New developments in first-principles excited-state dynamics simulations: unveiling the solvent specificity of excited anionic cluster relaxation and electron solvation

Charge-transfer-to-solvent excited iodide–polar solvent molecule clusters, [I^? (Solv)_n]^*, have attracted substantial interest over the past 20 years as they can undergo intriguing relaxation processes leading ultimately to the formation of gas-phase molecular analogues of the solvated electron. In this review article, we present a comprehensive overview of the development and application of state-of-the-art first-principles molecular dynamics simulation approaches to understand and interpret the results of femtosecond photoelectron spectroscopy experiments on [I^? (Solv)_n]^* relaxation, which point to a high degree of solvent specificity in the electron solvation dynamics. The intricate molecular details of the [I^? (Solv)_n]^* relaxation process are presented, and by contrasting the relaxation mechanisms of clusters with several different solvents (water, methanol and acetonitrile), the molecular basis of the solvent specificity of electron solvation in [I^? (Solv)_n]^* is uncovered, leading to a more refined view of the manifestation of electron solvation in small gas-phase clusters.     

Disproportionation of pentaammineruthenium(III)–nucleoside complexes leads to two-electron oxidation of nucleosides without involving oxygen molecules

Pentaammineruthenium(III) complexes of deoxyinosine (dIno) and xanthosine (Xao) ([Ru^III(NH₃)₅(L)], L?is?dIno, Xao) in basic solution were studied by UV?Cvis spectroscopy, liquid chromatography/electrospray ionization mass spectrometry, and high-performance liquid chromatography. Both Ru^III complexes disproportionate to Ru^II and Ru^IV. Disproportionation followed the rate law d[Ru^II]/dt?=?(k _o?+?k ₁[OH^?])[Ru^III]. k _o and k ₁ of disproportionation at 25?°C were 2.1 (±0.1)?×?10^?3?s^?1 and 21.4?±?3.2?M^?1 s^?1, respectively, for [Ru^III(NH₃)₅(dIno)], and 3.5 (±0.7)?×?10^?4?s^?1 and 59.7?±?3.6?M^?1?s^?1, respectively, for [Ru^III(NH₃)₅(Xao)]. The [Ru^III(NH₃)₅(Xao)] complex disproportionates at a faster rate than [Ru^III(NH₃)₅(dIno)] owing to the stronger electron-withdrawing effect of exocyclic oxygen in Xao. The activation parameters ??H ^? and ??S ^? for k ₁ of [Ru^III(NH₃)₅(dIno)] were 80.2?±?15.2?kJ?mol^?1 and 47.6?±?9.8?J?K^?1 mol^?1, respectively, indicating that the disproportionation of Ru^III to Ru^II and Ru^IV is favored owing to the positive entropy of activation. The final products of both complexes in basic solution under Ar were compared with those under O₂. Under both conditions [Ru(NH₃)₅(8-oxo-L)] was produced, but via different mechanisms. In both aerobic and anaerobic conditions, the deprotonation of highly positively polarized C8-H of Ru-L by OH^? initiates a two-electron redox reaction. For the next step, we propose a one-step two-electron redox reaction between L and Ru^IV under anaerobic conditions, which differentiates from Clarke??s mechanism of two consecutive one-electron redox reactions between L, Ru^III, and O₂.     

An analysis of the influence of age and school-attendance status on the spread of variola minor.

