31P NMR study of head group behavior in sonicated phosphatidylcholine liposomes in the gel and in the liquid state

We measured the ³¹P[¹H] Nuclear Overhauser Effect (NOE) as a function of temperature and of ¹H irradiation frequency, the linewidth $\text{Δν}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ as a function of temperature and the relaxation time T₁ above and below the thermal transition temperature, of the ³¹P-NMR signal in sonicated liposomes of 1,2-dimiristoyl-3-sn-phosphatidylcholine (DMPC), 1,2-dipalmitoyl-3-sn-phosphatidylcholine (DPPC) and 1,2-dimiristoyl-3-sn-phosphatidylcholine (DSPC). The same measurements were repeated in the presence of high molecular weight dextrans. They strongly reduce the NOE and produce longer relaxation times T₁. According to the current models, we were able to evaluate, in the different situations, the correlation time of the internal motion τ_G and the distance r between interacting groups in the region of the polar head groups. While the first parameter changes abruptly through the phase transition and under the effect of dextrans, the latter does not appear modified in any case. These results are discussed in terms of a conformational change of the phosphocholine head groups.     

Proton nuclear magnetic resonance studies of the manganese (II) binding site of concanavalin A. Effect of saccharides on the solvent relaxation rates

The longitudinal relaxation rate ($\text{1}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>p</mtext>}}$) of water protons was studied in solutions of Mn(II)-concanavalin A at a number of frequencies. These relaxation rates were lowered in the presence of a variety of saccharides which have affinities for concanavalin A which range over two orders of magnitude. A good correlation was found in which saccharides which bind tightly have the greatest effect and saccharides which bind weakly or not at all have little effect on the $\text{1}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>p</mtext>}}$ values. The temperature dependence of the proton relaxation rates showed that the lowering of these rates in the presence of saccharides was most likely due to a change in the exchange rate of solvent interacting with protein-bound Mn(II), $\text{1}\text{T}{}_{\mathrm{<mtext>m</mtext>}}$.An analysis of the temperature and frequency dependence of the $\text{1}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>p</mtext>}}$ and $\text{1}\text{T}{}_{\mathrm{<mtext>2</mtext><mtext>p</mtext>}}$ (transverse) solvent proton relaxation rates resulted in evaluation of a number of parameters for solvent water molecules interacting in the first coordination sphere of Mn(II) bound to concanavalin A. The ratio of the number of water molecules (q) to the Mn(II)-proton distance (r) obtained from a computer fit of the data over a limited temperature range is in accord with the findings of Koenig et al. ((1973) Proc. Nat. Acad. Sci.70, 475) and Meirovitch and Kalb ((1973) Biochim. Biophys. Acta303, 258). However, our studies of $\text{1}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>p</mtext>}}$ and $\text{1}\text{T}{}_{\mathrm{<mtext>2</mtext><mtext>p</mtext>}}$ of water over a more extensive temperature range are best fit with the following conclusions: at low temperatures (<20 °C), the data are consistent with an outer-sphere relaxation process. At higher temperatures (> 30 °C), the water molecule in the inner coordination sphere of the bound Mn(II) begins exchanging more rapidly and contributes to the relaxation processes ($\text{1}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>p</mtext>}}$ and $\text{1}\text{T}{}_{\mathrm{<mtext>2</mtext><mtext>p</mtext>}}$). The relaxation time of protons in the inner coordination shell, T_1M, contributes over the entire temperature range and produces a frequency dependence in the relaxivity data from 6 to 100 MH_z since the contributions to the correlation times are in the range 10^?9-10^?8 sec.     

Oxidation of anionic nitroalkanes by flavoenzymes,and participation of superoxide anion in the catalysis

