Solid-state NMR paramagnetic relaxation enhancement immersion depth studies in phospholipid bilayers

NMR as a "Gold Standard" Method in Drug Design and Discovery

Studying disease models at the molecular level is vital for drug development in order to improve treatment and prevent a wide range of human pathologies. Microbial infections are still a major challenge because pathogens rapidly and continually evolve developing drug resistance. Cancer cells also change genetically, and current therapeutic techniques may be (or may become) ineffective in many cases. The pathology of many neurological diseases remains an enigma, and the exact etiology and underlying mechanisms are still largely unknown. Viral infections spread and develop much more quickly than does the corresponding research needed to prevent and combat these infections; the present and most relevant outbreak of SARS-CoV-2, which originated in Wuhan, China, illustrates the critical and immediate need to improve drug design and development techniques. Modern day drug discovery is a time-consuming, expensive process. Each new drug takes in excess of 10 years to develop and costs on average more than a billion US dollars. This demonstrates the need of a complete redesign or novel strategies. Nuclear Magnetic Resonance (NMR) has played a critical role in drug discovery ever since its introduction several decades ago. In just three decades, NMR has become a "gold standard" platform technology in medical and pharmacology studies. In this review, we present the major applications of NMR spectroscopy in medical drug discovery and development. The basic concepts, theories, and applications of the most commonly used NMR techniques are presented. We also summarize the advantages and limitations of the primary NMR methods in drug development.

Using NMR spectroscopy to investigate the role played by copper in prion diseases

Prion diseases are a group of rare neurodegenerative disorders that develop as a result of the conformational conversion of normal prion protein (PrPC) to the disease-associated isoform (PrPSc). The mechanism that actually causes disease remains unclear. However, the mechanism underlying the conformational transformation of prion protein is partially understood-in particular, there is strong evidence that copper ions play a significant functional role in prion proteins and in their conformational conversion. Various models of the interaction of copper ions with prion proteins have been proposed for the Cu (II)-binding, cell-surface glycoprotein known as prion protein (PrP). Changes in the concentration of copper ions in the brain have been associated with prion diseases and there is strong evidence that copper plays a significant functional role in the conformational conversion of PrP. Nevertheless, because copper ions have been shown to have both a positive and negative effect on prion disease onset, the role played by Cu (II) ions in these diseases remains a topic of debate. Because of the unique properties of paramagnetic Cu (II) ions in the magnetic field, their interactions with PrP can be tracked even at single atom resolution using nuclear magnetic resonance (NMR) spectroscopy. Various NMR approaches have been utilized to study the kinetic, thermodynamic, and structural properties of Cu (II)-PrP interactions. Here, we highlight the different models of copper interactions with PrP with particular focus on studies that use NMR spectroscopy to investigate the role played by copper ions in prion diseases.

Extraction, Characterization, and Anticoagulant Activity of a Sulfated Polysaccharide from Bursatella leachii Viscera

Bioactive compounds for drug discovery are increasingly extracted and purified from natural sources including marine organisms. Heparin is a therapeutic agent that has been used for several decades as an anticoagulant. However, heparin is known to cause many undesirable complications such as thrombocytopenia and risk of hemorrhage. Hence, there is a need to find alternatives to current widely used anticoagulant drugs. Here, we extract a sulfated polysaccharide from sea hare, that is, Bursatella leachii viscera, by enzymatic digestion. Several analytical approaches including elemental analysis, Fourier-transform infrared spectroscopy, nuclear magnetic resonance, and high-performance liquid chromatography-mass spectrometry analysis show that B. leachii polysaccharides have chemical structures similar to glycosaminoglycans. We explore the anticoagulant activity of the B. leachii extract using the activated partial thromboplastin time and the thrombin time. Our results demonstrate that the extracted sulfated polysaccharide has heparin-like anticoagulant activity, thus showing great promise as an alternative anticoagulant therapy.