Definitions are proposed for the independent and joint contributions that the chemical groups A and B make to the free energy of association of the ligand A?B with a receptor. The definitions are independent of the choice of the standard state and are consistent with the basic thermodynamic cycle relating the association of the ligands A?B, A?Y and X?B to the receptor Rappaport 1976. The basic idea is the use of the excess free energy of association of the ligand A?Y over the free energy of association of the reference ligand X?Y as the measure of the “independent” contribution of the group A to the binding. This definition allows the free energy of association of the ligand A?B to be written as the sum of the independent contributions of the groups A and B, their joint contribution, and an invariant free energy of association of the reference ligand with any receptor. With the appropriate definition of the receptor-reference ligand complex, water can be chosen as the reference ligand. Using ΔG(A?OH)?AG(HOH), ΔG(H?B?H)?ΔG(HOH) and ΔG(HO?C)?ΔG(HOH) as the definitions of the “independent” contributions of the chemical groups A, B and C to the binding of the ligand A?B?C, the joint contribution of the groups A and C to the binding is ΔG(A?B?C) ? ΔG(A?B?H) ? ΔG(H-B-C) + ΔG(H?B?H).     

Evaluation of growth and phycobiliprotein composition of cyanobacteria isolates cultivated in different nitrogen sources

Phycobiliproteins, light-harvesting pigments found in cyanobacteria and in some eukaryotic algae, have numerous commercial applications in food, cosmetic, and pharmaceutical industries. Colorant production from cyanobacteria offers advantages over their production from higher plants, as cyanobacteria have fast growth rate and high photosynthetic efficiency and require less space. In this study, three cyanobacteria strains were studied for phycobiliprotein production and the influence of sodium nitrate, potassium nitrate and ammonium chloride on the growth and phycobiliprotein composition of the strains were evaluated. In the batch culture period of 12 days, Phormidium sp. and Pseudoscillatoria sp. were able to utilize all tested nitrogen sources; however, ammonium chloride was the best nitrogen source for both strains to achieve maximum growth rate μ?=?0.284?±?0.03 and μ?=?0.274?±?0.13 day^?1, chlorophyll a 16.2?±? 0.5 and 12.2?±? 0.2 mg L^?1, and phycobiliprotein contents 19.38?±?0.09 and 19.99?±?0.14% of dry weight, whereas, for Arthrospira platensis, the highest growth rate of μ?=?0.304?±?0.0 day^?1, chlorophyll a 19.1?±?0.5 mg L^?1, and phycobiliprotein content of 22.27?±?0.21% of dry weight were achieved with sodium nitrate. The phycocyanin from the lyophilized cyanobacterial biomass was extracted using calcium chloride and food grade purity (A₆₂₀/A₂₈₀ ratio >?0.7) was achieved. Furthermore, phycocyanin was purified using two-step chromatographic method and the analytical grade purity (A₆₂₀/A₂₈₀ ratio >?4) was attained. SDS-PAGE demonstrated the purity and presence of two bands corresponding to α- and β-subunits of the C-phycocyanin. The results showed that Phormidium sp. and Pseudoscillatoria sp. could be good candidates for phycocyanin production.     

Altitudinal Change in the Photosynthetic Capacity of Tropical Trees: A Case Study from Ecuador and a Pantropical Literature Analysis

In tropical mountains, trees are the dominant life form from sea level to above 4,000-m altitude under highly variable thermal conditions (range of mean annual temperatures: <8 to >28°C). How light-saturated net photosynthesis of tropical trees adapts to variation in temperature, atmospheric CO₂ concentration, and further environmental factors, that change along elevation gradients, is not precisely known. With gas exchange measurements in mature trees, we determined light-saturated net photosynthesis at ambient temperature (T) and [CO₂] (A _sat) of 40 tree species from 21 families in tropical mountain forests at 1000-, 2000-, and 3000-m elevation in southern Ecuador. We tested the hypothesis that stand-level averages of A _sat and leaf dark respiration (R _D) per leaf area remain constant with elevation. Stand-level means of A _sat were 8.8, 11.3, and 7.2?μmol?CO₂?m^?2?s^?1; those of R _D 0.8, 0.6, and 0.7?μmol?CO₂?m^?2?s^?1 at 1000-, 2000-, and 3000-m elevation, respectively, with no significant altitudinal trend. We obtained coefficients of among-species variation in A _sat and R _D of 20–53% (n?=?10–16 tree species per stand). Examining our data in the context of a pan-tropical A _sat data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based A _sat decreases in tropical mountains by, on average, 1.3?μmol?CO₂?m^?2?s^?1?per?km altitude increase (or by 0.2?μmol?CO₂?m^?2?s^?1 per K temperature decrease). The A _sat decrease occurred despite an increase in leaf mass per area with altitude. Local geological and soil fertility conditions and related foliar N and P concentrations considerably influenced the altitudinal A _sat patterns. We conclude that elevation is an important influencing factor of the photosynthetic activity of tropical trees. Lowered A _sat together with a reduced stand leaf area decrease canopy C gain with elevation in tropical mountains.     