The reactivities of anionic nitroalkanes with 2-nitropropane dioxygenase of Hansenula mrakii, glucose oxidase of Aspergillus niger, and mammalian d-amino acid oxidase have been compared kinetically. 2-Nitropropane dioxygenase is 1200 and 4800 times more active with anionic 2-nitropropane than d-amino acid oxidase and glucose oxidase, respectively. The apparent K_m values for anionic 2-nitropropane are as follows: 2-nitropropane dioxygenase, 1.61 mm; glucose oxidase, 16.7 mm; and d-amino acid oxidase, 11.1 mm. Anionic 2-nitropropane undergoes an oxygenase reaction with 2-nitropropane dioxygenase and glucose oxidase, and an oxidase reaction with d-amino acid oxidase. In contrast, anionic nitroethane is oxidized through an oxygenase reaction by 2-nitropropane dioxygenase, and through an oxidase reaction by glucose oxidase. All nitroalkane oxidations by these three flavoenzymes are inhibited by Cu and Zn-superoxide dismutase of bovine blood, Mn-superoxide dismutases of bacilli, Fe-superoxide dismutase of Serratia marcescens, and other $\text{O}{}_2{}^{\mathrm{?}}$ scavengers such as cytochrome c and NADH, but are not affected by hydroxyl radical scavengers such as mannitol. None of the $\text{O}{}_2{}^{\mathrm{?}}$ scavengers tested affected the inherent substrate oxidation by glucose oxidase and d-amino acid oxidase. Furthermore, the generation of $\text{O}{}_2{}^{\mathrm{?}}$ in the oxidation of anionic 2-nitropropane by 2-nitropropane dioxygenase was revealed by ESR spectroscoy. The ESR spectrum of anionic 2-nitropropane plus 2-nitropropane dioxygenase shows signals at g₁ = 2.007 and g₁₁ = 2.051, which are characteristic of $\text{O}{}_2{}^{\mathrm{?}}$. The $\text{O}{}_2{}^{\mathrm{?}}$ generated is a catalytically essential intermediate in the oxidation of anionic nitroalkanes by the enzymes.     

Respiration-driven proton translocation with nitrite and nitrous oxide in Paracoccus denitrificans

(1) $\text{H}$⁺/electron acceptor ratios have been determined with the oxidant pulse method for cells of denitrifying Paracoccus denitrificans oxidizing endogenous substrates during reduction of O₂, NO^?₂ or N₂O. Under optimal H⁺-translocation conditions, the ratios $\text{H}{}^{\mathrm{+}}\text{O}$, $\text{H}{}^{\mathrm{+}}\text{N}{}_2\text{O}$, $\text{H}{}^{\mathrm{+}}\text{NO}{}^{\mathrm{?}}{}_2$ for reduction to N₂ and $\text{H}{}^{\mathrm{+}}\text{NO}{}^{\mathrm{?}}{}_2$ for reduction to N₂O were 6.0–6.3, 4.02, 5.79 and 3.37, respectively. (2) With ascorbate/N,N,N′,N′-tetramethyl-p-phenylenediamine as exogenous substrate, addition of NO^?₂ or N₂O to an anaerobic cell suspension resulted in rapid alkalinization of the outer bulk medium. $\text{H}{}^{\mathrm{+}}\text{N}{}_2\text{O}$, $\text{H}{}^{\mathrm{+}}\text{NO}{}^{\mathrm{?}}{}_2$ for reduction to N₂ and $\text{H}{}^{\mathrm{+}}\text{NO}{}^{\mathrm{?}}{}_2$ for reduction to N₂O were ?0.84, ?2.33 and ?1.90, respectively. (3) The $\text{H}{}^{\mathrm{+}}\text{oxidant}$ ratios, mentioned in item 2, were not altered in the presence of $\text{valinomycin}\text{K}{}^{\mathrm{+}}$ and the triphenylmethylphosphonium cation. (4) A simplified scheme of electron transport to O₂, NO^?₂ and N₂O is presented which shows a periplasmic orientation of the nitrite reductase as well as the nitrous oxide reductase. Electrons destined for NO^?₂, N₂O or O₂ pass two H⁺-translocating sites. The $\text{H}{}^{\mathrm{+}}\text{electron}$ acceptor ratios predicted by this scheme are in good agreement with the experimental values.     

A general method for the direct isolation of the 3'-terminal fragments derived from transfer RNA molecules

A general procedure for the isolation of 3′-linked fragments derived from tRNA molecules is described. Purified N-2-naphthoxyacetylglycyl derivatives of the $\text{tRNA}{}_1{}^{\mathrm{Gly}}$ and $\text{tRNA}{}_2{}^{\mathrm{Gly}}$ of yeast were exhaustively digested with RNase T₁ and the 3′-linked fragments (bearing the derivative) were separated from other degradation products (lacking the derivative) by stepwise chromatography on BD-cellulose. Subsequent chromatographic resolution and base-composition analysis allowed tentative identification of the 3′-terminals of $\text{tRNA}{}_1{}^{\mathrm{Gly}}$ and $\text{tRNA}{}_2{}^{\mathrm{Gly}}$ as Gp(Cp,Ap)CpCpA and Gp(Cp,Cp,Up,Ap)CpCpA, respectively. The potential utility of this procedure for development of a novel approach to nucleic acid sequence analysis is discussed.     

