Application of Spin Labels To Membrane Bioenergetics

DFT study of nitroxide radicals: explicit modeling of solvent effects on the structural and electronic characteristics of 4‐amino‐2,2,6,6‐tetramethyl‐piperidine‐N‐oxyl

“…This method gives significant gain in the computation efficiency: the total run time for PBE/ccpvnz was ∼20-30 times shorter as compared with rather expensive B3LYP/EPR-II and PBE0/EPR-II methods. For complete aqueous sphere around TA(H + ) (n H 2 O = 45), calculations with B3LYP/EPR-II, PBE0/EPR-II and PBE/ccpvnz lead to a iso (ρ N2 ) ≈ 15.26-15.84 G; these values are somewhat lower (by ≈1 G) than experimental constant for protonated TA in water solution, a exp iso (water) = 16.9 G. [14,17,22] There may be several reasons for certain underestimation of calculated constant. One of them might be an insufficiently high size of a water cluster for simulating the 'bulk-like' water surroundings.…”

“…[1 -22] One of the most informative parameters of the EPR spectrum of a spin-label is the hyperfine splitting constant, a iso , the value of which is determined by the Fermi contact interaction of the unpaired electron with the nitrogen nucleus in the nitroxide fragment >N-O • . [16] Among a great variety of different spin-labels, the nitroxide radicals with the proton-accepting groups (the amine-type, [2,4,5,14,17] imidazolinebased and imidazolidine-based nitroxides [6,8,13,16,19] ) are of special interest as molecular probes for measuring pH values in chemical and biological systems, including the energy-transducing organelles of the living cell [2,4,12 -14,17,22] and polarity profiles in the intraprotein cavities and biomembranes. [9 -12,15 -18,20,21] There are two different approaches to the measurements of the local pH and polarity with pH-sensitive spin-labels.…”

“…[22] Another approach of measuring the pH values inside closed vesicles comes from the pH-dependent partitioning of the amine-type spin-labels between external medium and internal compartments of the vesicles. [2,4,14,17] An amine distribution technique is based on the assumption that the partition of probing molecules between the vesicles' interior and the surrounding medium is determined by the ratio of hydrogen ion activities inside to outside the vesicles. [2,14] One of the amine-type nitroxides, which is widely used for measuring the transmembrane pH difference in bioenergetic systems, [2,14,17] is a water-soluble spin-label 4-amino-2,2,6,6-tetramethyl-piperidine-N-oxyl (TA).…”

EPR Spectroscopy — A Valuable Tool to Study Photosynthesizing Organisms Exposed to Abiotic Stresses

“…By addition of a membrane-impermeable paramagnetic compound, chromium oxalate (which broadens the EPR signal from TEMPAMINE in the external medium), into chloroplast suspension, the probe molecules occurring outside and inside of the thylakoid can be visualized. The second method is based on the measurement of EPR spectra of pH-sensitive spin probes (imidazoline nitroxide radicals) loaded into the vesicles [158].…”

EPR at work: Part 1

“…The ⌬m s ϭ Ϯ2 transition, at g ϭ 4, also called the half-field transition, was shown in no. 30. Photolysis of tryptophan, tyrosine, and phenylalanine in frozen aqueous solution yield triplet-state EPR spectra for which the half-field transition and the formation and decay rates are shown in EPR in the World of Biochemistry.…”