Influence of the PsbA1/PsbA3, Ca2+/Sr2+ and Cl−/Br− exchanges on the redox potential of the primary quinone QA in Photosystem II from Thermosynechococcus elongatus as revealed by spectroelectrochemistry

Ca²⁺ and Cl^? ions are essential elements for the oxygen evolution activity of photosystem II (PSII). It has been demonstrated that these ions can be exchanged with Sr²⁺ and Br^?, respectively, and that these ion exchanges modify the kinetics of some electron transfer reactions at the Mn₄Ca cluster level (Ishida et al., J. Biol. Chem. 283 (2008) 13330–13340). It has been proposed from thermoluminescence experiments that the kinetic effects arise, at least in part, from a decrease in the free energy level of the Mn₄Ca cluster in the S₃ state though some changes on the acceptor side were also observed. Therefore, in the present work, by using thin-layer cell spectroelectrochemistry, the effects of the Ca²⁺/Sr²⁺ and Cl^?/Br^? exchanges on the redox potential of the primary quinone electron acceptor Q_A, E_m(Q_A/Q_A^?), were investigated. Since the previous studies on the Ca²⁺/Sr²⁺ and Cl^?/Br^? exchanges were performed in PsbA3-containing PSII purified from the thermophilic cyanobacterium Thermosynechococcus elongatus, we first investigated the influences of the PsbA1/PsbA3 exchange on E_m(Q_A/Q_A^?). Here we show that i) the E_m(Q_A/Q_A^?) was up-shifted by ca. + 38 mV in PsbA3-PSII when compared to PsbA1-PSII and ii) the Ca²⁺/Sr²⁺ exchange up-shifted the E_m(Q_A/Q_A^?) by ca. + 27 mV, whereas the Cl^?/Br^? exchange hardly influenced E_m(Q_A/Q_A^?). On the basis of the results of E_m(Q_A/Q_A^?) together with previous thermoluminescence measurements, the ion-exchange effects on the energetics in PSII are discussed.     

Studies of mononuclear and dinuclear complexes of dibromodimethylplatinum(IV): Preparation, characterization and crystal structures

A number of complexes of the types [PtBr₂Me₂(N^?N)] (N^?N = 4,4′-di-Me-2,2′-bpy (1); 4,4′-di-t-Bu-2,2′-bpy (2); 2,2′-bpz (3); bpym (4)) and [PtBr₂Me₂(L)₂] (L = H-pz (5); 4-Me-H-pz (6); H-idz (7); H-im (8); H-bim (9); quaz (10)) are reported. Characterization by NMR (¹H, ¹³C and ¹⁹⁵Pt), IR and EI-MS is given. In addition, crystal structures of several of these complexes are described. Furthermore, interactions within these structures including intramolecular hydrogen bonding and π-π stacking interactions are reported. The reactivity of selected mononuclear complexes was investigated and yielded two dinuclear complexes [PPh₄][(PtBrMe₂)₂(μ-Br)(μ-pz)₂] (11) and [(PtBr₂Me₂)₂(μ-bpym)] (12), respectively. The latter complex is accompanied by a solid-state structure. Finally, the thermal stability of all complexes is reported.     

