Understanding Sterol-Membrane Interactions Part I: Hartree-Fock versus DFT Calculations of13C and1H NMR Isotropic Chemical Shifts of Sterols in Solution and Analysis of Hydrogen-Bonding Effects

17O NMR spectroscopy as a tool to study hydrogen bonding of cholesterol in lipid bilayers

Cholesterol is a crucial component of biological membranes and can interact with other membrane components through hydrogen bonding. NMR spectroscopy has been used previously to investigate this bonding, however this study represents the first 17O NMR spectroscopy study of isotopically enriched cholesterol. We demonstrate the 17O chemical shift is dependent on hydrogen bonding, providing a novel method for the study of cholesterol in bilayers.

Diversity of the Hydrogen Bond Network and Its Impact on NMR Parameters of Amylose B Polymorph: A Study Using Molecular Dynamics and DFT Calculations Within Periodic Boundary Conditions

Classical molecular dynamics simulations have been combined with quantum (DFT) calculations of ¹³C NMR parameters in order to relate the experimental spectrum of the double-helix form of the amylose B-polymorph in highly crystalline conditions not only to its 3D structure but also to the arrangement of atoms in the crystal lattice. Structures obtained from these simulations or from geometry optimization procedures at the DFT level have shown the presence of hydrogen bond networks between sugars of the same helix or between residues of the two chains of the double helix. ¹³C NMR parameter calculations have revealed the impact of such a network on the chemical shifts of carbon atoms. In addition, DFT calculations using periodic boundary conditions were compulsory to highlight the presence of two types of sugar within the crystal sample. It allows us to confirm, theoretically, the experimental hypothesis that the existence of two distinct sugar types in the NMR spectrum is a consequence of crystal packing.

Solid-state NMR investigation of effect of fluorination and methylation on prednisolone conformation

Prednisolone (Prd) is a polymorphous synthetic corticosteroid that has three crystalline forms mediated by different solvents. In this study, we have demonstrated that solid-state {(1)H}(13)C cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy is able to resolve the effects of methylation and fluorination on the conformation of the steroidal rings in Prd. Two compounds were chosen for the study, 6-α-methylprednisolone (Prd-6M) and 6-α-fluoroprednisolone (Prd-6F). The (13)C signals of Prd-6F showed primarily doublet patterns, with splittings of 40-380 Hz, indicating multiple ring conformations, whereas the (13)C signals of Prd and Prd-6M exhibited a singlet pattern, indicating a unique conformation. Using evidence from chemical shift deviation and anisotropy analysis, we have demonstrated by solid-state NMR that Prd-6F adopts two different steroidal ring conformations that are different from that of Prd-6M, and less similar to that of unsubstituted Prd.

Two classes of cholesterol binding sites for the β2AR revealed by thermostability and NMR

Cholesterol binding to G protein-coupled receptors (GPCRs) and modulation of their activities in membranes is a fundamental issue for understanding their function. Despite the identification of cholesterol binding sites in high-resolution x-ray structures of the ?2 adrenergic receptor (β2AR) and other GPCRs, the binding affinity of cholesterol for this receptor and exchange rates between the free and bound cholesterol remain unknown. In this study we report the existence of two classes of cholesterol binding sites in β2AR. By analyzing the β2AR unfolding temperature in lipidic cubic phase (LCP) as a function of cholesterol concentration we observed high-affinity cooperative binding of cholesterol with sub-nM affinity constant. In contrast, saturation transfer difference (STD) NMR experiments revealed the existence of a second class of cholesterol binding sites, in fast exchange on the STD NMR timescale. Titration of the STD signal as a function of cholesterol concentration provided a lower limit of 100 mM for their dissociation constant. However, these binding sites are specific for both cholesterol and β2AR, as shown with control experiments using ergosterol and a control membrane protein (KpOmpA). We postulate that this specificity is mediated by the high-affinity bound cholesterol molecules and propose the formation of transient cholesterol clusters around the high-affinity binding sites.

