@article{chestnut_milikisiyants_chatterjee_kern_smirnov_2021, title={Electronic Structure of the Primary Electron Donor P-700(+center dot) in Photosystem I Studied by Multifrequency HYSCORE Spectroscopy at X- and Q-Band}, volume={125}, ISSN={["1520-5207"]}, url={https://doi.org/10.1021/acs.jpcb.0c09000}, DOI={10.1021/acs.jpcb.0c09000}, abstractNote={The primary electron donor P700 of the photosystem I (PSI) is a heterodimer consisting of two chlorophyll molecules. A series of electron-transfer events immediately following the initial light excitation leads to a stabilization of the positive charge by its cation radical form, P700+•. The electronic structure of P700+• and, in particular, its asymmetry with respect to the two chlorophyll monomers is of fundamental interest and is not fully understood up to this date. Here, we apply multifrequency X- (9 GHz) and Q-band (35 GHz) hyperfine sublevel correlation (HYSCORE) spectroscopy to investigate the electron spin density distribution in the cation radical P700+• of PSI from a thermophilic cyanobacterium Thermosynechococcus elongatus. Six 14N and two 1H distinct nuclei have been resolved in the HYSCORE spectra and parameters of the corresponding nuclear hyperfine and quadrupolar hyperfine interactions were obtained by combining the analysis of HYSCORE spectral features with direct numerical simulations. Based on a close similarity of the nuclear quadrupole tensor parameters, all of the resolved 14N nuclei were assigned to six out of total eight available pyrrole ring nitrogen atoms (i.e., four in each of the chlorophylls), providing direct evidence of spin density delocalization over the both monomers in the heterodimer. Using the obtained experimental values of the 14N electron-nuclear hyperfine interaction parameters, the upper limit of the electron spin density asymmetry parameter is estimated as RA/Bupper = 7.7 ± 0.5, while a tentative assignment of 14N observed in the HYSCORE spectra yields RB/A = 3.1 ± 0.5.}, number={1}, journal={JOURNAL OF PHYSICAL CHEMISTRY B}, publisher={American Chemical Society (ACS)}, author={Chestnut, Melanie M. and Milikisiyants, Sergey and Chatterjee, Ruchira and Kern, Jan and Smirnov, Alex I}, year={2021}, month={Jan}, pages={36–48} }
 @article{kovaleva_molochnikov_tambasova_marek_chestnut_osipova_antonov_kirilyuk_smirnov_2020, title={Electrostatic properties of inner nanopore surfaces of anodic aluminum oxide membranes upon high temperature annealing revealed by EPR of pH-sensitive spin probes and labels}, volume={604}, ISSN={["1873-3123"]}, url={https://doi.org/10.1016/j.memsci.2020.118084}, DOI={10.1016/j.memsci.2020.118084}, abstractNote={Anodic aluminum oxide (AAO) membranes are versatile nanomaterials that combine the chemically stable and mechanically robust properties of ceramics with homogeneous nanoscale organization that can be tuned to desirable pore diameters and lengths. The AAO substrates feature high surface area that is readily accessible to large and small molecules, making these nanostructures uniquely suited for many possible applications. Examples include templated self-assembly of macroscopically aligned biological membranes and substrates for heterogeneous catalysis. For further development of such applications, one would like to characterize and tune the electrostatic properties of the inner pore surface as well as the local acidity within the nanochannels. Here, we employed electron paramagnetic resonance (EPR) spectroscopy of a small molecule – ionizable nitroxide – as a reporter of the average local acidity in the nanochannels and the local electrostatic potential in the immediate vicinity of the pore surface. The former was achieved by measuring EPR spectra of this molecular probe diffusing in an aqueous phase confined in the AAO nanochannels while for the latter the nitroxide was covalently attached to the hydroxyl group of the alumina surface. We show that the local acidity within the nanochannels is increased by as much as ≈1.48 pH units vs. the pH of bulk solution by decreasing the pore diameter down to ca. 31 nm. Furthermore, the positive surface charge of the as-prepared AAO could be decreased and even switched to a negative surface charge upon annealing the membranes first to 700 °C and then to 1200 °C. For as-prepared AAO, the local electrostatic potential reaches ψ= (163 ± 5) mV for the nitroxide label covalently attached to AAO and located about 0.5 nm away from the surface. Overall, we demonstrate that the acid-based properties of the aqueous volume confined by the AAO nanopores pores can be tuned by either changing the pore diameter from ca. 71 to 31 nm or by thermal annealing to switch the sign of the surface charge. These observations provide a simple and robust means to tailor these versatile high-surface-area nanomaterials for specific applications that depend on acid-base equilibria.}, journal={JOURNAL OF MEMBRANE SCIENCE}, publisher={Elsevier BV}, author={Kovaleva, Elena G. and Molochnikov, Leonid S. and Tambasova, Daria and Marek, Antonin and Chestnut, Melanie and Osipova, Victoria A. and Antonov, Denis O. and Kirilyuk, Igor A. and Smirnov, Alex I}, year={2020}, month={Jun} }
 @article{acharya_chestnut_marek_smirnov_krim_2017, title={A Combined QCM and AFM Study Exploring the Nanoscale Lubrication Mechanism of Silica Nanoparticles in Aqueous Suspension}, volume={65}, ISSN={["1573-2711"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85026840191&partnerID=MN8TOARS}, DOI={10.1007/s11249-017-0898-5}, number={3}, journal={TRIBOLOGY LETTERS}, author={Acharya, B. and Chestnut, M. and Marek, A. and Smirnov, A. I. and Krim, J.}, year={2017}, month={Sep} }