Fast NMR data acquisition from bicelles containing a membrane-associated peptide at natural-abundance

On-Site Measurement of Fat and Protein Contents in Milk Using Mobile NMR Technology

Robust and easy-to-use NMR sensor technology is proposed for accurate, on-site determination of fat and protein contents in milk. The two parameters are determined using fast consecutive ¹H and ³⁵Cl low-field NMR experiments on milk samples upon the 1:1 addition of a low-cost contrast solution. Reliable and accurate measurements are obtained without tedious calibrations and the need for extensive database information and may readily be conducted by non-experts in production site environments. This enables on-site application at farms or dairies, or use in laboratories harvesting significant reductions in costs and time per analysis as compared to wet-chemistry analysis. The performance is demonstrated for calibration samples, various supermarket milk products, and raw milk samples, of which some were analyzed directly in the milking room. To illustrate the wide application range, the supermarket milk products included both conventionally/organically produced, lactose-free milk, cow's, sheep's and goat's milk, homogenized and unhomogenized milk, and a broad nutrient range (0.1-9% fat, 1-6% protein). Excellent agreement between NMR measurements and reference values, without corrections or changes in calibration for various products and during extensive periods of experiment conduction (4 months) demonstrates the robustness of the procedure and instrumentation. For the raw milk samples, correlations between NMR and IR, NMR and wet-chemistry, as well as IR and wet-chemistry results, show that NMR, in terms of accuracy, compares favorably with the other methods.

Lipid-nanodiscs formed by paramagnetic metal chelated polymer for fast NMR data acquisition

Lipid-nanodiscs have been shown to be an exciting innovation as a membrane-mimicking system for studies on membrane proteins by a variety of biophysical techniques, including NMR spectroscopy. Although NMR spectroscopy is unique in enabling the atomic-resolution investigation of dynamic structures of membrane-associated molecules, it, unfortunately, suffers from intrinsically low sensitivity. The long data acquisition often used to enhance the sensitivity is not desirable for sensitive membrane proteins. Instead, paramagnetic relaxation enhancement (PRE) has been used to reduce NMR data acquisition time or to reduce the amount of sample required to acquire an NMR spectra. However, the PRE approach involves the introduction of external paramagnetic probes in the system, which can induce undesired changes in the sample and on the observed NMR spectra. For example, the addition of paramagnetic ions, as frequently used, can denature the protein via direct interaction and also through sample heating. In this study, we show how the introduction of paramagnetic tags on the outer belt of polymer-nanodiscs can be used to speed-up data acquisition by significantly reducing the spin-lattice relaxation (T1) times with minimum-to-no alteration of the spectral quality. Our results also demonstrate the feasibility of using different types of paramagnetic ions (Eu3+, Gd3+, Dy3+, Er3+, Yb3+) for NMR studies on lipid-nanodiscs. Experimental results characterizing the formation of lipid-nanodiscs by the metal-chelated polymer, and their increased tolerance toward metal ions are also reported.

Metal-Chelated Polymer Nanodiscs for NMR Studies

Paramagnetic relaxation enhancement (PRE) is commonly used to speed up spin lattice relaxation time (T1 ) for rapid data acquisition in NMR structural studies. Consequently, there is significant interest in novel paramagnetic labels for enhanced NMR studies on biomolecules. Herein, we report the synthesis and characterization of a modified poly(styrene-co-maleic acid) polymer which forms nanodiscs while showing the ability to chelate metal ions. Cu2+ -chelated nanodiscs are demonstrated to reduce the T1 of protons for both polymer and lipid-nanodisc components. The chelated nanodiscs also decrease the proton T1 values for a water-soluble DNA G-quadruplex. These results suggest that polymer nanodiscs functionalized with paramagnetic tags can be used to speed-up data acquisition from lipid bilayer samples and also to provide structural information from water-soluble biomolecules.