Cation ion specifically induces a conformational change in trans-dehydroandrosterone - a solid-state NMR study

In this work, we demonstrated that calcium (Ca(+2)) is able to induce a conformational change in trans-dehydroandrosterone (DHEA). To this respect, solid-state NMR spectroscopy was applied to a series of DHEA molecules that were incubated with Ca(+2) under different concentrations. The high-resolution (13)C NMR spectra of the DHEA/Ca(+2) mixtures exhibited two distinct sets of signals; one was attributed to DHEA in the free form, and the second set was due to the DHEA/Ca(+2) complex. Based on chemical shift isotropy and anisotropy analyses, we postulated that Ca(+2) might have associated with the oxygen attached to C17 via a lone-pair of electrons, which induced a conformational change in DHEA. Apart from Ca(+2), we also incubated DHEA with magnesium (Mg(+2)) to determine whether Mg(+2) was able to interact with DHEA in a similar manner to Ca(+2). We found that Mg(+2) was able to induce a conformational change in DHEA deviated from that of Ca(+2). These solid-state NMR observations indicate that DHEA is able to interact with cations, such as Mg(+2) and Ca(+2), with specificity.

Hydrogen bonding of cholesterol in the lipidic cubic phase

The addition of cholesterol to the monoolein-based lipidic cubic phase (LCP) has been instrumental in obtaining high-resolution crystal structures of several G protein-coupled receptors. Here, we report the use of high-resolution magic angle spinning NMR spectroscopy to record and assign the isotropic (13)C chemical shifts of cholesterol in lipidic lamellar and cubic phases at different hydration levels with monoolein and chain-deuterated DMPC as host lipids. The hydrogen-bonding patterns of cholesterol in these phases were determined from the NMR data by quantum chemical calculations. The results are consistent with the normal orientation of cholesterol in lipid bilayers and with the cholesterol hydroxyl group located at the hydrophobic/hydrophilic interface. The (13)C chemical shifts of cholesterol are mostly affected by the host lipid identity with little or no dependency on the hydration (20% vs 40%) or the phase identity (lamellar vs LCP). In chain-deuterated DMPC bilayers, the hydroxyl group of cholesterol forms most of its hydrogen bonds with water, while in monoolein bilayers it predominately interacts with monoolein. Such differences in the hydrogen-bonding network of cholesterol may have implications for the design of experiments in monoolein-based LCP.

Solid-state NMR analysis of steroidal conformation of 17α- and 17β-estradiol in the absence and presence of lipid environment

Solid-state {(1)H}(13)C cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy has been applied to 17β-estradiol (E2) and 17α-estradiol (E2α), to analyze the steroidal ring conformations of the two isomers in the absence and presence of lipids at the atomic level. In the absence of lipid, the high-resolution (13)C NMR signals of E2 in a powdered form show only singlet patterns, suggesting a single ring conformation. In contrast, the (13)C signals of E2α reveal multiplet patterns with splittings of 20-300Hz, implying multiple ring conformations. In the presence of a mimic of the lipid environment, made by mixing 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC) in a molar ratio 3:1, E2 and E2α revealed multiplet patterns different from those seen in the absence of lipids, indicating that the two isomers adopt multiple conformations in the lipid environment. In this work, on the basis of chemical shift isotropy and anisotropy analysis, we demonstrated that E2 and E2α prefer to adopt multiple steroidal ring conformations in the presence of a lipid environment, distinct from that observed in solution phase and powdered form.