Heterozygote frequencies in small subpopulations.

The mean fixation index within subpopulations (F_IS) has been defined as $\text{F}\text{̄}{}_{\mathrm{IS}}\text{= ∑w}{}_{\mathrm{i}}\text{F}{}_{\mathrm{ISi}}\text{or as}\text{F}\text{̂}{}_{\mathrm{IS}}\text{=}\text{∑w}{}_{\mathrm{i}}\text{p}{}_{\mathrm{i}}\text{q}{}_{\mathrm{i}}\text{F}{}_{\mathrm{ISi}}\text{∑w}{}_{\mathrm{i}}\text{p}{}_{\mathrm{i}}\text{q}{}_{\mathrm{i}}$. The latter definition is preferred because it can be obtained from the two other fixation indices, F_ST and F_IT and because it is unaffected by the mean gene frequency. The expected frequency of heterozygotes in small subpopulations of dioecious organisms will exceed Hardy-Weinberg expectations and this can be measured by $\text{F}\text{̂}{}_{\mathrm{IS}}$. In an isolated subpopulation of constant variance effective size $\text{N,}\text{F}\text{̂}{}_{\mathrm{IS}}$ rapidly tends to $\text{1 − 4N}{}^2\text{(N − 1 + [N}{}^2\text{+ 1]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}^2$. In the Island model of population structure, $\text{F}\text{̂}{}_{\mathrm{IS}}$ is approximately $\text{−(1 − m)}\text{N}\text{where}\text{m}$ is the immigration rate.When a sample is drawn from a natural population, the observed F_IS will depend upon the genetic structure of the population. The values of F_IS expected in three different types of population structure are discussed.     

Sodium exchange between two sites the binding of sodium to halotolerant bacteria

The longitudinal and transverse relaxation curves of sodium undergoing exchange between two sites are presented. When the two sites are ‘bound’ and ‘free’ sodium respectively, the relaxation curves are, in general, not exponential. It is shown that in some cases only one exponential decay could be detected experimentally though the true decay curve is much more complicated. In such cases where the population of ‘free’ and ‘bound’ sodium are equal, only 40–70% of the total intensity would be detected, depending on the lifetime of sodium in the two sites. It is also shown that the fast exchange approximation, usually applied in the interpretation of sodium relaxation curves, might lead to wrong conclusions.Measurements of sodium relaxation times in halotolerant bacteria show that T₁ and T₂ are different and frequency-dependent. The intensity of the sodium signal is 40% of the tota sodium concentration. It was possible to simulate the relaxation behaviour and intensity measurements by applying the following model. There are three types of sodium: the extracellular sodium (A) which exchanges with part of the intracellular sodium (B) and a fraction (C) which is bound but does not exchange with the extracellular sodium. It was possible to estimate the physical properties of sodium at site B. The quadrupole coupling constant $\text{(e}{}^2\text{qQ/}\text{h}\text{?}\text{)}{}_{\mathrm{<mtext>B</mtext>}}\text{= 9 · 10}{}^6\text{rad/s}$, the correlation time ⊥_cB = 5.5 · 10^?7 s and the lifetime of sodium at site B, ⊥_B = 6 · 10^?4 s.     

Effects of 5-hydroxylysine on acetylene reduction and NH4+-assimilation in the cyanobacterium Anabaena cylindrica.

5-hydroxylysine, an analogue of glutamate and lysine, causes $\text{NH}{}_4{}^{\mathrm{+}}$ production by N₂-fixing A. cylindrica; it also reversibly inhibits GS activity in vitro but has no effect on alanine dehydrogenase or GOGAT. On adding 5-hydroxylysine intracellular pools of glutamine, glutamate and aspartate decrease; those of alanine and serine increase. 5-hydroxylysine alleviates the inhibitory effect of $\text{NH}{}_4{}^{\mathrm{+}}$ on heterocyst production and C₂H₂ reduction and in $\text{NH}{}_4{}^{\mathrm{+}}\text{-grown}$ cultures results in heterocyst synthesis and in C₂H₂ reduction. The data suggest that the GS-GOGAT pathway is the sole route of importance in primary $\text{NH}{}_4{}^{\mathrm{+}}$ assimilation in A. cylindrica, that $\text{NH}{}_4{}^{\mathrm{+}}$ alone does not inhibit nitrogenase and heterocyst production, and that GS and/or a product is involved in regulating the production of both.     

