Low temperature EPR studies

Wasielewski and coworkers have studied a series of molecules in which porphyrins are linked to quinones and similar electron acceptors, and determined that when a solvent such as 2-methyltetrahydrofuran freezes, about 0.8 eV of driving force is lost [116]. These researchers have prepared dyads with a driving force for photoinduced electron transfer greater than this value in polar solvents, and found that they are capable of displaying photoinduced electron transfer even in frozen organic glasses. This, in turn, has opened the door to low-temperature EPR studies of the charge-separated states in various donor-acceptor systems and their decay pathways [66, 99, 117]. 

It is interesting to note that upon dilution of the apoprotein in 0.05 M sodium dodecyl sulfate at pH 7.3, a loss of the rotatory perturbations in the 250—300 nm range was detected while the iron-conalbumin complex dissociated, as detected by a disappearance of the absorption band at 270 nm, suggesting the detergent affects the protein secondary structure in a way such that the resulting conformation is unfavorable for metal-binding 154). Recent low temperature EPR studies have... 

The alkyl halide radical anion is very unstable and short-lived, decomposing to an alkyl radical and a halide ion. In fact, the electron transfer very likely tK curs dissociatively, forming the radical directly with the transfer of the electron (eq. 2.15). This is supported theoretically [43 j, and is consistent with the fact (see below) that the ease of reduction increases with the stability of the radical formed. Low temperature EPR studies indicate only weak polarization interaction between the radical and the halide ion 44). Nevertheless, it has been suggested that even this weak interaction might affect the partition of nidical species during the lifetime of a radical pair 43). In contrast. 

The chemical properties of BA have been studied in detail (Lapin et al., 1984). Low temperature epr spectroscopy shows clearly that the ground state of BA is the triplet (3BA). The zero field parameters (Table 3) reveal some details of this structure. When the irradiation is performed at 4.6 K in a 2-methyltetrahydrofuran glass no epr signals from radical species are apparent. The optical spectrum under these conditions shows absorptions (Table 4) which disappear when the glass is warmed. From these findings the absorption bands are assigned tentatively to 3BA. This conclusion is strongly supported by results from laser flash photolysis experiments. 

Diphenylmethylene is certainly the most exhaustively studied of the aromatic carbenes. Low temperature epr spectroscopy (Trozzolo et al., 1962) clearly established the ground state of this carbene as the triplet. The optical spectrum of the triplet was recorded first in a 1,1-diphenylethylene host crystal (Closs et al., 1966) and later in frozen solvents (Trozzolo and Gibbons, 1967). 

Carbene fluorescence in solution is usually red shifted by 25-30 nm with respect to the band position observed in matrix at 77 K. This shift is attributed to emission from nonequilibrated conformations at low temperature. In matrices, the carbene is produced in a locked conformation similar to that for the precursor diazo compound but, in solution, it approaches the thermodynamically favored configuration. This difference has been demonstrated by variable temperature EPR studies of sterically congested carbenes (see Section 3.1.1.3). So, in solution, the equilibrium conformation is reached rapidly and only fluorescence from the relaxed state is observed. In support of this suggestion, the shift for dimesitylcarbene is smaller than for other carbenes, indicating that shifts are smaller when the carbene structure is such that it restricts conformational change. 

Although both haemoglobin [155,231] and myoglobin produce ferryl iron and free radicals upon addition of H2O2, the latter reaction has been studied more extensively. In contrast to most peroxidases, at least two myoglobin-bound free radicals have been detected immediately after peroxide addition, both by room-temperature [142] and low-temperature EPR[137]. These decay rapidly and independently of the decay of the ferryl iron, which is stable for at least an hour. It is not necessary to add exogenous reductants to reduce the radicals it is possible that some of the electrons come from elsewhere on the protein as different, more stable, free radicals can be detected one hour subsequent to peroxide treatment [137,236]. 

FROM THE PRIMARY RADIATION INDUCED RADICALS IN DNA CONSTITUENTS TO STRAND BREAKS LOW TEMPERATURE EPR/ENDOR STUDIES... 

The potentials of the redox centers of xanthine oxidase have been investigated by titrations in the presence of redox mediator dyes. An early study (245) used dithionite to generate reducing equivalents and quantified the reduced species by EPR measurements at low temperature. Subsequent studies as a function of pH showed that the potential of the molybdenum center was sensitive to pH (246). Concern over the effect of temperature on the observed potentials led to redox titrations monitored by room temperature CD and EPR spectroscopy (247). These experiments indicated that the redox potentials of all of the prosthetic... 

In the ferrichromes the iron is high spin ferric, with a magnetic moment of 5.73 Bohr magnetons (22). The nature of the iron locus has been studied by Mossbauer (23, 24) and far infrared (25) techniques. The low temperature EPR spectrum of ferrichrome A exhibits characteristic g =4.3 and g=9.6 bands (26). These studies have indicated a strong crystal field of low symmetry. 

EPR studies of polycrystalline samples (also other hydrazyls) temperature dependence of line broadening effects, exchange interaction, phase transition 0.05 ) Susceptibility, H-NMR and low field EPR studies of polycrystalline samples. 

PSII membranes were isolated from market spinach leaves as previously described (11,12). For storage at 77 K and for low-temperature EPR ex[)eriments, PSII membranes were susi)ended in a buffer solution containing 20 mM 2-(N-morpholino)ethanesulfonic acid (MES)-NaOH, pH 6.0, 15 mM NaCl, and 30% (v/v) ethylene glycol (as a cryoprotectant). For studies involving the S2 state, the S2-state multiline EPR signal was generated in extensively daik-adapted S -state PSII membranes by a 2 min illumination at 210 K, as previously described (12). 

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