Conformational analysis of steroid hormone molecules in the lipid environment--a solid-state NMR approach

Solid-state (1)H/(13)C cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy has been applied to two steroid compounds: dehydroepiandrosterone (DHEA) and spironolactone (SPI), to analyze their conformations at the atomic level. In the absence of lipid, the high-resolution (13)C CP/MAS NMR signals of DHEA and SPI in a powder form reveal multiple patterns, with splittings of 30-160 Hz, indicating the existence of multiple conformations. In the mimic lipid environment formed by mixing 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC) in a molar ratio 3:1, the resulting DHEA and SPI spectra revealed mostly singlet patterns, suggesting that these steroids undergo a conformational change leading to a specific conformation in the lipid environment. Evidence from chemical shift isotropy and anisotropy analysis indicates that DHEA might adopt conformations subtly different from that seen in solution and in the powder form. In conclusion, we demonstrate by solid-state NMR that the structures of DHEA and SPI may adopt slightly different conformations in different chemical environments.

DFT 2H quadrupolar coupling constants of ruthenium complexes: a good probe of the coordination of hydrides in conjuction with experiments

Transition metal (TM) hydrides are of great interest in chemistry because of their reactivity and their potential as catalysts for hydrogenation reactions. 2H solid-state NMR can be used in order to get information about the local environment of hydrogen atoms, and more particularly the coordination mode of hydrides in such complexes. In this work we will show that it is possible to establish at the level of density functional theory (DFT) a viable methodological strategy that allows the determination of 2H NMR parameters, namely the quadrupolar coupling constant (C(Q)) respectively the quadrupolar splitting (deltanuQ) and the asymmetry parameter (etaQ). The reliability of the method (B3PW91-DFT) and basis set effects have been first evaluated for simple organic compounds (benzene and fluorene). A good correlation between experimental and theoretical values is systematically obtained if the large basis set cc-pVTZ is used for the computations. 2H NMR properties of five mononuclear ruthenium complexes (namely Cp*RuD3(PPh3), Tp*RuD(THT)2, Tp*RuD(D2)(THT) and Tp*RuD(D2)2 and RuD2(D2)2(PCy3)2) which exhibit different ligands and hydrides involved in different coordination modes (terminal-H or eta2-H2), have been calculated and compared to previous experimental data. The results obtained are in excellent agreement with experiments. Although 2H NMR spectra are not always easy to analyze, assistance by quantum chemistry calculations allows unambiguous assignment of the signals of such spectra. As far as experiments can be achieved at very low temperatures in order to avoid dynamic effects, this hybrid theoretical/experimental tool may give useful insights in the context of the characterization of ruthenium surfaces or nanoparticles with solid-state NMR.

Ligand effect on the NMR, vibrational and structural properties of tetra- and hexanuclear ruthenium hydrido clusters: a theoretical investigation

Structural and spectroscopic properties of tetranuclear ruthenium hydrido clusters, and to a less extent, of hexanuclear ruthenium hydrido clusters, are investigated theoretically. Some of these (H)(n)Ru(k)(L)(m) (k = 4, 6) clusters were experimentally synthesized and characterized. Non-existing structures are also considered in order to examine the role of ligands on their structure, vibrational spectra and (1)H NMR chemical shifts. The calculated properties are found in very good agreement with experimental data, when available. Beyond the intrinsic interest elicited by transition metal clusters, these compounds are also considered in this paper as relevant to diamagnetic ruthenium nanoparticles as well as building blocks of hcp surfaces, which is the ruthenium nanoparticle lattice. On the basis of the very good agreement between experiments and theory, the structural and spectroscopic properties of several model clusters are also predicted in order to bring additional data which may help to analyze the spectral signature of ruthenium nanoparticles. A particular emphasis is put on (1)H NMR, which is of high practical importance for characterizing the presence of hydrides in ruthenium clusters and nanoparticles. Several topics are discussed: the structural preference of surface hydrides for terminal-, edge-bridging or face-capping coordination modes, hydrides adsorption energies, the possible presence of interstitial hydrogen atoms, the dependence of (1)H chemical shifts on ligands and on electron counting.