Biological membranes in EV biogenesis, stability, uptake, and cargo transfer: an ISEV position paper arising from the ISEV membranes and EVs workshop

Paracrine and endocrine roles have increasingly been ascribed to extracellular vesicles (EVs) generated by multicellular organisms. Central to the biogenesis, content, and function of EVs are their delimiting lipid bilayer membranes. To evaluate research progress on membranes and EVs, the International Society for Extracellular Vesicles (ISEV) conducted a workshop in March 2018 in Baltimore, Maryland, USA, bringing together key opinion leaders and hands-on researchers who were selected on the basis of submitted applications. The workshop was accompanied by two scientific surveys and covered four broad topics: EV biogenesis and release; EV uptake and fusion; technologies and strategies used to study EV membranes; and EV transfer and functional assays. In this ISEV position paper, we synthesize the results of the workshop and the related surveys to outline important outstanding questions about EV membranes and describe areas of consensus. The workshop discussions and survey responses reveal that while much progress has been made in the field, there are still several concepts that divide opinion. Good consensus exists in some areas, including particular aspects of EV biogenesis, uptake and downstream signalling. Areas with little to no consensus include EV storage and stability, as well as whether and how EVs fuse with target cells. Further research is needed in these key areas, as a better understanding of membrane biology will contribute substantially towards advancing the field of extracellular vesicles.

NMR Spectroscopy for Metabolomics Research

Over the past two decades, nuclear magnetic resonance (NMR) has emerged as one of the three principal analytical techniques used in metabolomics (the other two being gas chromatography coupled to mass spectrometry (GC-MS) and liquid chromatography coupled with single-stage mass spectrometry (LC-MS)). The relative ease of sample preparation, the ability to quantify metabolite levels, the high level of experimental reproducibility, and the inherently nondestructive nature of NMR spectroscopy have made it the preferred platform for long-term or large-scale clinical metabolomic studies. These advantages, however, are often outweighed by the fact that most other analytical techniques, including both LC-MS and GC-MS, are inherently more sensitive than NMR, with lower limits of detection typically being 10 to 100 times better. This review is intended to introduce readers to the field of NMR-based metabolomics and to highlight both the advantages and disadvantages of NMR spectroscopy for metabolomic studies. It will also explore some of the unique strengths of NMR-based metabolomics, particularly with regard to isotope selection/detection, mixture deconvolution via 2D spectroscopy, automation, and the ability to noninvasively analyze native tissue specimens. Finally, this review will highlight a number of emerging NMR techniques and technologies that are being used to strengthen its utility and overcome its inherent limitations in metabolomic applications.

Peridinin Is an Exceptionally Potent and Membrane-Embedded Inhibitor of Bilayer Lipid Peroxidation

Antilipoperoxidant protein dysfunction is associated with many human diseases, suggesting that bilayer lipid peroxidation may contribute broadly to pathogenesis. Small molecule inhibitors of this membrane-localized chemistry could in theory enable better understanding and/or treatment of such diseases, but currently available compounds have important limitations. Many biological questions thus remain unanswered, and clinical trials have largely been disappointing. Enabled by efficient, building block-based syntheses of three atypical carotenoid natural products produced by microorganisms that thrive in environments of extreme oxidative stress, we found that peridinin is a potent inhibitor of nonenzymatic bilayer lipid peroxidation in liposomes and in primary human endothelial cells. We also found that peridinin blocks monocyte-endothelial cell adhesion, a key step in atherogenesis. A series of frontier solid-state NMR experiments with a site-specifically 13C-labeled isotopolog synthesized using the same MIDA boronate building block-based total synthesis approach revealed that peridinin is completely embedded within and physically spans the hydrophobic core of POPC membranes, maximizing its effective molarity at the site of the targeted lipid peroxidation reactions. Alternatively, the widely used carotenoid astaxanthin is significantly less potent and was found to primarily localize extramembranously. Peridinin thus represents a promising and biophysically well-characterized starting point for the development of small molecule antilipoperoxidants that serve as more effective biological probes and/or therapeutics.