Interaction of lymphocytes and macrophage cell line cells (M1 cells). I. Functional maturation and appearance of Fc receptors im M1 cells

M₁ cells, which are cell line cells established from myeloid leukemia cells of the SL strain mouse, can differentiate from blast cells ($\text{M}{}_1{}^{\mathrm{?}}$) to mature macrophages ($\text{M}{}_1{}^{\mathrm{+}}$) within 48 hr, when they are cultured with conditioned medium (CM) obtained from murine embryonic fibroblasts. While $\text{M}{}_1{}^{\mathrm{?}}$ cells have no phagocytic activity nor Fc receptor (FcR), $\text{M}{}_1{}^{\mathrm{+}}$ cells possess both characteristics. The appearance of FcR is temperature-dependent and inhibited by a metabolic inhibitor, cycloheximide. FcR on $\text{M}{}_1{}^{\mathrm{+}}$ cells is resistant to trypsin and pronase. $\text{M}{}_1{}^{\mathrm{+}}$ cells improve the viability of macrophage-depleted SL splenic lymphocytes and restore the in vitro secondary plaque forming cell response of macrophage-depleted spleen cells to particulate and soluble antigens. $\text{M}{}_1{}^{\mathrm{?}}$ cells lack this macrophage-substituting capacity. Mm₁ cells, mutant cells from M₁ cells, having FcR and higher phagocytic activity than $\text{M}{}_1{}^{\mathrm{+}}$ cells, are also devoid of this capacity.     

Nuclear Overhauser enhancement evidence for inverse temperature dependence of hydrophobic side chain proximity in the polytetrapeptide of tropoelastin

Previous studies on aqueous solutions of HCO-(Val-Pro-Gly-Gly)₄₀-Val-OMe indicated an increase in secondary structure on increasing the temperature implying a concomitant intramolecular hydrophobic association. Nuclear Overhauser enhancement (NOE) studies are reported which explicitly demonstrate an increase in association of $\text{γ}\text{CH}{}_3$ of Val and $\text{δ}\text{CH}{}_2$ of Pro protons on increasing temperature. The analogue where Ala replaces Val does not show this inverse temperature transition. These results provide direct demonstration of the “hydrophobic effect” responsible for inverse temperature transitions in aqueous systems.     

Some dilute-solution parameters of the levan of streptococcus salivarius in various solvents

The intrinsic viscosities, weight-average molecular weights (M?_w), and radii of gyration [($\text{R}{}^2{}_{\mathrm{g}}$)^{$\text{1}\text{2}$}≈] of Streptococcus salivarius levan in various solvents were respectively obtained from viscosity and light-scattering measurements. The data showed that the levan in water is not aggregated by hydrogen bonds, and that the values of both the refractive index and ($\text{R}{}^2{}_{\mathrm{g}}$)^{$\text{1}\text{2}$} are lower in water than in aqueous solutions of urea. Urea may break intramolecular hydrogen-bonds, e.g., between branches, allowing the molecule to expand.     

Optimizing enzyme assays with one or two coupling enzymes

The cost of assays using one or two coupling enzymes is optimized by using equations to calculate the minimum amount(s) of enzyme(s) which should be used to obtain a given time (t₉₉) in which 99% of the rate V₀ of the first reaction is obtained. Using two coupling enzymes and given a value of t₉₉, the induction period L = L₁ + L₂ fulfills the requirement $\text{t}{}_{99}\text{2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{4.6}\text{≥ L ≥}\text{t}{}_{99}\text{4.6}$, allowing one to choose a cost lower than that derived from the until-now generally applied assumption of t₉₉ = 4.6L. Being $\text{α =}\text{L}{}_1\text{L}{}_2$, in optimized assays the values α, t₉₉, and L are related by $\text{T}{}_{99}\text{=4.6}\text{(1+α)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{1+α}\text{L}$, thus allowing (graphical) calculation of the amounts of coupling enzymes which will minimize the cost for every chosen t₉₉ or L. Maximum practical rates, allowed in some supposed interesting cases, have been calculated.     

A thermodynamic description of the self-association of flavin mononucleotide.

