High-frequency Electron Nuclear Double Resonance Research Articles

Identifikatsiya opticheski aktivnykh kvartetnykh spinovykh tsentrov na osnove vakansii kremniya v SiC, perspektivnykh dlya kvantovykh tekhnologiy

Optically active (bright) and optically inactive (dark) quartet S = 3/2 spin color centers including a negatively charged Si vacancy have been identified in silicon carbide using high-frequency electron nuclear double resonance on the nuclei of the 13C isotope, enhanced by a tenfold increase in its content. The alignment of populations of spin levels is optically induced in a bright center promising for quantum technologies, whereas the populations of spin levels in a dark center, which is an isolated negatively charged Si vacancy V-Si, correspond to a Boltzmann distribution and do not change under optical excitation.

Manifestations of Electron–Nuclear Interactions in the High-Frequency ENDOR/ODMR Spectra for Triplet Si–C Divacancies in 13C-Enriched SiC

The frequencies of electron–nuclear interactions with 13C and 29Si nuclei on remote coordination spheres are determined in triplet spin centers in the form of neutral VSi–VC divacancies in a silicon carbide crystal of the hexagonal polytype 6H–SiC enriched tenfold in the 13C isotope. High-frequency electron–nuclear double resonance and optically detected magnetic resonance under conditions of optical alignment of spins are used. Oscillations of the electron spin density on 29Si and 13C nuclei are found. Nuclear magnetic resonance transitions at Larmor and close-to-Larmor frequencies of 13C and 29Si cause giant changes in the populations of spin sublevels with the transformation of these resonances into electron paramagnetic resonance and optical signals.

Use of spin labels to study membrane proteins by high-frequency electron nuclear double resonance spectroscopy

The applicability of spin labels to study membrane proteins by high-frequency electron nuclear double resonance spectroscopy is demonstrated. With the use of bacteriorhodopsin embedded in a lipid membrane as an example, the spectra of protons of neighboring amino acids are recorded, electric field gradients at the membrane surface are detected, and the constant of hyperfine interaction with the chlorine nucleus at the site of ion trapping is measured.

High-Frequency 94 GHz ENDOR Characterization of the Metal Binding Site in Wild-Type Ras·GDP and Its Oncogenic Mutant G12V in Frozen Solution

The guanine nucleotide binding protein Ras plays a central role as molecular switch in cellular signal transduction. Ras cycles between a GDP-bound "off" state and a GTP-bound "on" state. Specific oncogenic mutations in the Ras protein are found in up to 30% of all human tumors. Previous 31P NMR studies had demonstrated that in liquid solution different conformational states in the GDP-bound as well as in the GTP-bound form coexist. High-field EPR spectroscopy of the GDP complexes in solution displayed differences in the ligand sphere of the wild-type complex as compared to its oncogenic mutant Ras(G12V). Only three water ligands were found in the former with respect to four in the G12V mutant [Rohrer, M. et al. (2001) Biochemistry 40, 1884-1889]. These differences were not detected in previous X-ray structures in the crystalline state. In this paper, we employ high-frequency electron nuclear double resonance (ENDOR) spectroscopy to probe the ligand sphere of the metal ion in the GDP-bound state. This technique in combination with selective isotope labeling has enabled us to detect the resonances of nuclei in the first ligand sphere of the ion with high spectral resolution. We have observed the 17O ENDOR spectra of the water ligands, and we have accurately determined the 17O hyperfine coupling with a(iso) = -0.276 mT, supporting the results of previous line shape analysis in solution. Further, the distinct resonances of the alpha-, beta-, and gamma-phosphorus of the bound nucleotides are illustrated in the 31P ENDOR spectra, and their hyperfine tensors lead to distances in agreement with the X-ray structures. Finally, 13C ENDOR spectra of uniformly 13C-labeled Ras(wt) x GDP and Ras(G12V) x GDP complexes as well as of the Ras(wt) x GppNHp and the selectively 1,4-13C-Asp labeled Ras(wt) x GDP complexes have revealed that in frozen solution only one amino acid is ligated to the ion in the GDP state, whereas two are bound in the GppNHp complex. Our results suggest that a second conformational state of the protein, if correlated with a different ligand sphere of the Mn2+ ion, is not populated in the GDP form of Ras at low temperatures in frozen solution.

Electron Transfer Pathways and Protein Response to Charge Separation in Photosynthetic Reaction Centers: Time-Resolved High-Field ENDOR of the Spin-Correlated Radical Pair P865+QA-

Recently we reported the first observation of time-resolved (TR) high-frequency (HF) electron nuclear double resonance (ENDOR) of the transient charge separated state P865(+)Q(-)A in purple photosynthetic bacterial reaction centers (RC) (Poluektov, O. G., et al. J. Am. Chem. Soc. 2004, 126, 1644-1645). The high resolution and orientational selectivity of HF ENDOR allows us to directly probe protein environments by spectrally selecting specific nuclei in isotopically labeled samples. A new phenomenon associated with the spin correlated radical pair (SCRP) nature of P865(+)Q(-)A was observed. The TR-HF ENDOR spectra of protein nuclei (protons) surrounding deuterated QA(-) exhibit a derivative-like, complicated line shape, which differs considerably from the HF ENDOR spectrum of the protein nuclei surrounding thermally equilibrated QA(-). Here, a theoretical analysis of these observations is presented that shows that the positions and amplitudes of ENDOR lines contain information on hyperfine interactions (HFI) of a particular nucleus (a proton of the protein) with both correlated electron spins. Thus, spin density delocalization in the protein environment between the SCRP donor and acceptor molecules can be revealed via HF ENDOR. Novel approaches for acquiring and analyzing SCRP ENDOR that simplify interpretation of the spectra are discussed. Furthermore, we report here that the positions of the ENDOR lines of the SCRP shift with an increase in the time after laser flash, which initiates electron transfer. These shifts provide direct spectroscopic evidence of reorganization of the protein environment to accommodate the donor-acceptor charge-separated state P865(+)QA(-).

Spatial distribution of the wave function of the self-trapped exciton in AgCl.

The results are presented of a high-frequency electron nuclear double resonance (ENDOR) study of the lowest triplet state of the self-trapped exciton in AgCl. Direct information about the spatial distribution of the wave function is obtained, which confirms that the exciton in its triplet state is built up from a strongly localized self-trapped hole and a very diffuse electron. The electron is mainly contained in a hydrogenlike 1s orbital with a Bohr radius of 15.1\ifmmode\pm\else\textpm\fi{}0.6 \AA{}, but near its center the electron density deviates from spherical symmetry and reflects the ${\mathit{D}}_{4\mathit{h}}$ charge distribution of the hole. From a comparison with the results of an ENDOR study of the self-trapped hole, it is concluded that the microscopic and electronic structure of a self-trapped hole is not significantly altered by the attraction of a shallow electron. No evidence is obtained for the formation of Frenkel pairs in AgCl upon ultraviolet irradiation at low temperatures. \textcopyright{} 1996 The American Physical Society.