Iodine contributes to osmotic acclimatisation in the kelp Laminaria digitata (Phaeophyceae)

Iodide (I^?) retained by the brown macroalga Laminaria digitata at millimolar levels, possesses antioxidant activities, but the wider physiological significance of its accumulation remains poorly understood. In its natural habitat in the lower intertidal, L. digitata experiences salinity changes and osmotic homeostasis is achieved by regulating the organic osmolyte mannitol. However, I^? may also holds an osmotic function. Here, impacts of hypo- and hypersaline conditions on I^? release from, and accumulation by, L. digitata were assessed. Additionally, mannitol accumulation was determined at high salinities, and physiological responses to externally elevated iodine concentrations and salinities were characterised by chl a fluorometry. Net I^? release rates increased with decreasing salinity. I^? was accumulated at normal (35 S _A) and high salinities (50 S _A); this coincided with enhanced rETR_max and q_P causing pronounced photoprotection capabilities via NPQ. At 50 S _A elevated tissue iodine levels impeded the well-established response of mannitol accumulation and prevented photoinhibition. Contrarily, low tissue iodine levels limited photoprotection capabilities and resulted in photoinhibition at 50 S _A, even though mannitol was accumulated. The results indicate a, so far, undescribed osmotic function of I^? in L. digitata and, thus, multifunctional principles of this halogen in kelps. The osmotic function of mannitol may have been substituted by that of I^? under hypersaline conditions, suggesting a complementary role of inorganic and organic solutes under salinity stress. This study also provides first evidence that iodine accumulation in L. digitata positively affects photo-physiology.     

Absorption spectrum of cattle hypsorhodopsin

On irradiation at liquid helium temperatures, rhodopsin is converted into hypsorhodopsin which decays to bathorhodopsin above 23 K. The absorption spectrum of cattle hypsorhodopsin (λ_max = 435 nm) is found to include a new sideband around 540 nm. This sideband may be due to π^* ← π transition to ¹A^?_g like state, which is made partially allowed by distortion of polyene chain of the retinylidene chromophore.     

SCN− and HSCN transport through lipid bilayer membranes: A model for SCN− inhibition of gastric acid secretion

Diffusion of thiocyanate (SCN?) and thiocyanic acid (HSCN) (pK=?1.8) through lipid bilayer membranes was studied as a function of pH. Membranes were made of egg phosphatidylcholine or phosphatidylcholine plus cholesterol (1:1 mol ratio) dissolved in decane or tetradecane. Tracer fluxes and electrical conductances were used to estimate the permeabilities to HSCN and SCN?. Over the pH range 1.0 to 3.3 only HSCN crosses the membrane at a significant rate. The relation between the total SCN flux (JA), concentrations and permeabilities is: 1/JA=1/Pul([A?]+[HA])+1/PHAm[HA], where [A?] and [HA] are the concentrations of SCN? and HSCN, Pul is permeability coefficient of the unstirred layer, and PHAm is the membrane permeability to HSCN. By fitting this equation to the data we find that PHAm = 2.6 cm · s?1 and Pul = 9.0 · 10?4 cm · s?1. Conductance measurements indicate that PA?m is 5 · 10?9 cm · s?1. Addition of cholesterol to phosphatidylcholine (1:1 mol ratio) reduces PHAm by a factor of 0.4 but has no effect on PA?m. SCN? is potent inhibitor of acid secretion in gastric mucosa, but the mechanism of SCN? action is unknown. Our results suggest that SCN? acts by combining with H+ in the mucosal unstirred layer (secretory pits) and diffusing back into the cells as HSCN, thus dissipating the proton gradient across the secretory membrane. A similar mechanism of action is proposed for some other inhibitors of gastric acid secretion, e.g. nitrite (NO2?), cyanate (CNO?) and NH4+.     