Systematic heat of dilution studies of the self-association of flavin mononucleotide (FMN) have been conducted as a function of ionic strength (0.05 – 2.0 m) and pH (5–9) in aqueous solution. The data are adequately described by the expression $\text{Q}{}_{\mathrm{T}}\text{= ΔH ? (}\text{ΔH}\text{K}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(}\text{Q}{}_{\mathrm{T}}\text{c}{}_{\mathrm{T}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ for an isodesmic self-association. Q_T is the molar heat of dilution, ΔH and K are the derived enthalpy and equilibrium constants for the process FMN + (FMN)_i?1 ? (FMN)_i, and c_T is the concentration of FMN expressed in monomer units. Typical values derived for the various thermodynamic parameters at 25 °C are ΔG = ?3.56 kcal mol^?1, ΔH = ?3.72 kcal mol^?1, and ΔS = ?0.54 cal (mol · deg)^?1. These data, plus nuclear magnetic resonance evidence (Yagi, K., Ohishi, N., Takai, A., Kawano, K., and Kyogoku, Y., 1976, Biochemistry15, 2877–2880) argue in favor of an open-ended association of flavin molecules. The signs of the various thermodynamic parameters suggest that both hydrophobic and surface energy forces contribute significantly to the association, while the lack of any significant ionic strength dependence indicates the lack of any ionic centers in the association.     

Preferential solvation of methylmercurated calf thymus deoxyribonucleic acid.

The changes in polymer-solvent interactions that occur when native calf thymus DNA is dialyzed against Na₂SO₄ solutions of a given ionic strength and buffer concentration but of varying concentrations in methylmercuric hydroxide have been investigated with the help of solution density measurements at 25 °C and pH 6.8–7.0. From measurements executed under equilibrium dialysis conditions at the three salt levels 5 mm, 0.05 m, and 0.5 m Na₂SO₄ (m refers to molality) and in the presence of 5 mm cacodylic acid buffer, the density increments $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ for native calf thymus DNA were determined as a function of CH₃HgOH concentration. $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ was found not to vary with organomercurial concentration, irrespective of the concentration of supporting electrolyte, until a certain CH₃HgOH concentration level has been reached, viz., , beyond which $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ increases strongly with increasing concentration of CH₃HgOH. As is shown by optical melting, $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ becomes a function of organomercurial concentration the moment DNA undergoes denaturation brought about by the complexing of CH₃HgOH with the various N-binding sites of the base residues in the DNA double helix.Polymer-solvent interactions, expressed in terms of preferential water interactions (“net hydration”) and preferential salt interactions (“salt solvation”), were derived from the $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ data in combination with data obtained on the preferential interaction of CH₃HgOH with denatured DNA and data on the partial specific volumes of all major solution components, gathered from density measurements on solutions with fixed concentrations of diffusible components. Evidence is presented which shows that denaturation in general decreases the net hydration while salt becomes preferentially associated with the polyelectrolyte. This process is further amplified by the interaction of CH₃HgOH with denatured DNA: Methylmercurated DNA alters the redistribution of diffusible components at dialysis equilibrium to such an extent that in a formal sense large amounts of water are rejected from the immediate vicinity of the polymer. The molecular implications of these findings are explored. The results are further discussed in the light of previous findings where the methylmercury-induced denaturation of DNA had been studied with the help of buoyant density measurements in a Cs₂SO₄ density gradient and by velocity-sedimentation in a variety of sulfate media.     

Influence of ionic strength upon the proton inventory for the water-catalyzed hydrolysis of N-acetylbenzotriazole

Proton inventory investigations of the hydrolysis N-acetylbenzotriazole at pH 3.0 (or the equivalent point on the pD rate profile) have been conducted at two different temperatures and at ionic strengths ranging from 0 to 3.0 M. The solvent deuterium isotope effects and proton inventories are remarkably similar over this wide range of conditions. The proton inventories suggest a cyclic transition state involving four protons contributing to the solvent deuterium isotope effect for the water-catalyzed hydrolysis. The hydrolysis data are described by the equation $\text{k}{}_{\mathrm{n}}\text{= k}{}_{\mathrm{<mtext>o</mtext>}}\text{(1 ? n + nπ}{}_{\mathrm{<mtext>a</mtext>}}{}^1\text{)}{}^4$ with $\text{π}{}_{\mathrm{<mtext>a</mtext>}}{}^1\text{～ 0.74}$, where k_o is the observed first-order rate constant in protium oxide, n is the atom fraction of deuterium in the solvent, k_n is the rate constant in a protium oxide-deuterium oxide mixture, and $\text{π}{}_{\mathrm{<mtext>a</mtext>}}{}^1$ is the isotopic fractionation factor.     

