Conventional Electron Spin Resonance Spectroscopy Research Articles

Coherent Addressing of Single Molecular Electron Spin Qubits

AbstractWith rational designability, versatile tunability, and quantum coherence, molecular electron spin qubits could offer new opportunities for quantum information science, enabling simplified implementation of quantum algorithms and chemical‐specific quantum sensing. The development of these transformative technologies relies on coherent addressing of single molecular electron spin qubits with high initialization, manipulation, and readout fidelities. This is unfeasible to conventional electron spin resonance spectroscopy, which is widely used for coherent addressing of ensemble electron spins, due to its low initialization efficiency and readout sensitivity. Taking advantage of single spin detectability of single‐molecule spectroscopy, scanning tunneling microscopy, atomic force microscopy, and quantum metrology, several strategies have been developed to empower electron spin resonance spectroscopy with single qubit addressability. In this Emerging Topic, we introduce principles and technical implementation of strategies for coherent addressing of single molecular electron spin qubits, discuss their potential applicability in quantum technologies, and point out their challenges in terms of scalability, molecular design, and/or decoherence suppression. We discuss future directions to overcome these challenges and to improve single qubit addressing technologies, which will facilitate the advancement of molecular quantum information science.

Paramagnetic defects of silicon nanowires

The paramagnetic defects in and on Si nanowires (SiNWs) obtained by oxide-assisted growth were studied by conventional electron spin resonance spectroscopy. For the as-grown nanowires, three different defects were found: Dangling bonds or Pb-centers with g=2.0065, located at the interface of the crystalline core to the surrounding oxide, E′-centers with g=2.0005 and EX-centers with g=2.00252, located in the oxide. For the EX-centers, the characteristic hyperfine lines separated by 16.4G were detected. The as-grown SiNWs showed a spin density of about 1018cm−3. H termination of the nanowires via hydrofluoric acid decreases the spin density drastically to 3×1016cm−3. The optical absorption spectra of SiNWs determined by photothermal deflection spectroscopy are comparable to those of microcrystalline silicon and show a similar decrease of defect density upon H termination.

Anomalous metallic lithium phases: Identification by ESR, ENDOR, and the bistable Overhauser effect

Metallic lithium particles precipitated in lithium hydride by UV irradiation are investigated by electron spin resonance (ESR), electron-nuclear double resonance (ENDOR), and ESR at low temperatures under conditions of bistable Overhauser effect [Phys. Rev. B 47, 15 023 (1993)]. Although the conventional ESR spectroscopy gives only a single structureless line for conduction electrons, ENDOR clearly shows the existence of two well-defined populations of lithium particles with different crystallographic structures. The presence of a quadrupole structure in the ENDOR spectrum indicates that one of the two populations belongs to the $9R$ close-packed phase of metallic lithium, normally stable below 80 K. The other population belongs to a cubic phase. At low temperatures the saturation of the ESR line produces a bistable Overhauser effect which gives a hysteresis of the ESR trace. This bistable conduction electron spin resonance clearly confirms the existence of two different populations of lithium particles. The presence of a broad hysteresis of the resonance at 4 K is the consequence of a very long ${T}_{2},$ which amounts to $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{s}$ and ${10}^{\ensuremath{-}6}\mathrm{s}$ in the cubic and the noncubic phases, respectively. These values are much larger than those deduced from the linewidth of the unsaturated ESR line. It is concluded that ENDOR combined with bistable ESR could provide very selective methods for the investigation of the lithium anode in lithium batteries.

Structural information on a membrane transport protein from nuclear magnetic resonance spectroscopy using sequence-selective nitroxide labeling.

The lactose transport protein (LacS) from Streptococcus thermophilus bearing a single cysteine mutation, K373C, within the putative interhelix loop 10-11 has been overexpressed in native membranes. Cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy (NMR) could selectively distinguish binding of (13)C-labeled substrate to just 50-60 nmol of LacS(K373C) in the native fluid membranes. Nitroxide electron spin-label at the K373C location was essentially immobile on the time scale of both conventional electron spin resonance spectroscopy (ESR) (<10(-8)s) and saturation-transfer ESR (<10(-3)s), under the same conditions as used in the NMR studies. The presence of the nitroxide spin-label effectively obscured the high-resolution NMR signal from bound substrate, even though (13)C-labeled substrate was shown to be within the binding center of the protein. The interhelix loop 10-11 is concluded to be in reasonably close proximity to the substrate binding site(s) of LacS (<15 A), and the loop region is expected to penetrate between the transmembrane segments of the protein that are involved in the translocation process.