Reduction of the 13C cross-polarization experimental time for pharmaceutical samples with long T1 by ball milling in solid-state NMR

Many pharmaceutical samples have notably long ¹H T₁ (proton spin-lattice relaxation time), leading to lengthy experiments lasting several days in solid-state NMR studies. In this work, we propose the use of ball milling on the pharmaceutical samples to reduce the ¹H T₁, which also leads to enhanced sensitivity in {¹H}-¹³C Cross-Polarization (CP) experiments due to reduced particle sizes and increased surface areas of the samples. Experimentally, we determined that depending on the substrates and milling time, the signal-to-noise ratio (S/N) of a 1D ¹³C CP spectrum can be increased by a factor of 3-6, which means that the experimental time can be shortened by a factor of 9-36. Furthermore, the application of simple ball-milling within a short time avoids the amorphization of the studied samples such that no signal due to amorphous state is observed in the ¹³C CP spectrum. This simple ball milling method used for sensitivity enhancement can be further applied in the SS-NMR studies of pharmaceutical samples.

Sensitivity enhancement for membrane proteins reconstituted in parallel and perpendicular oriented bicelles obtained by using repetitive cross-polarization and membrane-incorporated free radicals

Multidimensional separated local-field and spin-exchange experiments employed by oriented-sample solid-state NMR are essential for structure determination and spectroscopic assignment of membrane proteins reconstituted in macroscopically aligned lipid bilayers. However, these experiments typically require a large number of scans in order to establish interspin correlations. Here we have shown that a combination of optimized repetitive cross polarization (REP-CP) and membrane-embedded free radicals allows one to enhance the signal-to-noise ratio by factors 2.4-3.0 in the case of Pf1 coat protein reconstituted in magnetically aligned bicelles with their normals being either parallel or perpendicular to the main magnetic field. Notably, spectral resolution is not affected at the 2:1 radical-to-protein ratio. Spectroscopic assignment of Pf1 coat protein in the parallel bicelles has been established as an illustration of the method. The proposed methodology will advance applications of oriented-sample NMR technique when applied to samples containing smaller quantities of proteins and three-dimensional experiments.

Efficient DNP NMR of membrane proteins: sample preparation protocols, sensitivity, and radical location

Although dynamic nuclear polarization (DNP) has dramatically enhanced solid-state NMR spectral sensitivities of many synthetic materials and some biological macromolecules, recent studies of membrane-protein DNP using exogenously doped paramagnetic radicals as polarizing agents have reported varied and sometimes surprisingly limited enhancement factors. This motivated us to carry out a systematic evaluation of sample preparation protocols for optimizing the sensitivity of DNP NMR spectra of membrane-bound peptides and proteins at cryogenic temperatures of ~110 K. We show that mixing the radical with the membrane by direct titration instead of centrifugation gives a significant boost to DNP enhancement. We quantify the relative sensitivity enhancement between AMUPol and TOTAPOL, two commonly used radicals, and between deuterated and protonated lipid membranes. AMUPol shows ~fourfold higher sensitivity enhancement than TOTAPOL, while deuterated lipid membrane does not give net higher sensitivity for the membrane peptides than protonated membrane. Overall, a ~100 fold enhancement between the microwave-on and microwave-off spectra can be achieved on lipid-rich membranes containing conformationally disordered peptides, and absolute sensitivity gains of 105-160 can be obtained between low-temperature DNP spectra and high-temperature non-DNP spectra. We also measured the paramagnetic relaxation enhancement of lipid signals by TOTAPOL and AMUPol, to determine the depths of these two radicals in the lipid bilayer. Our data indicate a bimodal distribution of both radicals, a surface-bound fraction and a membrane-bound fraction where the nitroxides lie at ~10 Å from the membrane surface. TOTAPOL appears to have a higher membrane-embedded fraction than AMUPol. These results should be useful for membrane-protein solid-state NMR studies under DNP conditions and provide insights into how biradicals interact with phospholipid membranes.

The strengths and weaknesses of NMR spectroscopy and mass spectrometry with particular focus on metabolomics research

Mass spectrometry (MS) and nuclear magnetic resonance (NMR) have evolved as the most common techniques in metabolomics studies, and each brings its own advantages and limitations. Unlike MS spectrometry, NMR spectroscopy is quantitative and does not require extra steps for sample preparation, such as separation or derivatization. Although the sensitivity of NMR spectroscopy has increased enormously and improvements continue to emerge steadily, this remains a weak point for NMR compared with MS. MS-based metabolomics provides an excellent approach that can offer a combined sensitivity and selectivity platform for metabolomics research. Moreover, different MS approaches such as different ionization techniques and mass analyzer technology can be used in order to increase the number of metabolites that can be detected. In this chapter, the advantages, limitations, strengths, and weaknesses of NMR and MS as tools applicable to metabolomics research are highlighted.