Use of paramagnetic systems to speed-up NMR data acquisition and for structural and dynamic studies

NMR spectroscopy is a powerful experimental technique to study biological systems at the atomic resolution. However, its intrinsic low sensitivity results in long acquisition times that in extreme cases lasts for days (or even weeks) often exceeding the lifetime of the sample under investigation. Different paramagnetic agents have been used in an effort to decrease the spin-lattice (T1) relaxation times of the studied nuclei, which are the main cause for long acquisition times necessary for signal averaging to enhance the signal-to-noise ratio of NMR spectra. Consequently, most of the experimental time is "wasted" in waiting for the magnetization to recover between successive scans. In this review, we discuss how to set up an optimal paramagnetic relaxation enhancement (PRE) system to effectively reduce the T1 relaxation times avoiding significant broadening of NMR signals. Additionally, we describe how PRE-agents can be used to provide structural and dynamic information and can even be used to follow the intermediates of chemical reactions and to speed-up data acquisition. We also describe the unique challenges and benefits associated with the application of PRE to solid-state NMR spectroscopy, explaining how the use of PREs is more complex for membrane mimetic systems as PREs can also be exploited to change the alignment of oriented membrane systems. Functionalization of membrane mimetics, such as bicelles, can provide a controlled region of paramagnetic effect that has the potential, together with the desired alignment, to provide crucial biologically relevant structural information. And finally, we discuss how paramagnetic metals can be utilized to further increase the dynamic nuclear polarization (DNP) effects and how to preserve the enhancements when dissolution DNP is implemented.

How Do Nuclei Couple to the Magnetic Moment of a Paramagnetic Center? A New Theory at the Gauntlet of the Experiments

The recent derivation, based on pure quantum chemistry (QC) first-principles, of the pseudocontact shifts (PCSs) caused by a paramagnetic metal center on far away nuclei has cast doubts on the validity of the semiempirical (SE) theory, predicting PCSs to arise from the metal magnetic susceptibility anisotropy. The SE theory has been used and applied countless times, especially in the last 2 decades, to obtain structural information on proteins containing paramagnetic metal ions. We show here that the QC and SE predictions can be directly tested against experiments, provided a suitable macromolecular system is used. The SE approach yields a good prediction of the experimental PCSs while the QC one does not. It appears that the classic theory is able to grasp satisfactorily the underlying physics.

Paramagnetic relaxation enhancement for protein-observed 19F NMR as an enabling approach for efficient fragment screening

Protein-observed 19F (PrOF) NMR is an emerging tool for ligand discovery. To optimize the efficiency of PrOF NMR experiments, paramagnetic relaxation enhancement through the addition of chelated Ni(II) was used to shorten longitudinal relaxation time without causing significant line broadening. Thus enhancing relaxation time leads to shorter experiments without perturbing the binding of low- or high-affinity ligands. This method allows for time-efficient screening of potential ligands for a wide variety of proteins in the growing field of fragment-based ligand discovery.

Detergent-type membrane fragmentation by MSI-78, MSI-367, MSI-594, and MSI-843 antimicrobial peptides and inhibition by cholesterol: a solid-state nuclear magnetic resonance study

Multidrug resistance against the existing antibiotics is becoming a global threat, and any potential drug that can be designed using cationic antimicrobial peptides (AMP) could be an alternate solution to alleviate this existing problem. The mechanism of action of killing bacteria by an AMP differs drastically in comparison to that of small molecule antibiotics. The main target of AMPs is to interact with the lipid bilayer of the cell membrane and disrupt it to kill bacteria. Consequently, the modes of membrane interaction that lead to the selectivity of an AMP are very important to understand. Here, we have used different membrane compositions, such as negatively charged, zwitterionic, or mixed large unilamellar vesicles (LUVs), to study the interaction of four different synthetically designed cationic, linear antimicrobial peptides: MSI-78 (commercially known as pexiganan), MSI-367, MSI-594, and MSI-843. Our solid-state nuclear magnetic resonance (NMR) experiments confirmed that the MSI peptides fragmented LUVs through a detergent-like carpet mechanism depending on the amino acid sequence of the MSI peptide and/or the membrane composition of LUVs. Interestingly, the fragmented lipid aggregates such as SUVs or micelles are sufficiently small to produce an isotropic peak in the (31)P NMR spectrum. These fragmented lipid aggregates contain only MSI peptides bestowed with lipid molecules as confirmed by NMR in conjunction with circular dichroism spectroscopy. Our results also demonstrate that cholesterol, which is present only in the eukaryotic cell membrane, inhibits the MSI-induced fragmentation of LUVs, suggesting that the MSI peptides can discriminate the bacteria and the eukaryotic cell membranes, and this selectivity could be used for further development of novel antibiotics.

