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Abstract— The use of sodium dodecyl sulfate to dissociate photosynthetic membranes followed by standard fractionation techniques yields chlorophyll-proteins and reaction center complexes with molecular weights of 500,000 or less. Much about the structure and function of photo-synthetic units in vivo can be deduced from the properties of the isolated complexes. The Bchl-protein from green bacteria is approximated by an incompletely filled sphere ? 80 Â in diameter consisting of four identical subunits. The five Bchl molecules in each subunit are 14 to 20Â apart. The related Chl a-proteins from a blue-green alga and various eukaryotic plants may have similar structural characteristics. The Chl a-protein from a blue-green alga contains one molecule of P700 per 60–90 Chl a molecules. The quantum requirement for P700 oxidation is 2.6 or less. The midpoint potential in various preparations ranges from 0.38 V to 0.42 V. Green algae and higher plants yield a Chl a-protein similar to that from the blue-green alga; in addition they yield another Chl-protein (mol. wt. = 2–3×104) which contains an equal amount of Chl a and Chl b. These two Chl-proteins account for most of the chlorophyll in these organisms. Two photosynthetic bacteria (Rhodopseudomonas viridis and Chromatium) yield protein complexes containing Bchl, carotenoid, and bound cytochromes. The reaction center complex from R. viridis contains P960 (Em, 8= 0.39 to 0.42 V), cytochrome 558 (Em,8= 0.33 V) and cytochrome 553 (Em,7=— 0.02 V). Quantum requirements for P960 and C558 oxidation are ?2.2 and 3.0, respectively. Complex A from Chromatium contains Bchl 890, P883, cytochrome 556 (Em,8= 0.34 V) and cytochrome 552 (Em,7=?0.04 V). The quantum requirement for C556 oxidation is about 15. Both high- and low-potential cytochromes can donate electrons to the reaction center chlorophyll present in either complex. This fact supports the idea that only one kind of photochemical reaction center functions in photosynthetic bacteria. An hypothesis about the nature of the photosynthetic unit in purple bacteria is outlined.  相似文献   
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The reflection of waves from an isolated transition point isgeneralized to embrace an infinite number of evenly spaced non-isolatedtransition points lying on a straight line perpendicular tothe propagation axis in the complex plane. The comparison functionssuited to the problem are Bessel functions, yielding uniformapproximate solutions along the axis, from which reflectioncoefficients may be calculated. The theory is illustrated bycomparison with an exact analytical solution in terms of Whittakerfunctions, for a model containing two parallel lines of transitionpoints.  相似文献   
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A polynomial is said to be of type (p1, p2, p3) relative tothe unit circle if it has p1 zeros interior to, p2 on, and p3exterior to the unit circle. Stability criteria frequently arisewhere a polynomial or a family of polynomials must be shownto be of type (p1, p2, 0) or of type (p1, 0, 0). Here we reconsiderthe practical problem of showing that a polynomial is of oneor other of these types, and we show that the testing of a polynomialof degree n may always be reduced to the testing of one of degreen–1. The simplicity of the method is illustrated by itsapplication to several well known difference schemes for partialdifferential equations.  相似文献   
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The frequency and temperature dependence of the real (?′) and imaginary (?″)parts of the dielectric constant of α-cyclodrextrin (form 1; α-CD. 5.9H2O) and α-cyclodextrin (form III; α-CD.7.6H2O) and of the corresponding dried forms (α-CD.1.1H2O, α-CD.2.9H2O, respectively) has been investigated over the frequency range 0–100 k Hz and temperature range 130–350 K. In addition the dielectric behaviour has been investigated of heptakis-(2,3,6-tri-O-methyl)-β-cyclodextrin (β-CD.TRIME. 0.3H2O) and randomly methylated β-cyclodextrin (β-CD.RAME.0.8H2O). The dielectric behaviour is described well by Debyetype relaxation (α-disperson). All α-CD systems exhibit an additional ω-dispersion at low frequencies, which is attributed to proton transport. The fact that the ?′ values of α-CD form III are larger than those of α-CD form I is attributed to the different numbers and different strengths of the intramolecular hydrogen bonds. Form III has a stable conformation which is shown by the constant values of ?′ in the temperature range 125–250 K. By contrast, in form I the ?′ values increased linearly with temperature, indicating that the system passed through succesive conformations. The temperature dependence of ?″ and phase shift ∞ in all the specimens of α-CD (forms I, III) and fully methylated β-CD do not reveal any orderdisorder transition, because the developed hydrogen bonds ae not of the flip-flop type according to their crystal structures. The partially methylated β-CD reveals the characteristics of the order-disorder transition, which was observed before in the systems dried β-CD.2H2O and non-dried β-CD.9.8H2O. There is a direct relation between the hydroxyl groups of β-CD and the order-disorder transition. The order-disorder transition could also be shown in dried β-CD.2.4H2O and non-dried β-CD.9.8H2O samples but not in dried α-CD.1.1H2O and non-dried a-CD.5.9H2O samples, by low frequency Raman spectroscopy (< 180 cm?1). The step-like temperature dependence of the band at 33.7cm?1 reveals a transition at about 223K for both β-CD samples. In the case of α-CD samples the almost linear temperature dependence of the band at 49.1 cm?1 does not reveal any transition.  相似文献   
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The influence of exact exchange incorporated into exchange—correlation functionals on the predictions of relative energies, structures, electronic states, and vibrational spectra is examined numerically. Failures of widely used hybrid exchange—correlation functionals due to either the physical unacceptability of including exact exchange or an unbalanced mixing of exact exchange are considered. One set of examples involves tetraatomic chalcogen clusters and charge transfer complexes between diatomic chalcogens and diatomic oxygen. Poor energetic predictions from Hartree-Fock rule against the inclusion of exact exchange into the exchange—correlation functionals for these systems with significant left—right electron correlation effects. The energies of the conformers of [10]annulene are considered from an unusual viewpoint, namely, the empirical adjustment of the admixture of exact exchange to match the predictions of very high level theoretical methods. For this annulene with insignificant left-right electron correlation effects, a greater (50%) percentage of exact exchange should be included. The relationship of symmetry breaking to the inclusion of exact exchange is examined for seven linear radicals, OXO (X = B, Al, Ga, In, TI). AIOS, and OAIS. exchange—correlation functionals generate symmetry adapted solutions at the expense of an unusual ordering of the Kohn-Sham orbitals, which can cause uncharacteristic electronic states, incorrect vibrational spectra, and poorer predictions of energetics. These effects are greater when exact exchange is included. In all the examples considered, the appropriate focus for a detailed discussion of molecular properties involves consideration of the effects of exact exchange.  相似文献   
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The conformational structures of noradrenaline, isolated in the gas phase, have been explored through a combination of electronic structure computation (at the B3LYP/6-31 + G?, MP2/6-31 + G?, MP2/aug-cc-pVDZ and CIS/6-31 + G? levels of theory) and mass selected ultraviolet and infrared ion dip spectroscopy (following laser ablation of the neurotransmitter into a pulsed supersonic argon expansion). Despite the many possible low-lying conformational possibilities predicted by theory, almost the entire population of jet-cooled noradrenaline adopts the global minimum structure, associated with an extended, AG1a, ethanolamine side chain conformation. Intramolecular hydrogen bonds are formed between the neighbouring hydroxyl groups on the catechol ring and between the hydroxyl and amino groups on the side chain.  相似文献   
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