Maleimide, Iodoacetamide, Indanedione, and Chloromercuric Spin Label Reagents with Derivatized Nitroxide Rings as ESR Reporter Groups for Protein Conformation and Dynamics

The syntheses of eight nitroxide spin labels which bear maleimide, iodoacetamide, indanedione, or chloromercuric reactive groups and, in addition, a second substituent in the nitroxide ring are presented. The second substituent groups range from hydrophobic and hydrophilic esters to carboxylic acid and secondary and tertiary amine groups. The resulting spin labels are characterized with respect both to protein covalent modification and to the electron spin resonance spectral properties of the bound labels. The effect of the various substituents in the spin label on the reactivity toward the membrane-bound shark rectal gland and pig kidney Na,K-ATPase is described. The spectral differences between immobilized and mobile groups observed by electron spin resonance for the different protein-bound spin labels show that, by selecting an appropriate derivative for modification, a large range of different motional sensitivities of the reporter group can be obtained. Such different series of spin labels should therefore be useful for detecting mobility changes arising from conformational transitions in proteins by conventional electron spin resonance spectroscopy or for measurement of protein rotational diffusion using saturation transfer electron spin resonance spectroscopy. The chloromercuric series is found to be particularly useful because of the high reactivity, the lack of reversibility that potentially is associated with the Michael addition reaction, and the wide range of rotational mobility that is exhibited by the different derivatives.

Oxygen diffusion-concentration product in rhodopsin as observed by a pulse ESR spin labeling method

Permeation of molecular oxygen in rhodopsin, an integral membrane protein, has been investigated by monitoring the bimolecular collision rate between molecular oxygen and the nitroxide spin label using a pulse electron spin resonance (ESR) T1 method. Rhodopsin was labeled by regeneration with the spin-labeled 9-cis retinal analogue in which the beta-ionone ring of retinal is replaced by the nitroxide tetramethyl-oxypyrrolidine ring. The bimolecular collision rate was evaluated in terms of an experimental parameter W(x), defined as T1(-1)(air,x)--T1(-1)(N2,x) where T1's are the spin-lattice relaxation times of the nitroxide in samples equilibrated with atmospheric air and nitrogen respectively, which is proportional to the product of local oxygen concentration and local diffusion coefficient (transport). W-values at the beta-ionone binding site in spin-labeled rhodopsin are in the range of 0.02-0.13 microseconds-1, which are 10-60 times smaller than W's in water and 1.1-20 times smaller than in model membranes in the gel phase, indicating that membrane proteins create significant permeation resistance to transport of molecular oxygen inside and across the membrane. W(thereby the oxygen diffusion-concentration product) is larger in the meta II-enriched sample than in rhodopsin, indicating light-induced conformational changes of opsin around the beta-ionone binding site. W decreases with increase of temperature for both rhodopsin and meta II-enriched samples, suggesting that temperature-induced conformational changes take place in both samples. These changes were not observable using conventional ESR spectroscopy. It is concluded that W is a sensitive monitor of conformational changes of proteins.

A spin-trapping database implemented on the IBM PC/AT

Spin trapping is a technique in which highly reactive free radicals are detected by reaction with a diamagnetic molecule (usually a nitrone or nitroso compound) to form more stable free radicals (spin adducts) that can be observed by conventional electron spin resonance spectroscopy (1). Since its inception in 1969, spin trapping has gained wide acceptance in the study of short-lived reactive free radical species generated in chemical reactions, as well as in biochemical and biological systems (26). More recently, the technique has been employed to detect free radicals in isolated organs and intact animals (7, 8). The breadth of spin-trapping applications ranges from the undergraduate physical chemistry laboratory (9) to high-energy physics (IO). The number of useful spin traps has also increased and now includes compounds such as unsaturated substituted alkanes (10) and quinones (1 I). At present, Chemical Abstracts lists nearly 1000 references that contain the key word “spin trap(ping).” Unfortunately, abstracts often do not contain the ESR parameters of the spin adducts and other such useful information. Even if these data are present, it is usually necessary to consult the full publication for the experimental conditions, since ESR parameters are known to be system dependent (12). Buettner (2) has recently published a list of spin adducts derived from DMPO,’ PBN, MNP, and other popular spin traps. Each entry contains the type of adduct( the solvent system, the ESR parameters, experimental information, and reference. This compilation is very useful for finding the range of hyperfine parameters for a known adduct. However, in order to search for spin adducts with a given set of hyperfine parameters, obtained under defined experimental conditions, it is necessary to peruse the entire list. This approach is similar to trying to find an address in a telephone book using only a telephone number. The chore of manually searching a vast amount of printed information, plus the inflexibility in arranging such information, warrants the use of a computer. For information retrieval purposes a computer system is much less tedious, less time consuming, and less error prone than a paper-based system.