Gas chromatography-mass spectrometry of biofluids and extracts

Gas chromatography-mass spectrometry (GC-MS) has been widely used in metabonomics analyses of biofluid samples. Biofluids provide a wealth of information about the metabolism of the whole body and from multiple regions of the body that can be used to study general health status and organ function. Blood serum and blood plasma, for example, can provide a comprehensive picture of the whole body, while urine can be used to monitor the function of the kidneys, and cerebrospinal fluid (CSF) will provide information about the status of the brain and central nervous system (CNS). Different methods have been developed for the extraction of metabolites from biofluids, these ranging from solvent extracts, acids, heat denaturation, and filtration. These methods vary widely in terms of efficiency of protein removal and in the number of metabolites extracted. Consequently, for all biofluid-based metabonomics studies, it is vital to optimize and standardize all steps of sample preparation, including initial extraction of metabolites. In this chapter, recommendations are made of the optimum experimental conditions for biofluid samples for GC-MS, with a particular focus on blood serum and plasma samples.

Sample collection and preparation of biofluids and extracts for gas chromatography-mass spectrometry

To maximize the utility of gas chromatography-mass spectrometry (GC-MS) in metabonomics research, all stages of the experimental design should be standardized, including sample collection, storage, preparation, and sample separation. Moreover, the prerequisite for any GC-MS analysis is that a compound must be volatile and thermally stable if it is to be analyzed using this technique. Since many metabolites are nonvolatile and polar in nature, they are not readily amenable to analysis by GC-MS and require initial chemical derivatization of the polar functional groups in order to reduce the polarity and to increase the thermal stability and volatility of the analytes. In this chapter, an overview is presented of the optimum approach to sample collection, storage, and preparation for gas chromatography-mass spectrometry-based metabonomics with particular focus on urine samples as example of biofluids.

Recent advances in magic angle spinning solid state NMR of membrane proteins

Membrane proteins mediate many critical functions in cells. Determining their three-dimensional structures in the native lipid environment has been one of the main objectives in structural biology. There are two major NMR methodologies that allow this objective to be accomplished. Oriented sample NMR, which can be applied to membrane proteins that are uniformly aligned in the magnetic field, has been successful in determining the backbone structures of a handful of membrane proteins. Owing to methodological and technological developments, Magic Angle Spinning (MAS) solid-state NMR (ssNMR) spectroscopy has emerged as another major technique for the complete characterization of the structure and dynamics of membrane proteins. First developed on peptides and small microcrystalline proteins, MAS ssNMR has recently been successfully applied to large membrane proteins. In this review we describe recent progress in MAS ssNMR methodologies, which are now available for studies of membrane protein structure determination, and outline a few examples, which highlight the broad capability of ssNMR spectroscopy.

Solid-state NMR (31)P paramagnetic relaxation enhancement membrane protein immersion depth measurements

Paramagnetic relaxation enhancement (PRE) is a widely used approach for measuring long-range distance constraints in biomolecular solution NMR spectroscopy. In this paper, we show that (31)P PRE solid-state NMR spectroscopy can be utilized to determine the immersion depth of spin-labeled membrane peptides and proteins. Changes in the (31)P NMR PRE times coupled with modeling studies can be used to describe the spin-label position/amino acid within the lipid bilayer and the corresponding helical tilt. This method provides valuable insight on protein-lipid interactions and membrane protein structural topology. Solid-state (31)P NMR data on the 23 amino acid α-helical nicotinic acetylcholine receptor nAChR M2δ transmembrane domain model peptide followed predicted behavior of (31)P PRE rates of the phospholipid headgroup as the spin-label moves from the membrane surface toward the center of the membrane. Residue 11 showed the smallest changes in (31)P PRE (center of the membrane), while residue 22 shows the largest (31)P PRE change (near the membrane surface), when compared to the diamagnetic control M2δ sample. This PRE SS-NMR technique can be used as a molecular ruler to measure membrane immersion depth.

