Primary excitation

Oberlin E, Amara A, Bachelerie F et al (1996) The CXC chemokine SDF-1 is the ligand for LESTR/fusin and prevents infection by T-cell-line-adapted HIV-1. Nature 382 833-835 Oh SB, Endoh T, Simen AA, Ren D, Miller RJ (2002) Regulation of calcium currents by chemok-ines and their receptors. J Neuroimmunol 123 66-75 Oh SB, Tran PB, GiUard SE, Hurley RW, Hammond DL, Miller RJ (2001) Chemokines and glycoprotein 120 produce pain hypersensitivity by directly exciting primary nociceptive neurons. J Neurosci 21 5027-5035... 

Oh SB, Tran PB, GiUard SE, Hurley RW, Hammond DL, Miller RJ (2001) Chemokines and glycoprotein 120 produce pain hypersensitivity by directly exciting primary nociceptive neurons. J Neurosci 21 5027-5035... 

The partition of the dimerization yields at 77 °K. into RPD and DD yields is shown in Table VIII. DD was obtained at 20°C. from the yields in the presence of added I2 and at 77 °K. by utilization of the efficiency factor equal to 2.3. Note the low yields at 20°C. for products of secondary radical combinations. We postulate that the intermediate excited primary radicals formed after neutralization in the UPD mechanism at 77 °K. may rearrange to secondary radicals before the final cross bond is formed. Evidence for this postulate is indicated in Table VIII by the observation that the n-C12 and 5-MeCu yields appear to be almost entirely due to the DD mechanism. Such isomerization of primary to secondary alkyl radicals in the solid phase at —196 °C. has recently been observed by electron spin resonance.62... 

The de-excitation of the vibrationally excited state may take place within the time ofa few vibrations (< 10-12sec). It should be remembered that on the conventional chemical time scale one seldom considers intermediates with lifetimes < 10 12 sec thus most of these excited primary products may be overlooked from the chemical standpoint. One cannot, however, neglect these excited products when considering the detailed mechanism of the electron transfer. 

The de-excitation of the excited primary products may produce secondary excited products. The latter, in turn, may undergo deexcitation or dissociation within < 10-1° sec. The species that survive this time interval may still be highly reactive intermediates that react with each other, with the solvent or with other solutes. These intermediates seldom survive longer than milliseconds thereafter they form the secondary or tertiary more stable products. 

Products which can be ascribed to the intermediate formation of radicals have long been observed in carbene reactions. In the gas phase these products could arise by homolytic decomposition of excited primary products before collisional deactivation rather than from radicals generated in the course of insertion. This is not so in solution. It is found that, in the thermal decomposition of diphenyldiazomethane (Bethell et al., 1965) or photolysis of diphenylketene (Nozaki et al., 1966) in toluene solution, the product of insertion of diphenylmethylene into the benzylic carbon-hydrogen bonds, 1,1,2-triphenylethane, is accompanied by substantial amounts of 1,1,2,2-tetraphenylethane and bibenzyl. This is a strong indication that discrete diphenylmethyl and benzyl radicals are formed, and, taken in conjunction with EPR (Section IIB) and other evidence (Etter et al., 1959) that diphenylmethylene is a ground-state triplet, would support the view that equation (20) is an adequate representation of triplet insertion. 

Methane, which is formed by molecular elimination, may be produced either from the excited parent or from the excited primary product, e.g., reactions (157) and (158)... 

The absence of sufficient amounts of allene and propyne in the products argues against the mechanism involving the excited primary product. 

Primary photochemical events in reaction centers from the Rb. sphaeroides wild type and site directed mutant RCs, where the tyrosine at the M210 position was replaced by phenylalanine and leucine, were investigated by femtosecond time-resolved absorbance and ENDOR/TRIPLE spectroscopy techniques (Wachtveitl, et al., 1998). The results allowed the authors to suggest that primary electron transfer follows a stepwise mechanism and P+Bcl state is the first electron transfer intermediate in these mutants. Independent evidence in favor of the anion radical Bchl as the first material (but not virtual ) intermediate, was obtained (Yakovlev et al., 2000). It was demonstrated that in the porphyrin-modified RCs of Rb. spheroidas R-26, the femtosecond oscillations in the excited primary donor emission occur (Vos et al, 1994). 

One of the enigmatic problems of photsynthesis is the drastic difference between the rate of photelectron transfer in the active (M) and inactive branches of bacterial reaction centers. The quantum mechanical calculation (Kolbasov and Scherz, 2000) showed that the square of electronic matrix element VA2 for the electron transfer from the excited primary donor, P, to bacteriochlorophyl in the active brunch is larger by three order of magnitude than that in the inactive part Vb2. Therefore, the electron transfer rate in the RC inactive L-brunch should be essentially slower than that in the M-brunch. 

Fig. 4.1. Scheme of the electron transfer steps within the reaction center complex. Electrons from the excited primary donor are sequentially transferred to different electron acceptors characterized by increasingly positive oxidoreduction potentials. The energy barrier thus obtained prevents a rapid charge recombination process. The rate of back reaction must be small in comparison with the forward reaction for an effective electron transfer process. The rates of the forward reactions and of some back reactions in RC of Rps. sphaeroides are indicated as an example. 

SCHEME 17.6 Proposed mechanism for the deactivation of excited primary and secondary amine oxygen CTCs. 

PAIN The kinins are powerful algesic agents that cause an intense burning pain when apphed to the exposed base of a blister. Bradykinin excites primary sensory neurons and provokes the release of neuropeptides such as substance P, neurokinin A, and calcitonin gene-related peptide. Although there is overlap, receptors generally mediate acute bradykinin algesia, whereas the pain of chronic inflammation appears to involve increased numbers of Bj receptors. 

The profound alterations of the mechanistic picture revealed by comparison of liquid-phase and gas-phase results, and by the pressure effect, are a serious caveat against straightforward generalization of gas-phase data, for example, those from mass spectrometric studies of ion-molecule reactions, obtained under conditions that cannot ensure effective stabihzation and quenching of excited primary intermediates. 

The cross sections are calculated in a natural way. A large number of events (trajectories) are tested for a given impact parameter. Each event in studied reactions ends in the exit channel with two excited primary fragments. The corresponding double differential cross-section is calculated as follows... 

The mass absorption coefBdent// plays a very important role in quantitative XRF analysis. Both the exciting primary radiation and the fluorescence radiation are attenuated in the sample. To relate the observed fluorescence intensity to the concentration, this attenuation must be taken into account As illustrated in Fig. 11.1, the absorption of radiation in matter is the cumulative effect of several types of photon—matter interaction processes that take place in parallel. Accordingly, in the X-ray range the mass attenuation coefficient of element i can be expressed as ... 

Oh, S.B., Tran, P.B., Gillard, S.E., Hurley, R.W., Hammond, D.L., and Miller, R.J. (2001) Chemokines and glycoproteinl20 produce pain hypersensivity by direct exciting primary nociceptive neurons. The Journal of Neuroscience, 21, 5027-5035. 

Qin, X., Wan, Y, and Wang, X. (2005) CCL2 and CXCLl trigger calctonin gene-related peptide release by exciting primary nociceptive neurons. Journal of Neurosdence Research, 82, 51—62. 

The photochemical charge separation in photosystem I (PS I)(see (1) for a recent review), starting from the excited primary donor, P700 (a chlorophyll dimer), involves five membrane-bound electron acceptors the primary acceptor Aq (chlorophyll a), a secondary acceptor (presumably vitamin Kj ) and three iron sulfur centers, F, Fg and F. ... 

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