DIEL PATTERNS IN LIGHT ABSORPTION AND ABSORPTION EFFICIENCY FACTORS OF ISOCHRYSIS GALBANA (PRYMNESIOPHYCEAE)1

The chl a specific absorption coefficients [a* (λ), m²·mg chl a ^{? 1}] were examined in chemostat culture of the Prymnesiophyceae Isochrysis galbana (Parke) under a 12:12‐h light:dark cycle at low light (75 μmol photons·m ^{? 2}·s ^{? 1}) and high light (500 μmol photons· m ^{? 2}·s ^{? 1}) conditions. Other associated measurements such as pigment composition, cell density, and diameter as the measure of cell size were also made at the two light regimes every 2 h for 2 days to confirm the periodicity. A distinct diel variability was observed for the a* (λ) with maxima near dawn and minima near dusk. The magnitude of diel variation in a* (440) was 15% at low light and 22% at high light. Pronounced diel patterns were observed for cell size with minima near dawn and maxima near dusk. The magnitude of diel variation in cell size was 9.3% at low light and 21% at high light. The absorption efficiency factors [Q _a (440)] were determined by reconstruction using intracellular concentrations of pigments and cell size. The Q _a (440) also showed a distinct diel variability, with minima near dawn and maxima near dusk. The diel variation in a* (λ) and Q _a (λ) was primarily caused by changes in cell size due to growth, although there was some influence from diel variations in the intracellular pigment concentrations. The results presented here indicated that diel variation in a* (λ) was an important component of the optical characterization of phytoplankton.     

Kinetics and thermodynamics of the P870+Q−A → P870+Q−B reaction in isolated reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides

The temperature dependences of the P870⁺Q^?_A → P870Q_A and P870⁺Q^?_B → P870Q_B recombination reactions were measured in reaction centers from Rhodopseudomonas sphaeroides. The data indicate that the P870⁺Q^?_B state decays by thermal repopulation of the P870⁺Q^?_A state, followed by recombination. ΔG° for the P870⁺Q^?_A → P870⁺Q^?_B reaction is ?6.89 kJ · mol^?1, while ΔH° = ?14.45 kJ · mol^?1 and ?TΔS° = + 7.53 kJ · mol^?1. The activation ethalpy, $\text{H}{}^3$, for the P870⁺Q^?_A Δ P870⁺Q^?_B reaction is +56.9 kJ · mol^?1, while the activation entropy is near zero. The results permit an estimate of the shape of the potential energy curve for the P870⁺Q^?_A → P870⁺Q^?_B electron transfer reaction.     

Kinetic studies on the reaction of ethylenediaminetetraacetatochromium(III) complex with hydrogen peroxide

The rate of reaction of [Cr(III)Y]_aq (Y is EDTA anion) with hydrogen peroxide was studied in aqueous nitrate media [μ = 0.10 M (KNO₃)] at various temperatures. The general rate equation, Rate = $\text{k}{}_1\text{+}\text{k}{}_2\text{K}{}_1\text{[H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{?1}}\text{1 +}\text{K}{}_1\text{[H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{?1}}$ [Cr(III)Y]_aq[H₂O₂] holds over the pH range 5–9. The decomposition reaction of H₂O₂ is believed to proceed via two pathways where both the aquo and hydroxo-quinquedentate EDTA complexes are acting as the catalyst centres. Substitution-controlled mechanisms are suggested and the values of the second-order rate constants k₁ and k₂ were found to be 1.75 × 10^?2 M^?1 s^?1 and 0.174 M^?1 s^?1 at 303 K respectively, where k₂ is the rate constant for the aquo species and k₂ is that for the hydroxo complex. The respective activation enthalpies (ΔH^*₁ = 58.9 and ΔH^*₂ = 66.5 KJ mol^?1) and activation entropies (ΔS^*₁ = ?85 and ΔS^*₂ = ?40 J mol^?1 deg^?1) were calculated from a least-squares fit to the Eyring plot. The ionisation constant pK₁, was inferred from the kinetic data at 303 K to be 7.22. Beyond pH 9, the reaction is markedly retarded and ceases completely at pH ? 11. This inhibition was attributed in part to the continuous loss of the catalyst as a result of the simultaneous oxidation of Cr(III) to Cr(VI).     