Measuring membrane penetration with depth-dependent fluorescence quenching: distribution analysis is coming of age

Depth-dependent fluorescence quenching by lipid-attached quenchers (e.g., bromine atoms and doxyl groups) is an important tool for determining the penetration of proteins and peptides into lipid bilayers. Extracting quantitative information and accurate calculations of the depth of the fluorophore are complicated by thermal disorder, resulting in broad distributions of the transverse positions of both quenchers and fluorophores. Twenty-one years ago a methodology called distribution analysis (DA) was introduced, based on the emerging view of the complexity of the transverse organization of lipid bilayer structure. The method is aimed at extracting quantitative information on membrane penetration, such as position and width of fluorophore's distribution along the depth coordinate and its exposure to the lipid phase. Here we review recent progress in refining the DA method and illustrate its applications to protein-membrane interactions. We demonstrate how basic assumptions of the DA approach can be validated using molecular dynamics simulations and how the precision of depth determination is improved by applying a new protocol based on a combination of steady-state and time-resolved fluorescence quenching. Using the example of the MPER fragment of the membrane-spanning domain of the HIV-1 gp41 fusion protein, we illustrate how DA applications and computer simulations can be used together to reveal the molecular organization of a protein-membrane complex. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Sensitivity enhancement and contrasting information provided by free radicals in oriented-sample NMR of bicelle-reconstituted membrane proteins

Elucidating structure and topology of membrane proteins (MPs) is essential for unveiling functionality of these important biological constituents. Oriented-sample solid-state NMR (OS-NMR) is capable of providing such information on MPs under nearly physiological conditions. However, two dimensional OS-NMR experiments can take several days to complete due to long longitudinal relaxation times combined with the large number of scans to achieve sufficient signal sensitivity in biological samples. Here, free radicals 5-DOXYL stearic acid, TEMPOL, and CAT-1 were added to uniformly (15)N-labeled Pf1 coat protein reconstituted in DMPC/DHPC bicelles, and their effect on the longitudinal relaxation times (T1Z) was investigated. The dramatically shortened T1Z's allowed for the signal gain per unit time to be used for either: (i) up to a threefold reduction of the total experimental time at 99% magnetization recovery or (ii) obtaining up to 74% signal enhancement between the control and radical samples during constant experimental time at "optimal" relaxation delays. In addition, through OS-NMR and high-field EPR studies, free radicals were able to provide positional constraints in the bicelle system, which provide a description of the location of each residue in Pf1 coat protein within the bicellar membranes. This information can be useful in the determination of oligomerization states and immersion depths of larger membrane proteins.

Membrane proteins structure and dynamics by nuclear magnetic resonance

Membrane proteins represent a challenging class of biological systems to study. They are extremely difficult to crystallize and in most cases they retain their structure and functions only in membrane environments. Therefore, commonly used diffraction methods fail to give detailed molecular structure and other approaches have to be utilized to obtain biologically relevant information. Nuclear magnetic resonance (NMR) spectroscopy, however, can provide powerful structural and dynamical constraints on these complicated systems. Solution- and solid-state NMR are powerful methods for investigating membrane proteins studies. In this work, we briefly review both solution and solid-state NMR techniques for membrane protein studies and illustrate the applications of these methods to elucidate proteins structure, conformation, topology, dynamics, and function. Recent advances in electronics, biological sample preparation, and spectral processing provided opportunities for complex biological systems, such as membrane proteins inside lipid vesicles, to be studied faster and with outstanding quality. New analysis methods therefore have emerged, that benefit from the combination of sample preparation and corresponding specific high-end NMR techniques, which give access to more structural and dynamic information.

Structure and dynamic properties of membrane proteins using NMR

Integral membrane proteins are one of the most challenging groups of macromolecules despite their apparent conformational simplicity. They manage and drive transport, circulate information, and participate in cellular movements via interactions with other proteins and through intricate conformational changes. Their structural and functional decoding is challenging and has imposed demanding experimental development. Solution nuclear magnetic resonance (NMR) spectroscopy is one of the techniques providing the capacity to make a significant difference in the deciphering of the membrane protein structure-function paradigm. The method has evolved dramatically during the last decade resulting in a plethora of new experiments leading to a significant increase in the scientific repertoire for studying membrane proteins. Besides solving the three-dimensional structures using state-of-the-art approaches, a large variety of developments of well-established techniques are available providing insight into membrane protein flexibility, dynamics, and interactions. Inspired by the speed of development in the application of new strategies, by invention of methods to measure solvent accessibility and describe low-populated states, this review seeks to introduce the vast possibilities solution NMR can offer to the study of membrane protein structure-function analyses with special focus on applicability.

