Luminita Duma

Through-space contributions to two-dimensional double-quantum J correlation NMR spectra of magic-angle-spinning solids

A routinely used assumption when interpreting two-dimensional NMR spectra obtained with a commonly used double-quantum (DQ) magic-angle-spining (MAS) pulse sequence referred to as the refocused incredible natural abundance double-quantum transfer experiment (INADEQUATE) [A. Lesage, M. Bardet, and L. Emsley, J. Am. Chem. Soc. 121, 10987 (1999)] has been that correlation peaks are only observed for pairs of nuclei with a through-bond connectivity. The validity of this assumption is addressed here by theory, experiment, and computer simulations. If the isotropic chemical shifts of the two nuclei are different and the MAS frequency is far from rotational resonance, the theoretical description demonstrates that DQ correlation peaks are indeed indicative of a J coupling. However, if the isotropic chemical shifts are the same, it is shown that DQ peaks can appear for pairs of nuclei even in the absence of a through-bond J coupling. These peaks appear in the specific case of a pair of nuclei with a nonzero through-space dipole-dipole coupling and chemical shift anisotropy tensors having different principal magnitudes or orientations, provided that the MAS frequency is comparable to or smaller than the chemical shift anisotropies. Experimental 31P spectra recorded on a sample of TiP2O7 and computer simulations show that the magnitude of these anomalous peaks increases with increasing B0 magnetic field and that they decrease with increasing MAS frequency. This behavior is explained theoretically.

Spin-state selection in solid-state NMR.

Spin-state selection in solid-state NMR is demonstrated, using similar pulse sequences as used in liquid-state NMR. The different transitions of all three carbon resonances in fully 13C-labeled L-alanine are separated in different spectra. By selecting spin-states, the contribution of the J-coupling to the linewidth is removed, leading to a considerable enhancement in resolution. The spin-state-selective technique is demonstrated for magic-angle spinning frequencies from 6 to 35kHz. Other experimental conditions affecting the sensitivity of the experiments are discussed. Sensitivity losses due to the introduction of the spin-state-selective filter are shown to be acceptable. Finally, spin-state selection was used to experimentally confirm the differential broadening expected for the two transitions of the CH3 resonance.

Investigation of the interface in silica-encapsulated liposomes by combining solid state NMR and first principles calculations.

In the context of nanomedicine, liposils (liposomes and silica) have a strong potential for drug storage and release schemes: such materials combine the intrinsic properties of liposome (encapsulation) and silica (increased rigidity, protective coating, pH degradability). In this work, an original approach combining solid state NMR, molecular dynamics, first principles geometry optimization, and NMR parameters calculation allows the building of a precise representation of the organic/inorganic interface in liposils. {(1)H-(29)Si}(1)H and {(1)H-(31)P}(1)H Double Cross-Polarization (CP) MAS NMR experiments were implemented in order to explore the proton chemical environments around the silica and the phospholipids, respectively. Using VASP (Vienna Ab Initio Simulation Package), DFT calculations including molecular dynamics, and geometry optimization lead to the determination of energetically favorable configurations of a DPPC (dipalmitoylphosphatidylcholine) headgroup adsorbed onto a hydroxylated silica surface that corresponds to a realistic model of an amorphous silica slab. These data combined with first principles NMR parameters calculations by GIPAW (Gauge Included Projected Augmented Wave) show that the phosphate moieties are not directly interacting with silanols. The stabilization of the interface is achieved through the presence of water molecules located in-between the head groups of the phospholipids and the silica surface forming an interfacial H-bonded water layer. A detailed study of the (31)P chemical shift anisotropy (CSA) parameters allows us to interpret the local dynamics of DPPC in liposils. Finally, the VASP/solid state NMR/GIPAW combined approach can be extended to a large variety of organic-inorganic hybrid interfaces.

Exchange Rate Constants of Invisible Protons in Proteins Determined by NMR Spectroscopy

Although labile protons that are exchanging rapidly with those of the solvent cannot be observed directly, their exchange rate constants can be determined by indirect detection of scalar‐coupled neighboring nuclei. We have used heteronuclear NMR spectroscopy to measure the exchange rate constants of labile protons in the side chains of lysine and arginine residues in ubiquitin enriched in carbon‐13 and nitrogen‐15 at neutral pH. Exchange rate constants as fast as 40×103 s−1 were thus measured. These results demonstrate that NMR spectroscopy is a powerful tool for the characterization of lysine NH3+ and arginine NH groups in proteins at physiologically relevant pH values.

Molecularly Imprinted Polymer Coated Quantum Dots for Multiplexed Cell Targeting and Imaging.

Advanced tools for cell imaging are of great interest for the detection, localization, and quantification of molecular biomarkers of cancer or infection. We describe a novel photopolymerization method to coat quantum dots (QDs) with polymer shells, in particular, molecularly imprinted polymers (MIPs), by using the visible light emitted from QDs excited by UV light. Fluorescent core-shell particles specifically recognizing glucuronic acid (GlcA) or N-acetylneuraminic acid (NANA) were prepared. Simultaneous multiplexed labeling of human keratinocytes with green QDs conjugated with MIP-GlcA and red QDs conjugated with MIP-NANA was demonstrated by fluorescence imaging. The specificity of binding was verified with a non-imprinted control polymer and by enzymatic cleavage of the terminal GlcA and NANA moieties. The coating strategy is potentially a generic method for the functionalization of QDs to address a much wider range of biocompatibility and biorecognition issues.