Temperature-resistant bicelles for structural studies by solid-state NMR spectroscopy

Three-dimensional structure determination of membrane proteins is important to fully understand their biological functions. However, obtaining a high-resolution structure has been a major challenge mainly due to the difficulties in retaining the native folding and function of membrane proteins outside of the cellular membrane environment. These challenges are acute if the protein contains a large soluble domain, as it needs bulk water unlike the transmembrane domains of an integral membrane protein. For structural studies on such proteins either by nuclear magnetic resonance (NMR) spectroscopy or X-ray crystallography, bicelles have been demonstrated to be superior to conventional micelles, yet their temperature restrictions attributed to their thermal instabilities are a major disadvantage. Here, we report an approach to overcome this drawback through searching for an optimum combination of bicellar compositions. We demonstrate that bicelles composed of 1,2-didecanoyl-sn-glycero-3-phosphocholine (DDPC) and 1,2-diheptanoyl-sn-glycero-3-phosphocholin (DHepPC), without utilizing additional stabilizing chemicals, are quite stable and are resistant to temperature variations. These temperature-resistant bicelles have a robust bicellar phase and magnetic alignment over a broad range of temperatures, between -15 and 80 °C, retain the native structure of a membrane protein, and increase the sensitivity of solid-state NMR experiments performed at low temperatures. Advantages of two-dimensional separated-local field (SLF) solid-state NMR experiments at a low temperature are demonstrated on magnetically aligned bicelles containing an electron carrier membrane protein, cytochrome b5. Morphological information on different DDPC-based bicellar compositions, varying q ratio/size, and hydration levels obtained from (31)P NMR experiments in this study is also beneficial for a variety of biophysical and spectroscopic techniques, including solution NMR and magic-angle-spinning (MAS) NMR for a wide range of temperatures.

Cellular solid-state NMR investigation of a membrane protein using dynamic nuclear polarization

While an increasing number of structural biology studies successfully demonstrate the power of high-resolution structures and dynamics of membrane proteins in fully understanding their function, there is considerable interest in developing NMR approaches to obtain such information in a cellular setting. As long as the proteins inside the living cell tumble rapidly in the NMR timescale, recently developed in-cell solution NMR approaches can provide 3D structural information. However, there are numerous challenges to study membrane proteins inside a cell. Research in our laboratory is focused on developing a combination of solid-state NMR and biological approaches to overcome these challenges in order to obtain high-resolution structural insights into electron transfer processes mediated by membrane-bound proteins like mammalian cytochrome-b5, cytochrome-P450 and cytochrome-P450-reductase. In this study, we demonstrate the feasibility of using dynamic nuclear polarization (DNP) magic angle spinning (MAS) NMR spectroscopy for in-cell studies on a membrane-anchored protein. Our experimental results obtained from ¹³C-labeled membrane-anchored cytochrome-b5 in native Escherichia coli cells show a ~16-fold DNP signal enhancement. Further, results obtained from a 2D ¹³C/¹³C chemical shift correlation MAS experiment demonstrate the feasibility of suppressing the background signals from other cellular contents for high-resolution structural studies on membrane proteins. We believe that this study would pave new avenues for high-resolution structural studies on a variety of membrane-associated proteins and their complexes in the cellular context to fully understand their functional roles in physiological processes.

Acceleration of natural-abundance solid-state MAS NMR measurements on bone by paramagnetic relaxation from gadolinium-DTPA

Reducing the data collection time without affecting the signal intensity and spectral resolution is one of the major challenges for the widespread application of multidimensional nuclear magnetic resonance (NMR) spectroscopy, especially in experiments conducted on complex heterogeneous biological systems such as bone. In most of these experiments, the NMR data collection time is ultimately governed by the proton spin-lattice relaxation times (T1). For over two decades, gadolinium(III)-DTPA (Gd-DTPA, DTPA=Diethylene triamine pentaacetic acid) has been one of the most widely used contrast-enhancement agents in magnetic resonance imaging (MRI). In this study, we demonstrate that Gd-DTPA can also be effectively used to enhance the longitudinal relaxation rates of protons in solid-state NMR experiments conducted on bone without significant line-broadening and chemical-shift-perturbation side effects. Using bovine cortical bone samples incubated in different concentrations of Gd-DTPA complex, the (1)H T1 values were calculated from data collected by (1)H spin-inversion recovery method detected in natural-abundance (13)C cross-polarization magic angle spinning (CPMAS) NMR experiments. Our results reveal that the (1)H T1 values can be successfully reduced by a factor of 3.5 using as low as 10mM Gd-DTPA without reducing the spectral resolution and thus enabling faster data acquisition of the (13)C CPMAS spectra. These results obtained from (13)C-detected CPMAS experiments were further confirmed using (1)H-detected ultrafast MAS experiments on Gd-DTPA doped bone samples. This approach considerably improves the signal-to-noise ratio per unit time of NMR experiments applied to bone samples by reducing the experimental time required to acquire the same number of scans.

