Comparing Membrane Simulations to Scattering Experiments: Introducing the SIMtoEXP Software

SIMtoEXP is a software package designed to facilitate the comparison of biomembrane simulations with experimental X-ray and neutron scattering data. It has the following features: (1) Accepts number density profiles from simulations in a standard but flexible format. (2) Calculates the electron density ε(z) and neutron scattering length density ν(z) profiles along the z direction (i.e., normal to the membrane) and their respective Fourier transforms (i.e., F _ε [q _z ] and F _ν [q _z ]). The resultant four functions are then displayed graphically. (3) Accepts experimental F _ε (q _z ) and F _ν (q _z ) data for graphical comparison with simulations. (4) Allows for lipids and other large molecules to be parsed into component groups by the user and calculates the component volumes following Petrache et al. (Biophys J 72:2237–2242, 1997). The software then calculates and displays the contributions of each component group as volume probability profiles, ρ(z), as well as the contributions of each component to ε(z) and ν(z).     

Photoinhibition of colonial and unicellular <Emphasis Type="Italic">Microcystis</Emphasis> cells in a summer bloom in Lake Taihu

To evaluate the photoinhibition of colonial and unicellular cells of Microcystis aeruginosa under natural conditions, the maximum and effective quantum yields of photosystem II were measured from variable chlorophyll a fluorescence in samples from Lake Taihu during a summer bloom from June 19 to 21, 2006. Diurnal changes in the photoinhibition of Microcystis cells incubated immediately below the surface in clear bottles for 30 min and in situ samples under natural conditions were measured. At solar noon during the three days, the mean values of maximum quantum yield (F _v/F _m) and effective quantum yield (ΔF/F _m′) for unicellular cells (F _v/F _m = 0.15, ΔF/F _m′ = 0.10) were lower than those for colonial cells (F _v/F _m = 0.25, ΔF/F _m′ = 0.15). For in situ samples, the values of F _v/F _m and ΔF/F _m′ for colonial cells at solar noon on the three days (F _v/F _m 0.30, 0.25, 0.29; ΔF/F _m′ 0.24, 0.21, 0.22) were also higher than those of unicellular cells (F _v/F _m 0.26, 0.18, 0.25; ΔF/F _m′ 0.15, 0.11, 0.14). The results indicate that colony formation has a protective effect on Microcystis cells by reducing the occurrence of photoinhibition under high light intensities.     

Dombrock blood group (DO): assignment to chromosome 12p

The Dombrock blood group system (DO) is a common polymorphism in Caucasians, represented by two red cell antigen alleles. In a linkage study in our family material of 832 families from the Copenhagen area, we found a strong indication of tight linkage with the two flanking DNA polymorphisms D12S358 (z = 7.66; at θ _M = 0.001, θ _F = 0.031) and D12S364 (z = 8.53; at θ _M = 0.068, θ _F = 0.031). DO is assigned to the region 12p13.2– 12p12.1 by physically localised markers. Received: 18 April 1996 / Revised: 4 July 1996     

Ambient levels of UV-B in Hawaii combined with nutrient deficiency decrease photosynthesis in near-isogenic maize lines varying in leaf flavonoids: Flavonoids decrease photoinhibition in plants exposed to UV-B

