty-first century. Polymer science progressively addresses questions that deal with tuning mesoscopic and macroscopic structures and functions of polymers by understanding the effects that govern these systems on the nanoscopic level. EPR spectroscopy as a local, sensitive, and extremely specific magnetic resonance technique in many cases shows sensitivity on well-suited length- (0-10 nm) and time scales (µs-ps) and can deliver unique information on structure, dynamics, and in particular function of polymeric systems. A short review of recent literature is given and the power of simple EPR methods, especially CW EPR performed on a low-cost benchtop spectrometer, to elucidate complex polymeric materials is shown with specific examples from thermoresponsive polymer systems. These bear great potential in molecular transport and biomedical applications (e.g., drug delivery) and insights into interactions between carrier and small molecule are fundamental for designing and tuning these materials.

Aggregation behavior of amphiphilic p(HPMA)-co-p(LMA) copolymers studied by FCS and EPR spectroscopy.

A combined study of fluorescence correlation spectroscopy and electron paramagnetic resonance spectroscopy gave a unique picture of p(HPMA)-co-p(LMA) copolymers in aqueous solutions, ranging from the size of micelles and aggregates to the composition of the interior of these self-assembled systems. P(HPMA)-co-p(LMA) copolymers have shown high potential as brain drug delivery systems, and a detailed study of their physicochemical properties can help to elucidate their mechanism of action. Applying two complementary techniques, we found that the self-assembly behavior as well as the strength of hydrophobic attraction of the amphiphilic copolymers can be tuned by the hydrophobic LMA content or the presence of hydrophobic molecules or domains. Studies on the dependence of the hydrophobic lauryl side chain content on the aggregation behavior revealed that above 5 mol % laury side-chain copolymers self-assemble into intrachain micelles and larger aggregates. Above this critical alkyl chain content, p(HPMA)-co-p(LMA) copolymers can solubilize the model drug domperidone and exhibit the tendency to interact with model cell membranes.

Nanoscale inhomogeneities in thermoresponsive triblock copolymers.

We present continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy data of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in aqueous solutions of poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronic or Poloxamer). TEMPO is notably smaller than the spin probes conventionally used in the context of polymer science and reveals the early emergence of small hydrophobic cavities when PPO strands of several molecules aggregate and collapse upon temperature increase. The occurrence of hydrophobic cavities appears independent of the overall molecular weight of the Pluronics, but clearly depends on the relative PPO/PEO contents. In all the cases studied, the volume fraction of hydrophobic cavities increases in a broad temperature range of ≥40 °C. The appearance of the hydrophobic regions does not seem to be directly correlated to micellization of the polymers. A decrease of the relative PPO amount in the polymers not only hinders collapse of the PPO strands, it can also be described as a site exchange of the spin probes between hydrophobic cavities and the surrounding medium. On the other hand, in cases of high PPO contents, spin probe exchange could not be observed. This suggests that one may potentially control the diffusion of small molecules between the micellar cores and the surrounding medium by adjusting the PEO/PPO ratio of the used Pluronics.

Influence of different polymer belts on lipid properties in nanodiscs characterized by CW EPR spectroscopy.

With this study we aim at comparing the well-known lipid membrane model system of liposomes and polymer-encapsulated nanodiscs regarding their lipid properties. Using differential scanning calorimetry (DSC) and continuous-wave electron paramagnetic resonance (CW EPR) spectroscopy, we characterize the temperature-dependent lipid behavior within 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes and nanodiscs made from such liposomes by application of various polymers based on styrene-co-maleic acid (SMA), diisobutylene-alt-maleic acid (DIBMA), and styrene-co-maleic amide sulfobetaine (SMA-SB), a new SMA-derived copolymer containing sulfobetaine side chains. By incorporating a spin label doxyl moiety into the lipid bilayer in position 16 or 5 we were able to study the micropolarity as well as rotational restrictions onto the lipids in the apolar bilayer center and the chain region adjacent to the carbonyl groups, respectively. Our results suggest that all polymers broaden the main melting transition of DMPC, change the water accessibility within the lipid bilayer, and exhibit additional constraints onto the lipids. Independent of the used polymer, the rotational mobility of both spin-labeled lipids decreased with DIBMA exerting less restraints probably due to its aliphatic side chains. Our findings imply that the choice of the solubilizing polymer has to be considered an important step to form lipid nanodiscs which should be included into research of lipid membranes and membrane proteins in the future.