Bicelles exhibiting magnetic alignment for a broader range of temperatures: a solid-state NMR study

Bicelles are increasingly used as model membranes to suitably mimic the biological cell membrane for biophysical and biochemical studies by a variety of techniques including NMR and X-ray crystallography. Recent NMR studies have successfully utilized bicelles for atomic-resolution structural and dynamic studies of antimicrobial peptides, amyloid peptides, and membrane-bound proteins. Though bicelles composed with several different types of lipids and detergents have been reported, the NMR requirement of magnetic alignment of bicelles limits the temperature range in which they can be used and subsequently their composition. Because of this restriction, low-temperature experiments desirable for heat-sensitive membrane proteins have not been conducted because bicelles could not be aligned. In this study, we characterize the magnetic alignment of bicelles with various compositions for a broad range of temperatures using (31)P static NMR spectroscopy in search of temperature-resistant bicelles. Our systematic investigation identified a temperature range of magnetic alignment for bicelles composed of 4:1 DLPC:DHexPC, 4:1:0.2 DLPC:DHexPC:cholesterol, 4:1:0.13 DLPC:DHexPC:CTAB, 4:1:0.13:0.2 DLPC:DHexPC:CTAB:cholesterol, and 4:1:0.4 DLPC:DHexPC:cholesterol-3-sulfate. The amount of cholesterol-3-sulfate used was based on mole percent and was varied in order to determine the optimal amount. Our results indicate that the presence of 75 wt % or more water is essential to achieve maximum magnetic alignment, while the presence of cholesterol and cholesterol-3-sulfate stabilizes the alignment at extreme temperatures and the positively charged CTAB avoids the mixing of bicelles. We believe that the use of magnetically aligned 4:1:0.4 DLPC:DHexPC:cholesterol-3-sulfate bicelles at as low as -15 °C would pave avenues to study the structure, dynamics, and membrane orientation of heat-sensitive proteins such as cytochrome P450 and could also be useful to investigate protein-protein interactions in a membrane environment.

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.

Paramagnetic doping of a 7TM membrane protein in lipid bilayers by Gd³⁺-complexes for solid-state NMR spectroscopy

A considerable limitation of NMR spectroscopy is its inherent low sensitivity. Approximately 90 % of the measuring time is used by the spin system to return to its Boltzmann equilibrium after excitation, which is determined by (1)H-T1 in cross-polarized solid-state NMR experiments. It has been shown that sample doping by paramagnetic relaxation agents such as Cu(2+)-EDTA accelerates this process considerably resulting in enhanced sensitivity. Here, we extend this concept to Gd(3+)-complexes. Their effect on (1)H-T1 has been assessed on the membrane protein proteorhodopsin, a 7TM light-driven proton pump. A comparison between Gd(3+)-DOTA, Gd(3+)-TTAHA, covalently attached Cu(2+)-EDTA-tags and Cu(2+)-EDTA reveals a 3.2-, 2.6-, 2.4- and 2-fold improved signal-to-noise ratio per unit time due to longitudinal paramagnetic relaxation enhancement. Furthermore, Gd(3+)-DOTA shows a remarkably high relaxivity, which is 77-times higher than that of Cu(2+)-EDTA. Therefore, an order of magnitude lower dopant concentration can be used. In addition, no line-broadening effects or peak shifts have been observed on proteorhodopsin in the presence of Gd(3+)-DOTA. These favourable properties make it very useful for solid-state NMR experiments on membrane proteins.

Shortening spin-lattice relaxation using a copper-chelated lipid at low-temperatures - A magic angle spinning solid-state NMR study on a membrane-bound protein

Inherent low sensitivity of NMR spectroscopy has been a major disadvantage, especially to study biomolecules like membrane proteins. Recent studies have successfully demonstrated the advantages of performing solid-state NMR experiments at very low and ultralow temperatures to enhance the sensitivity. However, the long spin-lattice relaxation time, T1, at very low temperatures is a major limitation. To overcome this difficulty, we demonstrate the use of a copper-chelated lipid for magic angle spinning solid-state NMR measurements on cytochrome-b5 reconstituted in multilamellar vesicles. Our results on multilamellar vesicles containing as small as 0.5mol% of a copper-chelated lipid can significantly shorten T1 of protons, which can be used to considerably reduce the data collection time or to enhance the signal-to-noise ratio. We also monitored the effect of slow cooling on the resolution and sensitivity of (13)C and (15)N signals from the protein and (13)C signals from lipids.