Near-isogenic lines of maize varying in their genes for flavonoid biosynthesis were utilized to examine the effects of foliar flavonoids and nutrient deficiency on maximum net photosynthetic rate (P _N) and chlorophyll (Chl) fluorescence (F_v/F_m) in response to ultraviolet-B (UV-B) radiation. Plants with deficient (30 to 70 % lower N, K, Mn, Fe, and Zn) and sufficient nutrients were exposed to four irradiation regimes: (1) no UV-B with solar photosynthetically active radiation (PAR), (2) two day shift to ambient artificial UV-B, 8.2–9.5 kJ m⁻² d⁻¹ (21–25 mmol m⁻² d⁻¹); (3) continuous ambient artificial UV-B; (4) continuous solar UV-B in Hawaii 12–18 kJ m⁻² d⁻¹ (32–47 mmol m⁻² d⁻¹). The natural ratio of UVB: PAR (0.25–0.40) was maintained in the UV-B treatments. In the adequately fertilized plants, lines b and lc had higher contents of flavonoids and anthocyanins than did lines hi27 and dta. UV-B induced the accumulation of foliar flavonoids in lines hi27 and b, but not in the low flavonoid line dta or in the high flavonoid line lc. In plants grown on deficient relative to adequate nutrients, flavonoid and anthocyanin contents decreased by 30–40 and 40–50 %, respectively, and Chl a and Chl b contents decreased by 30 and 70 %, respectively. The UV-B treatments did not significantly affect P _N and F_v/F_m in plants grown on sufficient nutrients, except in the low flavonoid lines dta and hi27 in which P _N and F_v/F_m decreased by ∼15 %. P _N, F_v/F_m, and stomatal conductance decreased markedly (20–30 %) in all lines exposed to UV-B when grown on low nutrients. The decrease in F_v/F_m was 10 % less in higher flavonoid lines b and lc. The photosynthetic apparatus of maize readily tolerated ambient UV-B in the tropics when plants were adequately fertilized. In contrast, ambient UV-B combined with nutrient deficiency significantly reduced photosynthesis in this C₄ plant. Nutrient deficiency increased the susceptibility of maize to UV-B-induced photoinhibition in part by decreasing the contents of photoprotective compounds.     

The F<Subscript>X</Subscript> iron–sulfur cluster serves as the terminal bound electron acceptor in heliobacterial reaction centers

Phototrophs of the family Heliobacteriaceae contain the simplest known Type I reaction center (RC), consisting of a homodimeric (PshA)₂ core devoid of bound cytochromes and antenna proteins. Unlike plant and cyanobacterial Photosystem I in which the F_A/F_B protein, PsaC, is tightly bound to P₇₀₀–F_X cores, the RCs of Heliobacterium modesticaldum contain two F_A/F_B proteins, PshBI and PshBII, which are loosely bound to P₈₀₀–F_X cores. These two 2[4Fe–4S] ferredoxins have been proposed to function as mobile redox proteins, reducing downstream metabolic partners much in the same manner as does [2Fe–2S] ferredoxin or flavodoxin (Fld) in PS I. Using P₈₀₀–F_X cores devoid of PshBI and PshBII, we show that iron–sulfur cluster F_X directly reduces Fld without the involvement of F_A or F_B (Fld is used as a proxy for soluble redox proteins even though a gene encoding Fld is not identified in the H. modesticaldum genome). The reduction of Fld is suppressed by the addition of PshBI or PshBII, an effect explained by competition for the electron on F_X. In contrast, P₇₀₀–F_X cores require the presence of the PsaC, and hence, the F_A/F_B clusters for Fld (or ferredoxin) reduction. Thus, in H. modesticaldum, the interpolypeptide F_X cluster serves as the terminal bound electron acceptor. This finding implies that the homodimeric (PshA)₂ cores should be capable of donating electrons to a wide variety of yet-to-be characterized soluble redox partners.     

Effects of C-terminal truncation on autocatalytic processing of <Emphasis Type="Italic">Bacillus licheniformis</Emphasis> γ-glutamyl transpeptidase

The role of the C-terminal region of Bacillus licheniformis γ-glutamyl transpeptidase (BlGGT) was investigated by deletion analysis. Seven C-terminally truncated BlGGTs lacking 581–585, 577–585, 576–585, 566–585, 558–585, 523–585, and 479–585 amino acids, respectively, were generated by site-directed mutagenesis. Deletion of the last nine amino acids had no appreciable effect on the autocatalytic processing of the enzyme, and the engineered protein was active towards the synthetic substrate L-γ-glutamyl-p-nitroanilide. However, a further deletion to Val576 impaired the autocatalytic processing. In vitro maturation experiments showed that the truncated BlGGT precursors, pro-Δ(576–585), pro-Δ(566–585), and pro-Δ(558–585), could partially precede a time-dependent autocatalytic process to generate the L- and S-subunits, and these proteins showed a dramatic decrease in catalytic activity with respect to the wild-type enzyme. The parental enzyme (BlGGT-4aa) and BlGGT were unfolded biphasically by guanidine hydrochloride (GdnCl), but Δ(577–585), Δ(576–585), Δ(566–585), Δ(558–585), Δ(523–585), and Δ(479–585) followed a monophasic unfolding process and showed a sequential reduction in the GdnCl concentration corresponding to half effect and ΔG ₀ for the unfolding. BlGGT-4aa and BlGGT sedimented at ∼4.85 S and had a heterodimeric structure of approximately 65.23 kDa in solution, and this structure was conserved in all of the truncated proteins. The frictional ratio (f/f _o) of BlGGT-4aa, BlGGT, Δ(581–585), and Δ(577–585) was 1.58, 1.57, 1.46, and 1.39, respectively, whereas the remaining enzymes existed exclusively as precursor form with a ratio of less than 1.18. Taken together, these results provide direct evidence for the functional role of the C-terminal region in the autocatalytic processing of BlGGT.     