Theoretical description of the magnetic properties of μ3-hydroxo bridged trinuclear copper(II) complexes

A theoretical study of the magnetic properties, using density functional theory, of a family of trinuclear μ₃-OH copper(II) complexes reported in the literature is presented. The reported X-ray crystal structures of [Cu₃(μ₃-OH)(aat)₃(H₂O)₃](NO₃)₂·H₂O (HUKDUM), where aat: 3-acetylamine-1,2,4-triazole; [Cu₃(μ₃-OH)(aaat)₃(H₂SO₄)(HSO₄)(H₂O)] (HUKDOG), where aaat: 3-acetylamine-5-amine-1,2,4-triazole; [Cu₃(μ₃-OH)(PhPyCNO)₃(tchlphac)₂] (HOHQUR), where PhPyCNO: phenyl 2-pyridyl-ketoxime and tchlphac: acid 2,4,5-trichlorophenoxyacetic; [Cu₃(μ₃-OH)(PhPyCNO)₃(NO₃)₂(CH₃OH)] (ILEGEM); [Cu₃(μ₃-OH)(pz)₃(Hpz)₃(ClO₄)₂] (QOPJIP), where Hpz?=?pyrazole; [Cu₃(μ₃-OH)(pz)₃(Hpz)(Me₃CCOO)₂]?2Me₃CCOOH (DEFSEN) and [Cu₃(μ₃-OH)(8-amino-4-methyl-5-azaoct-3-en-2-one)₃][CuI₃] (RITXUO), were used in the calculations. The magnetic exchange constants were calculated using the broken-symmetry approach. The calculated J values are for HUKDUM J₁?=??68.6 cm^?1, J₂?=??69.9 cm^?1, J₃?=??70.4 cm^?1; for HUKDOG, J₁?=??73.5 cm^?1, J₂?=??58.9 cm^?1, J₃?=??62.1 cm^?1; for HOHQUR J₁?=??128.3 cm^?1, J₂?=??134.1 cm^?1, J₃?=??120.4 cm^?1; for ILEGEM J₁?=??151.6 cm^?1, J₂?=??173.9 cm^?1, J₃?=??186.9 cm^?1; for QOPJIP J₁?=??118.3 cm^?1, J₂?=??106.0 cm^?1, J₃?=??120.6 cm^?1; for DEFSEN J₁?=??74.9 cm^?1, J₂?=??64.0 cm^?1, J₃?=??57.7 cm^?1 and for RITXUO J₁?=??10.9 cm^?1, J₂?=?+14.3 cm^?1, J₃?=??35.4 cm^?1. The Kahn-Briat model was used to correlate the calculated magnetic properties with the overlap of the magnetic orbitals. Spin density surfaces show that the delocalization mechanism is predominant in all the studied compounds.

Hemodialysis Decreases Serum Brain-Derived Neurotrophic Factor Concentration in Humans