Solution NMR studies on the orientation of membrane-bound peptides and proteins by paramagnetic probes

Many peptides and proteins are attached to or immersed in a biological membrane. In order to understand their function not only the structure but also their topology in the membrane is important. Solution NMR spectroscopy is one of the most often used approaches to determine the orientation and localization of membrane-bound peptides and proteins. Here we give an application-oriented overview on the use of paramagnetic probes for the investigation of membrane-bound peptides and proteins. The examples discussed range from the large pool of antimicrobial peptides, bacterial toxins, cell penetrating peptides to domains of larger proteins or the calcium regulating protein phospholamban. Topological information is obtained in all these examples by the use of either attached or freely mobile paramagnetic tags. For some examples information obtained from the paramagnetic probes was included in the structure determination.

2D 1H/1H RFDR and NOESY NMR experiments on a membrane-bound antimicrobial peptide under magic angle spinning

There is significant interest in solving high-resolution dynamic structures of membrane-associated peptides using solid-state NMR spectroscopy. Previous solid-state NMR studies have provided valuable insights into the functional properties of an exciting class of biomacromolecules such as antimicrobial peptides and amyloid peptides. However, it has been a major challenge to apply solid-state NMR techniques to study peptides or proteins that are not labeled with specific isotopes such as (13)C, (15)N, and/or (2)H. This study utilizes 2D (1)H/(1)H radio frequency-driven recoupling (RFDR) and nuclear Overhauser effect spectroscopy (NOESY) pulse sequences under magic angle spinning to study a membrane-bound antimicrobial peptide MSI-78 (or also known as pexiganan). We demonstrate that proton resonances can be assigned and structural constraints, NOE and (1)H-(1)H dipolar couplings, can be measured without the need for any isotopic enrichment. The buildup curves, showing the dependence of the cross peak intensity against the mixing time, obtained from 2D (1)H/(1)H NOESY and RFDR experiments are compared. Our results reveal that the RFDR-recovered (1)H-(1)H dipolar couplings associated with alpha and side chain protons are larger than that with the amide-protons. This study provides a means to measure residual (1)H-(1)H dipolar couplings for the investigation of structure, dynamics, and aggregation of peptides using a suitable model membrane like micelles or bicelles.

On the role of NMR spectroscopy for characterization of antimicrobial peptides

Antimicrobial peptides (AMPs) provide a primordial source of immunity, conferring upon eukaryotic cells resistance against bacteria, protozoa, and viruses. Despite a few examples of anionic peptides, AMPs are usually relatively short positively charged polypeptides, consisting of a dozen to about a hundred amino acids, and exhibiting amphipathic character. Despite significant differences in their primary and secondary structures, all AMPs discovered to date share the ability to interact with cellular membranes, thereby affecting bilayer stability, disrupting membrane organization, and/or forming well-defined pores. AMPs selectively target infectious agents without being susceptible to any of the common pathways by which these acquire resistance, thereby making AMPs prime candidates to provide therapeutic alternatives to conventional drugs. However, the mechanisms of AMP actions are still a matter of intense debate. The structure-function paradigm suggests that a better understanding of how AMPs elicit their biological functions could result from atomic resolution studies of peptide-lipid interactions. In contrast, more strict thermodynamic views preclude any roles for three-dimensional structures. Indeed, the design of selective AMPs based solely on structural parameters has been challenging. In this chapter, we will focus on selected AMPs for which studies on the corresponding AMP-lipid interactions have helped reach an understanding of how AMP effects are mediated. We will emphasize the roles of both liquid- and solid-state NMR spectroscopy for elucidating the mechanisms of action of AMPs.

High-resolution structural insights into bone: a solid-state NMR relaxation study utilizing paramagnetic doping