A monte carlo approach to population dynamics of cells in a HIV immune response model

Summary Using a direct Monte Carlo simulation, population growth of helper T-cells (N _H) and viral cells (N _v) is studied for an immune response model with an enhanced spatial inter-cellular interaction relevant to HIV as a function of viral mutation. In the absence of cellular mobility (P _mob=0), the helper T-cells grow nonmonotonically before reaching saturation and the viral population grows monotonically before reaching a constant equilibrium. Cellular mobility (P _mob=1) enhances the viral growth and reduces the stimulative T-cell growth. Below a mutation threshold (P _c), the steady-state density of helper T-cell (p _H) is larger than that of the Virus (p _v); the density difference Δp _o(=pV−pH) remains a constant at P _mob=1 while −Δp _o→0 as P _mut→P _c at P _mob=0. Above the mutation threshold, the difference Δp _o in cell density, grows with ΔP=P _mut−P _c monotonically: ΔP _o ∞ (ΔP)^β ≃ with β≈0.574±0.016 in absence of mobility, while Δp _o≈6(ΔP) with P _mob=1.     

On the biosorption,by brown seaweed, <Emphasis Type="Italic">Lobophora variegata</Emphasis>, of Ni(II) from aqueous solutions: equilibrium and thermodynamic studies

The biosorption equilibrium isotherms of Ni(II) onto marine brown algae Lobophora variegata, which was chemically-modified by CaCl₂ were studied and modeled. To predict the biosorption isotherms and to determine the characteristic parameters for process design, twenty-three one-, two-, three-, four- and five-parameter isotherm models were applied to experimental data. The interaction among biosorbed molecules is attractive and biosorption is carried out on energetically different sites and is an endothermic process. The five-parameter Fritz–Schluender model gives the most accurate fit with high regression coefficient, R ² (0.9911–0.9975) and F-ratio (118.03–179.96), and low standard error, SE (0.0902–0.0.1556) and the residual or sum of square error, SSE (0.0012–0.1789) values to all experimental data in comparison to other models. The biosorption isotherm models fitted the experimental data in the order: Fritz–Schluender (five-parameter) > Freundlich (two-parameter) > Langmuir (two-parameter) > Khan (three-parameter) > Fritz–Schluender (four-parameter). The thermodynamic parameters such as ΔG ⁰, ΔH ⁰ and ΔS ⁰ have been determined, which indicates the sorption of Ni(II) onto L. variegata was spontaneous and endothermic in nature.     

Modulation of Bacillus pasteurii cytochrome c 553 reduction potential by structural and solution parameters