Molecular dynamics simulations of depth distribution of spin-labeled phospholipids within lipid bilayer

Spin-labeled lipids are commonly used as fluorescence quenchers in studies of membrane penetration of dye-labeled proteins and peptides using depth-dependent quenching. Accurate calculations of depth of the fluorophore rely on the use of several spin labels placed in the membrane at various positions. The depth of the quenchers (spin probes) has to be determined independently; however, experimental determination of transverse distributions of spin probe depths is difficult. In this Article, we use molecular dynamics (MD) simulations to study the membrane behavior and depth distributions of spin-labeled phospholipids in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. To probe different depths within the bilayer, a series containing five Doxyl-labeled lipids (n-Doxyl PC) has been studied, in which a spin moiety was covalently attached to nth carbon atoms (where n = 5, 7, 10, 12, and 14) of the sn-2 stearoyl chain of the host phospholipid. Our results demonstrate that the chain-attached spin labels are broadly distributed across the model membrane and their environment is characterized by a high degree of mobility and structural heterogeneity. Despite the high thermal disorder, the depth distributions of the Doxyl labels were found to correlate well with their attachment positions, indicating that the distribution of the spin label within the model membrane is dictated by the depth of the nth lipid carbon atom and not by intrinsic properties of the label. In contrast, a much broader and heterogeneous distribution was observed for a headgroup-attached Tempo spin label of Tempo-PC lipids. MD simulations reveal that, due to the hydrophobic nature, a Tempo moiety favors partitioning from the headgroup region deeper into the membrane. Depending on the concentration of Tempo-PC lipids, the probable depth of the Tempo moiety could span a range from 14.4 to 18.2 Å from the membrane center. Comparison of the MD-estimated immersion depths of Tempo and n-Doxyl labels with their suggested experimental depth positions allows us to review critically the possible sources of error in depth-dependent fluorescence quenching studies.

Utilizing NMR and EPR spectroscopy to probe the role of copper in prion diseases

Copper is an essential nutrient for the normal development of the brain and nervous system, although the hallmark of several neurological diseases is a change in copper concentrations in the brain and central nervous system. Prion protein (PrP) is a copper-binding, cell-surface glycoprotein that exists in two alternatively folded conformations: a normal isoform (PrP(C)) and a disease-associated isoform (PrP(Sc)). Prion diseases are a group of lethal neurodegenerative disorders that develop as a result of conformational conversion of PrP(C) into PrP(Sc). The pathogenic mechanism that triggers this conformational transformation with the subsequent development of prion diseases remains unclear. It has, however, been shown repeatedly that copper plays a significant functional role in the conformational conversion of prion proteins. In this review, we focus on current research that seeks to clarify the conformational changes associated with prion diseases and the role of copper in this mechanism, with emphasis on the latest applications of NMR and EPR spectroscopy to probe the interactions of copper with prion proteins.

Observing the translocation of a mitochondria-penetrating peptide with solid-state NMR

A new class of penetrating peptides that can target the mitochondria with high specificity was recently discovered. In this work, we developed a model inner mitochondrial membrane, equipped with a transmembrane gradient, suitable for solid-state NMR experiments. Using solid-state NMR, we observed a mitochondria-penetrating peptide interacting with the model inner mitochondrial membrane to gain insight into the mechanism of translocation. The paramagnetic relaxation effect due to Mn(2+) ions on (13)C magic angle spinning NMR was used to measure the insertion depth of the peptide and its distribution in each monolayer of the membrane. We found that at low peptide concentration the peptide binds to the outer leaflet and at high concentration, it crosses the hydrophobic bilayer core and is distributed in both leaflets. In both concentration regimes, the peptide binds at the C2 position on the lipid acyl chain. The mitochondria-penetrating peptide crossed to the inner leaflet of the model membranes without disrupting the lamellarity. These results provide evidence that supports the electroporation model of translocation. We estimated the energy associated with crossing the inner mitochondrial membrane. We found that the transmembrane potential provides sufficient energy for the peptide to cross the hydrophobic core, which is the most unfavorable step in translocation.