Membrane interaction of amphotericin B as single-length assembly examined by solid state NMR for uniformly 13C-enriched agent

The membrane interaction of amphotericin B (AmB), one of the most important anti-fungal drugs, was investigated by solid state NMR measurements of uniformly 13C-enriched AmB, which was prepared by the culture of the drug-producing microorganism in the presence of [u-13C6]glucose. All the 13C NMR signals of AmB upon binding to DLPC membrane were successfully assigned on the basis of the 13C-13C correlation spectrum. 13C-31P RDX (Rotational-Echo Double Resonance for X-clusters) experiments clearly revealed the REDOR dephasing effects for carbon atoms residing in the both terminal parts, whereas no dephasing was observed for the middle parts including polyolefinic C20-C33 and hydroxyl-bearing C8/C9 parts. These observations suggest that AmB binds to DLPC membrane with a high affinity to the phospholipid and spans the membrane with a single molecular length.

Multi-dimensional 1H-13C HETCOR and FSLG-HETCOR NMR study of sphingomyelin bilayers containing cholesterol in the gel and liquid crystalline states

(13)C cross polarization magic angle spinning (CP-MAS) and (1)H MAS NMR spectra were collected on egg sphingomyelin (SM) bilayers containing cholesterol above and below the liquid crystalline phase transition temperature (T(m)). Two-dimensional (2D) dipolar heteronuclear correlation (HETCOR) spectra were obtained on SM bilayers in the liquid crystalline (L(alpha)) state for the first time and display improved resolution and chemical shift dispersion compared to the individual (1)H and (13)C spectra and significantly aid in spectral assignment. In the gel (L(beta)) state, the (1)H dimension suffers from line broadening due to the (1)H-(1)H homonuclear dipolar coupling that is not completely averaged by the combination of lipid mobility and MAS. This line broadening is significantly suppressed by implementing frequency switched Lee-Goldburg (FSLG) homonuclear (1)H decoupling during the evolution period. In the liquid crystalline (L(alpha)) phase, no improvement in line width is observed when FSLG is employed. All of the observed resonances are assignable to cholesterol and SM environments. This study demonstrates the ability to obtain 2D heteronuclear correlation experiments in the gel state for biomembranes, expands on previous SM assignments, and presents a comprehensive (1)H/(13)C NMR assignment of SM bilayers containing cholesterol. Comparisons are made to a previous report on cholesterol chemical shifts in dimyristoylphosphatidylcholine (DMPC) bilayers. A number of similarities and some differences are observed and discussed.

Determination of the orientation and dynamics of ergosterol in model membranes using uniform 13C labeling and dynamically averaged 13C chemical shift anisotropies as experimental restraints

A new strategy was established to determine the average orientation and dynamics of ergosterol in dimyristoylphosphatidylcholine model membranes. It is based on the analysis of chemical shift anisotropies (CSAs) averaged by the molecular dynamics. Static (13)C CSA tensors were computed by quantum chemistry, using the gauge-including atomic-orbital approach within Hartree-Fock theory. Uniformly (13)C-labeled ergosterol was purified from Pichia pastoris cells grown on labeled methanol. After reconstitution into dimyristoylphosphatidylcholine lipids, the complete (1)H and (13)C assignment of ergosterol's resonances was performed using a combination of magic-angle spinning two-dimensional experiments. Dynamically averaged CSAs were determined by standard side-band intensity analysis for isolated (13)C resonances (C(3) and ethylenic carbons) and by off-magic-angle spinning experiments for other carbons. A set of 18 constraints was thus obtained, from which the sterol's molecular order parameter and average orientation could be precisely defined. The validity of using computed CSAs in this strategy was verified on cholesterol model systems. This new method allowed us to quantify ergosterol's dynamics at three molar ratios: 16 mol % (Ld phase), 30 mol % (Lo phase), and 23 mol % (mixed phases). Contrary to cholesterol, ergosterol's molecular diffusion axis makes an important angle (14 degrees) with the inertial axis of the rigid four-ring system.