Revealing molecular self-assembly and geometry of non-covalent halogen bonding by solid-state NMR spectroscopy

We report a new spectroscopic fingerprint of intermolecular contacts in halogen bond-driven self-assembling aggregates and a precise determination of intermolecular NI distances in microcrystalline samples.

Carbon-13 lineshapes in solid-state NMR of labeled compounds. Effects of coherent CSA-dipolar cross-correlation

The experimental lineshapes of the carboxyl and methyl carbon resonances of fully 13C enriched L-Alanine are studied in detail at different MAS frequencies and decoupling field strengths. Complex lineshapes at intermediate spinning speeds were explained by the joint effect of off rotational resonance and coherent CSA-dipolar cross-correlation. Whereas off rotational-resonance effects lead to complex lineshapes due to a splitting of some energy levels, coherent CSA-dipolar cross-correlation introduces either a differential intensity and/or a differential broadening of the lines of the J-multiplet. The conditions which lead to such effects are explained and experimentally verified. Additional simulations show that these effects can be expected over a wide range of static magnetic fields and are not restricted to L-Alanine.

Plastic Antibodies for Cosmetics: Molecularly Imprinted Polymers Scavenge Precursors of Malodors

Molecularly imprinted polymers (MIPs) are synthetic antibody mimics capable of specific molecular recognition. Advantageously, they are more stable, easy to tailor for a given application and less expensive than antibodies. These plastic antibodies are raising increasing interest and one relatively unexplored domain in which they could outplay these advantages particularly well is cosmetics. Here, we present the use of a MIP as an active ingredient of a cosmetic product, for suppressing body odors. In a dermo-cosmetic formulation, the MIP captures selectively the precursors of malodorous compounds, amidst a multitude of other molecules present in human sweat. These results pave the way to the fabrication of a novel generation of MIPs with improved selectivities in highly complex aqueous environments, and should be applicable to biotechnological and biomedical areas as well.

Broadband Dipolar Recoupling for Magnetization Transfer in Solid-State NMR Correlation Spectroscopy

Recent developments in solid-state nuclear magnetic resonance have opened the way to detailed structural and dynamic analysis of crystalline and non-crystalline biological solids. In isotopically enriched molecules of biological interest, C–C magnetization transfer via spin-exchange processes (also called spin diffusion) allows one both to assign the resonances and to determine internuclear distances. Chemical-shift correlation experiments commonly employed for such purposes exploit different means of compensating for the energy imbalances of C spins with different resonance frequencies and use a variety of recoupling schemes which inhibit the quenching of homoand heteronuclear dipolar interactions by magic angle spinning (MAS). Spin exchange can be promoted by any of three methods: 1) Proton-driven spin diffusion (PDSD) which does not require any radio-frequency (RF) irradiation and relies on line broadening due to carbon–proton dipolar couplings, so that its efficiency strongly depends on the spinning frequency and on local motions. 2) Rotor-driven spin exchange which occurs in rotational resonance (R) methods, and does not use any RF irradiation either. It strongly favors flip-flop processes between pairs of C spins which have a difference in isotropic chemical shifts that matches an integer multiple of the spinning frequency. 3) RF-driven spin exchange which relies on homoand heteronuclear recoupling of the dipolar interactions. A wide range of recoupling sequences has been developed, including a number of broadband homonuclear recoupling schemes. For optimal performance, especially at high MAS frequencies, most known recoupling methods require high RF power which can lead to excessive sample heating at longer transfer times. Some dipolar recoupling methods employ reduced RF power on C or on H, like dipolar assisted rotational resonance (DARR) or RF-assisted diffusion (RAD) experiments with RF amplitudes adjusted to the spinning frequency nr . This enhances C–C spin exchange through recoupling of homoand heteronuclear dipolar interactions. In fact, rotary resonance recoupling (R) using various ratios n= n1H /nr=1/2, 1 and 2 leads to the restoration of different anisotropic spin interactions. However, the efficiency of rotary resonance recoupling, including DARR/RAD-like experiments, is critically dependent on resonance offsets and on the homogeneity of the RF fields, and may lead to non-uniform C–C spin exchange between various sites. Herein, we introduce a broadband variant of rotary resonance recoupling (BR) aimed at promoting improved C–C transfer. In Figure 1, it is shown that the RF irradiation is ap-

Fast Proton Exchange in Histidine: Measurement of Rate Constants through Indirect Detection by NMR Spectroscopy

Owing to its imidazole side chain, histidine participates in various processes such as enzyme catalysis, pH regulation, metal binding, and phosphorylation. The determination of exchange rates of labile protons for such a system is important for understanding its functions. However, these rates are too fast to be measured directly in an aqueous solution by using NMR spectroscopy. We have obtained the exchange rates of the NH3+ amino protons and the labile NHε2 and NHδ1 protons of the imidazole ring by indirect detection through nitrogen-15 as a function of temperature (272 K<T<293 K) and pH (1.3<pH<4.9) of uniformly nitrogen-15- and carbon-13-labeled l-histidine⋅HCl⋅H2O. Exchange rates up to 8.5×104 s−1 could be determined (i.e., lifetimes as short as 12 μs). The three chemical shifts δHi of the invisible exchanging protons Hi and the three one-bond scalar coupling constants 1J(N,Hi) could also be determined accurately.