Myelin basic protein (MBP) charge variants show different sphingomyelin-mediated interactions with myelin-like lipid monolayers.

Multiple sclerosis (MS) is correlated with increased deimination of myelin basic protein (MBP) in the central nervous system. Here, the interaction of MBP C1 (charge: +19) and MBP C8 (charge: +13) with the major lipids of the cytoplasmic side of the oligodendrocyte membrane is analysed using monolayer adsorption experiments and epifluorescence microscopy. Our findings show that the electrostatic attraction between the positively charged proteins and negatively charged lipids in the myelin-like monolayers competes with the incorporation of MBP into regions directly bordering cholesterol-rich domains. The latter is favoured to avoid additional lipid condensation and reduction in fluidity of the phospholipid layer. We find that MBP C1 does not incorporate at the cholesterol-rich domains if sphingomyelin (SM) is absent from the lipid composition. In contrast, MBP C8 is still incorporated near cholesterol-enriched regions without SM. Thus, the highly charged C1 variant needs a specific interaction with SM, whereas for C8 the incorporation at the cholesterol-rich regions is ensured due to its reduced net positive charge. This phenomenon may be relevant for the correlation of higher amounts of MBP C8 in brains of adult MS patients and healthy children, in which the amount of SM is reduced compared to healthy adults.

EPR spectroscopy reveals nanoinhomogeneities in the structure and reactivity of thermoresponsive hydrogels.

The dynamic and chemical behavior of solute molecules inside new thermoresponsive hydrogels (photocrosslinked poly(N-isopropylacrylamide) (pNiPAAm) copolymers) is studied by continuous-wave electron paramagnetic resonance spectroscopy. Via addition of paramagnetic tracer molecules (so-called spin probes) a picture is obtained of the thermally induced collapse on the molecular scale, which proceeds over a substantially broader temperature range than indicated by the sharp macroscopic volume transition. The sampling of hydrophilic and hydrophobic environments suggests a discontinuous collapse mechanism with a coexistence of collapsed and expanded network regions. These structural inhomogeneities on the nanoscale also lead to an inhomogeneity in chemical reactivity. The hydrophilic regions form nanoreactors, which strongly accelerate the reaction while the hydrophobic regions act as nanoshelters, in which enclosed spin probes are protected from the decay. The results show that the system consisting of a statistical binary or tertiary copolymer displays remarkably complex behavior that mimics spatial and chemical inhomogeneities observed in functional biopolymers such as enzymes.

A polyphenylene dendrimer drug transporter with precisely positioned amphiphilic surface patches.

The design and synthesis of a polyphenylene dendrimer (PPD 3) with discrete binding sites for lipophilic guest molecules and characteristic surface patterns is presented. Its semi-rigidity in combination with a precise positioning of hydrophilic and hydrophobic groups at the periphery yields a refined architecture with lipophilic binding pockets that accommodate defined numbers of biologically relevant guest molecules such as fatty acids or the drug doxorubicin. The size, architecture, and surface textures allow to even penetrate brain endothelial cells that are a major component of the extremely tight blood-brain barrier. In addition, low to no toxicity is observed in in vivo studies using zebrafish embryos. The unique PPD scaffold allows the precise placement of functional groups in a given environment and offers a universal platform for designing drug transporters that closely mimic many features of proteins.

Structure and dynamics of self-assembled poly(ethylene glycol) based coiled-coil nano-objects.

Herein we describe the structure and dynamics of self-assembled nano-objects generated from poly(ethylene glycol) based (PEG-ylated) coiled-coil hybrid block copolymers. Electron paramagnetic resonance (EPR) experiments on spin-labeled samples provided a strong indication for a parallel alignment of the peptide helices in at least the dimeric coiled-coil nano-object and indicated that the PEG chains are folded rather closely around the peptide core of the nano-objects. The EPR results were supported by AFM studies, which revealed the presence of discrete nanosized objects in thin, spin cast films of the block copolymers on mica substrates. Since their size and structure may be engineered via directed mutations in the amino acid sequence, these nano-objects may be interesting building blocks for the development of supramolecular materials with various potential applications.