The hierarchical heterogeneous architecture of bone imposes significant challenges to structural and dynamic studies conducted by traditional biophysical techniques. High-resolution solid-state nuclear magnetic resonance (SSNMR) spectroscopy is capable of providing detailed atomic-level structural insights into such traditionally challenging materials. However, the relatively long data-collection time necessary to achieve a reliable signal-to-noise ratio (S/N) remains a major limitation for the widespread application of SSNMR on bone and related biomaterials. In this study, we attempt to overcome this limitation by employing the paramagnetic relaxation properties of copper(II) ions to shorten the (1)H intrinsic spin-lattice (T(1)) relaxation times measured in natural-abundance (13)C cross-polarization (CP) magic-angle-spinning (MAS) NMR experiments on bone tissues for the purpose of accelerating the data acquisition time in SSNMR. To this end, high-resolution solid-state (13)C CPMAS experiments were conducted on type I collagen (bovine tendon), bovine cortical bone, and demineralized bovine cortical bone, each in powdered form, to measure the (1)H T(1) values in the absence and in the presence of 30 mM Cu(II)(NH(4))(2)EDTA. Our results show that the (1)H T(1) values were successfully reduced by a factor of 2.2, 2.9, and 3.2 for bovine cortical bone, type I collagen, and demineralized bone, respectively, without reducing the spectral resolution and thus enabling faster data acquisition. In addition, paramagnetic quenching of particular (13)C NMR resonances on exposure to Cu(2+) ions in the absence of mineral was also observed, potentially suggesting the relative proximity of three main amino acids in the protein backbone (glycine, proline, and alanine) to the bone mineral surface.

Does cholesterol suppress the antimicrobial peptide induced disruption of lipid raft containing membranes?

The activity of antimicrobial peptides has been shown to depend on the composition of the target cell membrane. The bacterial selectivity of most antimicrobial peptides has been attributed to the presence of abundant acidic phospholipids and the absence of cholesterol in bacterial membranes. The high amount of cholesterol present in eukaryotic cell membranes is thought to prevent peptide-induced membrane disruption by increasing the cohesion and stiffness of the lipid bilayer membrane. While the role of cholesterol on an antimicrobial peptide-induced membrane disrupting activity has been reported for simple, homogeneous lipid bilayer systems, it is not well understood for complex, heterogeneous lipid bilayers exhibiting phase separation (or "lipid rafts"). In this study, we show that cholesterol does not inhibit the disruption of raft-containing 1,2-dioleoyl-sn-glycero-3-phosphocholine:1,2-dipalmitoyol-sn-glycero-3-phosphocholine model membranes by four different cationic antimicrobial peptides, MSI-78, MSI-594, MSI-367 and MSI-843 which permeabilize membranes. Conversely, the presence of cholesterol effectively inhibits the disruption of non-raft containing 1,2-dioleoyl-sn-glycero-3-phosphocholine or 1,2-dipalmitoyol-sn-glycero-3-phosphocholine lipid bilayers, even for antimicrobial peptides that do not show a clear preference between the ordered gel and disordered liquid-crystalline phases. Our results show that the peptide selectivity is not only dependent on the lipid phase but also on the presence of phase separation in heterogeneous lipid systems.

An analysis of NMR sensitivity enhancements obtained using non-uniform weighted sampling, and the application to protein NMR

Non-uniform weighted sampling (NUWS) is a sampling strategy, related to non-uniform sampling (NUS) in the limit of long acquisition times, in which each indirect increment of a multidimensional spectrum is sampled multiple times according to some weighting function. As the spectrum is fully sampled it can be processed in a conventional manner by the discrete Fourier transform, making the analysis of sensitivity much more straightforward than for NUS data. Previously, 2-3 fold increases in signal-to-noise ratio (SNR) have been reported using NUWS. However, as the sampling schedule acts as a window function, the observed SNR must be compared with uniformly sampled data apodized using the same weighting function. On doing this, we calculate more modest improvements of 10-20% in SNR, and these are verified experimentally for spectra of α-synuclein and YFP. Nevertheless, we prove that NUWS always improves the sensitivity compared with identically processed uniformly sampled data, and when combined with rapid recycling experiments such as the SOFAST-HMQC, NUWS methods have the potential to make a useful and practical contribution to sensitivity-limited measurements.

Effect of divalent cations on DMPC/DHPC bicelle formation and alignment

Many important classes of biomolecules require divalent cations for optimal activity, making these ions essential for biologically relevant structural studies. Bicelle mixtures composed of short-chain and long-chain lipids are often used in solution- and solid-state NMR structure determination; however, the phase diagrams of these useful orienting media and membrane mimetics are sensitive to other solution components. Therefore, we have investigated the effect of varying concentrations of four divalent cations, Ca(2+), Mg(2+), Zn(2+), and Cd(2+), on cholesterol sulfate-stabilized DMPC/DHPC bicelles. We found that low concentrations of all the divalent ions are tolerated with minimal perturbation. At higher concentrations Zn(2+) and Cd(2+) disrupt the magnetically aligned phase while Ca(2+) and Mg(2+) produce more strongly oriented phases. This result indicates that divalent cations are not only required to maintain the biological activity of proteins and nucleic acids; they may also be used to manipulate the behavior of the magnetically aligned phase.