In the present study we have evaluated the effect of a single hemodialysis session on the brain-derived neurotrophic factor levels in plasma [BDNF]_pl and in serum [BDNF]_s as well as on the plasma isoprostanes concentration [F₂ isoprostanes]_pl, plasma total antioxidant capacity (TAC) and plasma cortisol levels in chronic kidney disease patients. Twenty male patients (age 69.8?±?2.9?years (mean?±?SE)) with end-stage renal disease undergoing maintenance hemodialysis on regular dialysis treatment for 15?C71?months participated in this study. A single hemodialysis session, lasting 4.2?±?0.1?h, resulted in a decrease (P?=?0.014) in [BDNF]_s by ~42?% (2,574?±?322 vs. 1,492?±?327?pg?ml^?1). This was accompanied by an increase (P?<?10^?4) of [F₂-Isoprostanes]_pl (38?±?3 vs. 116?±?16?pg?ml^?1), decrease (P?<?10^?4) in TAC (1,483?±?41 vs. 983?±?35 trolox equivalents, ??mol?l^?1) and a decrease (P?=?0.004) in plasma cortisol level (449.5?±?101.2 vs. 315.3?±?196.3?nmol?l^?1). No changes (P?>?0.05) in [BDNF]_pl and the platelets count were observed after a single dialysis session. Furthermore, basal [BDNF]_s in the chronic kidney disease patients was significantly lower (P?=?0.03) when compared to the age-matched control group (n?=?23). We have concluded that the observed decrease in serum BDNF level after hemodialysis accompanied by elevated [F₂-Isoprostanes]_pl and decreased plasma TAC might be caused by enhanced oxidative stress induced by hemodialysis.     

Bence jones proteins and light chains of immunoglobulins

The availability of antisera with specificity forλ light chains which have the Mcg- or the non-Mcg-associated amino acid C-region sequence alternations has made possible our immunochemical differentiation of humanλ chains as Mcg⁺ or Mcg^?. One antiserum, prepared against an Mcg⁺ λ chain having the Mcg-associated C-region amino acid residues (asparaginyl, threonyl, and lysyl at positions 116, 118, and 167, respectively), had specificity forλ chains with this C-region sequence. A second antiserum, prepared against an Mcg^? λ chain having the non-Mcg-associated C-region residues (alanyl, seryl, and threonyl at these same three respective positions), had specificity forλ chains with this alternative type of C-region sequence. Immunodiffusion analyses ofλ chains of known amino acid sequence confirmed their chemical classification as Mcg or non-Mcg in type. No association between a particular V-regionλ-chain subgroup and the Mcg factor was evident. Based on sequence and serological analyses, ～ 11 percent ofλ light chains have the Mcg-related C-region sequence alternations. The immunochemical recognition of both Mcg⁺ and Mcg^? light chains isolated from the IgG of normal individuals corroborated the isotypic nature of the Mcg factor. Despite the fact that the Mcg-related substitutions are in the C_λ, the loss of Mcg antigenicity upon cleavage of Mcg⁺ and Mcg^? λ chains into V_L and C_L indicates that the intact light polypeptide chain is essential for expression of the Mcg antigenic factor.     

Electron transfer in reaction centers of Rhodopseudomonas sphaeroides. I. Determination of the charge recombination pathway of D+QAQ−B and free energy and kinetic relations between Q−AQB and QAQ−B

The electron-transfer reactions and thermodynamic equilibria involving the quinone acceptor complex in bacterial reaction centers from R. sphaeroides were investigated. The reactions are described by the scheme: We found that the charge recombination pathway of D⁺Q_AQ^?_B proceeds via the intermediate state D⁺Q^?_AQ_B, the direct pathway contributing less than approx. 5% to the observed recombination rate. The method used to obtain this result was based on a comparison of the kinetics predicted for the indirect pathway (given by the product k_AD-times the fraction of reaction centers in the Q^?_AQ_B state) with the observed recombination rate, k^obs_{D⁺ →D}. The kinetic measurements were used to obtain the pH dependence (6.1 ? pH ? 11.7) of the free energy difference between the states Q^?_AQ_B and Q_AQ^?_B. At low pH (less than 9) Q_AQ^?_B is stabilized relative to Q^?_AQ_B by 67 meV, whereas at high pH Q^?_AQ_B is energetically favored. Both Q^?_A and Q^?_B associate with a proton, with pK values of 9.8 and 11.3, respectively. The stronger interaction of the proton with Q^?_B provides the driving force for the forward electron transfer.     