New experimental approach to the estimation of rate of electron transfer from the primary to secondary acceptors in the photosynthetic electron transport chain of purple bacteria

A method for calculating the rate constant ($\text{K}{}_{\mathrm{<mtext>A</mtext><mtext>1</mtext><mtext>A</mtext><mtext>2</mtext>}}$) for the oxidation of the primary electron acceptor (A₁) by the secondary one (A₂) in the photosynthetic electron transport chain of purple bacteria is proposed.The method is based on the analysis of the dark recovery kinetics of reaction centre bacteriochlorophyll (P) following its oxidation by a short single laser pulse at a high oxidation-reduction potential of the medium. It is shown that in Ectothiorhodospira shaposhnikovii there is little difference in the value of $\text{K}{}_{\mathrm{<mtext>A</mtext><mtext>1</mtext><mtext>A</mtext><mtext>2</mtext>}}$ obtained by this method from that measured by the method of Parson ((1969) Biochim. Biophys. Acta 189, 384–396), namely: $\text{(4.5±1.4) · 10}{}^3\text{s}{}^{\mathrm{?1}}$ and $\text{(6.9±1.2) · 10}{}^3\text{s}{}^{\mathrm{?1}}$, respectively.The proposed method has also been used for the estimation of the $\text{K}{}_{\mathrm{<mtext>A</mtext><mtext>1</mtext><mtext>A</mtext><mtext>2</mtext>}}$ value in chromatophores of Rhodospirillum rubrum deprived of constitutive electron donors which are capable of reducing P⁺ at a rate exceeding this for the transfer of electron from A₁ to A₂. The method of Parson cannot be used in this case. The value of $\text{K}{}_{\mathrm{<mtext>A</mtext><mtext>1</mtext><mtext>A</mtext><mtext>2</mtext>}}$ has been found to be $\text{(2.7±0.8) · 10}{}^3\text{s}{}^{\mathrm{?1}}$.The activation energies for the A₁ to A₂ electron transfer have also been determined. They are 12.4 kcal/mol and 9.9 kcal/mol for E. shaposhnikovii and R. rubrum, respectively.     

Damping of oscillations in the semiquinone absorption in reaction centers after successive flashes. Determination of the equilibrium between QA−QB and QAQB−

A quantitative model for the damping of oscillations of the semiquinone absorption after successive light flashes is presented. It is based on the equilibrium between the states Q_A^?Q_B and Q_AQ_B^?. A fit of the model to the experimental results obtained for reaction centers from Rhodopseudomonas sphaeroides gave a value of $\text{α =}\text{[}\text{Q}{}_{\mathrm{<mtext>A</mtext>}}{}^{\mathrm{?}}\text{Q}{}_{\mathrm{<mtext>B</mtext>}}\text{]}\text{([}\text{Q}{}_{\mathrm{<mtext>A</mtext>}}{}^{\mathrm{?}}\text{Q}{}_{\mathrm{<mtext>B</mtext>}}\text{] + [}\text{Q}{}_{\mathrm{<mtext>A</mtext>}}\text{Q}{}_{\mathrm{<mtext>B</mtext>}}{}^{\mathrm{?}}\text{])}\text{= 0.065 ± 0.005}$ (T = 21°C, pH 8).     

Hydration of Escherichia coli lipids: Deuterium T1 relaxation time studies of phosphatidylglycerol,phosphatidylethanolamine and phosphatidylcholine

The hydration properties of Escherichia coli lipids (phosphatidylglycerol, phosphatidylethanolamine) and synthetic $\text{1,2-}\text{dioleoyl}\text{-sn-}\text{glycero-3-phosphocholine}$ in H₂O/²H₂O mixtures (9:1, v/v) were investigated with ²H-NMR. Comparison of the ²H₂O spin lattice relaxation time ($\text{T}{}_1$) as a function of the water content revealed a remarkable quantitative similarity of all three lipid-H₂O systems. Two distinct hydration regions could be discerned in the $\text{T}{}_1$ relaxation time profile. (1) A minimum of 11–16 water molecules was needed to form a primary hydration shell, characterized by an average relaxation time of $\text{T}{}_1\text{≈ 90}\text{ms}$. (2) Additional water was found to be in exchange with the primary hydration shell. The exchange process could be described in terms of a two-site exchange model, assuming rapid exchange between bulk water with $\text{T}{}_1\text{= 500}\text{ms}$ and hydration water with $\text{T}{}_1\text{= 80–120}\text{ms}$. Analysis of the linewidth and the residual quadrupole splitting (at low water content) confirmed the size of the primary hydration layer. However, each lipid-water system exhibited a somewhat different linewidth behavior, and a detailed molecular interpretation appeared to be preposterous.