Topology of the trans-membrane peptide WALP23 in model membranes under negative mismatch conditions

The organization and orientation of membrane-inserted helices is important for better understanding the mode of action of membrane-active peptides and of protein-membrane interactions. Here we report on the application of ESEEM (electron spin-echo envelope modulation) and DEER (double electron-electron resonance) techniques to probe the orientation and oligomeric state of an α-helical trans-membrane model peptide, WALP23, under conditions of negative mismatch between the hydrophobic cores of the model membrane and the peptide. Using ESEEM, we measured weak dipolar interactions between spin-labeled WALP23 and (2)H nuclei of either the solvent (D2O) or of lipids specifically deuterated at the choline group. The ESEEM data obtained from the deuterated lipids were fitted using a model that provided the spin label average distance from a layer of (2)H nuclei in the hydrophilic region of the membrane and the density of the (2)H nuclei in the layer. DEER was used to probe oligomerization through the dipolar interaction between two spin-labels on different peptides. We observed that the center of WALP23 does not coincide with the bilayer midplane and its N-terminus is more buried than the C-terminus. In addition, the ESEEM data fitting yielded a (2)H layer density that was much lower than expected. The DEER experiments revealed the presence of oligomers, the presence of which was attributable to the negative mismatch and the electrostatic dipole of the peptide. A discussion of a possible arrangement of the individual helices in the oligomers that is consistent with the ESEEM and DEER data is presented.

Jahn-Teller dynamics in a series of high-symmetry Co(II) chelates determine paramagnetic relaxation enhancements

NMR paramagnetic relaxation enhancements (PREs) of a series of structurally characterized, trigonal bis-trispyrazolylborate (Tp) chelates of high-spin Co(II), spanning 100-850 MHz in field, are reported. Prior knowledge of the metal-nucleus distances allows numerical extraction of position-dependent electron spin relaxation rates (τ(c)(-1)) from direct measurement of the individual PREs of the four symmetry distinct protons in Co(Tp)(2), using available closed-form expressions. The data for this electronically complex system where spin-orbit coupling defines the ground state electronic structure are analyzed in terms of the Solomon-Bloembergen-Morgan (SBM) relations, as well as available zero-field splitting limit theories. A simple angular correction is shown to be sufficient to reconcile the individual τ(c)(T) data for the four classes of protons. The data identify a previously unrecognized dynamic Jahn-Teller effect in these historically important complexes, with a barrier of ~230 cm(-1), pointing to a level of dynamics in trispyrazolylborate chemistry that has not been described before, and further show that it is the Jahn-Teller that is responsible for the PREs in fluid solution. A field-dependent component is also identified for the two protons nearest g(//), which is suggested to arise due to Zeeman mixing of excited state character into the ground level.

An ultrasensitive tool exploiting hydration dynamics to decipher weak lipid membrane-polymer interactions

We introduce a newly developed tool, (1)H Overhauser Dynamic Nuclear Polarization (ODNP), to sensitively explore weak macromolecular interactions by site-specifically probing the modulation of the translational dynamics of hydration water at the interaction interface, in the full presence of bulk water. Here, ODNP is employed on an illustrative example of a membrane-active triblock copolymer, poloxamer 188 (P188), which is known to restore the integrity of structurally compromised cell membranes. We observe a distinct change in the translational dynamics of the hydration layer interacting with the lipid membrane surface and the bilayer-interior as P188 is added to a solution of lipid vesicles, but no measurable changes in the dynamics or structure of the lipid membranes. This study shows that hydration water is an integral constituent of a lipid membrane system, and demonstrates for the first time that the modulation of its translational diffusivity can sensitively report on weak polymer-membrane interactions, as well as mediate essential lipid membrane functions. ODNP holds much promise as a unique tool to unravel molecular interactions at interfaces even in the presence of bulk water under ambient conditions.