Crystal structure, optical, magnetic, and photochemical properties of the complex pentakis(dimethyl sulfoxide)nitrosylchromium(2+) hexafluorophosphate

The nitrosyl complex [Cr(dmso)₅(NO)](PF₆)₂ (1) (dmso = dimethyl sulfoxide) has been prepared by the solvolysis of [Cr(NCCH₃)₅(NO)](PF₆)₂ in neat dmso. The optical absorption spectrum of 1 in dmso shows maxima at 734, 567, 450, 413, and 337 nm. Continuous photolysis of 1 with λ = 365-580 nm light in dmso solution results in a release of NO with quantum yield, Φ, in the range 0.034-0.108 mol Einstein⁻¹. Irradiation of a deoxygenated CH₃CN solution of [Cr(NCCH₃)₅(NO)](PF₆)₂ in the presence of excess of [Fe(S₂CNEt₂)₂] results in a transfer of NO to the iron centre as shown from the characteristic EPR spectrum of [Fe(S₂CNEt₂)₂(NO)] with A_iso(¹⁴N) = 12.2 × 10⁻⁴ cm⁻¹. The EPR parameters of 1 were determined: g_iso, g_‖ and g_⊥ : 1.96725, 1.91881(4) and 1.992763(2); A_iso(⁵³Cr), A_‖ (⁵³Cr) and A_⊥(⁵³Cr): 22.8 × 10⁻⁴, 39 × 10⁻⁴ and 15.8 × 10⁻⁴ cm⁻¹; A_iso(¹⁴N), A_‖ (¹⁴N) and A_⊥(¹⁴N): 5.9 × 10⁻⁴, 2 × 10⁻⁴ and 7.540(4) × 10⁻⁴ cm⁻¹.     

Isokinetic Relationship in the Oxidation of Cuprous Stellacyanin by Cobalt(III) Complexes

Rate parameters have been obtained for the oxidation of cuprous stellacyanin by cobalt(III) ions of the form cis(N)-[CoN₂O₄]^?, including cis(N)-[Co(NTA)(gly)]^?, cis(N)-[Co(IDA)₂]^?, [Co(en)(ox)₂]^?(μ 0.5 M(phosphate), pH 7.0), and Co(EDTA)^?(μ 0.1 M(NaCl), pH 7.2, 0.001 M phosphate). An excellent isokinetic correlation between the activation parameters ΔH^≠ and ΔS^≠ exists for the reactions of aminopolycarboxylatocobalt(III) ions with reduced stellacyanin (β = 300 ± 12 K; correlation coefficient = 0.995). It is concluded that enthalpy-entropy compensation in these reactions may be understood in terms of differing orientations preferred by the various oxidants in forming precursor complexes with the reduced blue protein. While ΔH^≠ and ΔS^≠ values for electron transfer from stellacyanin to cis(N)-[CoN₂O₄]^? ions vary over ranges of 10.7 kcal/mol and 34 cal/mol-deg, respectively, room temperature rate constants are relatively constant (3.6–34.5 M^?1 sec^?1), as expected from Marcus theory for outer sphere electron transfer.     

Evidence that the intermediate electron acceptor,A2, in Photosystem I is a bound iron-sulfur protein

Absorption changes accompanying the formation of light-induced P-700⁺ were investigated in a highly enriched Photosystem I preparation where an intermediate electron acceptor preceding P-430 could be detected. In an enriched Photosystem I particle, light-induced reversible absorption changes observed at 700 nm in the presence of dithionite resembled those previously seen at 703 nm and 820 nm [9], thus indicating the presence of a backreaction between P-700⁺ and A^?₂. After this same Photosystem I particle was treated to denature the bound iron-sulfur centers, the photochemical changes that could be attributed to P-700 A₂ were completely lost. These results provide evidence that the intermediate electron acceptor, A₂, is a bound iron-sulfur protein. Additional studies in the 400–500 nm region with Photosystem I particles prepared by sonication indicate that the spectrum of A₂ is different from that of P-430.