 Direct cyclic voltammetry and ¹H NMR spectroscopy have been combined to investigate the electrochemical and spectroscopic properties of cytochrome c ₅₅₃ isolated from the alkaliphilic soil bacterium Bacillus pasteurii. A quasi-reversible diffusion-controlled redox process is exhibited by cytochrome c ₅₅₃ at a pyrolitic graphite edge microelectrode. The temperature dependence of the reduction potential, measured using a non-isothermal electrochemical cell, revealed a discontinuity at 308 K. The thermodynamic parameters determined in the low-temperature range (275–308 K;ΔS°′=–162.7±1.2 J mol^–1 K^–1, ΔH°′=–53.0±0.5 kJ mol^–1, ΔG°′=–4.5±0.1 kJ mol^–1, E°′=+47.0±0.6 mV) indicate the presence of large enthalpic and entropic effects, leading, respectively, to stabilization and destabilization of the reduced form of cytochrome c ₅₅₃. Both effects are more accentuated in the high-temperature range (308–323 K;ΔS°′=–294.1±8.4 J mol^–1 K^–1, ΔH°′=–93.4±3.1 kJ mol^–1, ΔG°′=–5.8±0.6 kJ mol^–1, E°′=+60.3±5.8 mV), with the net result being a slight increase of the standard reduction potential. These thermodynamic parameters are interpreted using the compensation theory of hydration of biopolymers as indicating the extrusion, upon reduction, of water molecules from the hydration sphere of the cytochrome. The low-T and high-T conformers differ by the number of water molecules in the solvation sphere: in the high-T conformer, the number of water molecules extruded upon reduction increases, as compared to the low-T conformer. The ionic strength dependence of the reduction potential at 298 K, treated within the frame of extended Debye-Hückel theory, yields values of E ^°′ _(I=0) =–25.4±1.4 mV, z _red=–11.3, and z _ox=–10.3. The pH dependence of the reduction potential at 298 K shows a plateau in the pH range 7–10 and an increase at more acidic pH, allowing the calculation of pK _O=5.5 and pK _R=5.7, together with the estimate of the reduction potentials of completely protonated (+71 mV) and deprotonated (+58 mV) forms of cytochrome c ₅₅₃. ¹H NMR spectra of the oxidized paramagnetic cytochrome c ₅₅₃ indicate the presence of a His-Met axial coordination of the low-spin (S=1/2) heme iron, which is maintained in the temperature interval 288–340 K at pH 7 and in the pH range 4.8–10.0 at 298 K. The temperature dependence of the hyperfine-shifted signals shows both Curie-type and anti-Curie-type behavior, with marked deviations from linearity, interpreted as indicating the presence of a fast equilibrium between the low-T and high-T conformers, having slightly different heme electronic structures resulting from the T-induced conformational change. Increasing the NaCl concentration in the range 0–0.2 M causes a slight change of the ¹H NMR chemical shifts of the hyperfine-shifted signals, with no influence on their linewidth. The calculated lower limit value of the apparent affinity constant for specific ion binding is estimated as 5.2±1.1 M^–1. The pH dependence of the isotropically shifted ¹H NMR signals of the oxidized cytochrome displays at least one ionization step with pK _O=5.7. The thermodynamic and spectroscopic data indicate a large solvent-derived entropic effect as the main cause for the observed low reduction potential of B. pasteurii cytochrome c ₅₅₃. Received: 9 January 1998 / Accepted: 8 April 1998     

Surface Rheology Parameters of Source-Specific Surfactant Films as Indicators of Organic Matter Dynamics

The paper contains the results of natural film experiments carried out on inland and coastal waters in the Dead Vistula catchment area and mouth during 2000–2002, using the integrated Langmuir trough–Wilhelmy plate system. The static film parameters result from the generalized scaling procedures applied to the surface pressure–area isotherms. They appear to reflect in a quantitative and sensitive way the film composition (A_lim, M_w, E_isoth), film solubility and the miscibility of its components (via R, ΔS_c and y factors), and surface concentration (π_eq, Γ_eq). The adsorption kinetics parameters: effective diffusion coefficient D_eff/D and activation energy barrier E_a/RT are derived from dynamic surface pressure. There is a reason to suggest that certain classes of film-forming components or ‘end-members’ may dominate the static and dynamic surface properties. Variation in the surface rheological parameters of source-specific biosurfactants is postulated to reflect organic matter dynamics in natural waters and were measured for the Dead Vistula river, its tributaries and the adjacent coastal area.     

Leaf discs floated on water are different from intact leaves in photosynthesis and photoinhibition

Photoinhibition has been often evaluated with leaf discs floated on water or placed on wet papers to prevent desiccation. Under these conditions, there is a possibility that CO₂ diffusion is blocked by water, which may lead to reduction in photosynthetic CO₂ assimilation. Using Chenopodium album L. grown at two irradiances, photosynthesis, quantum yield of Photosystem II (ΔF/F _m′), non-photochemical quenching (qN), and photoinhibition were compared between detached leaves and leaf discs. In low-light-grown plants, photoinhibition was greater in leaf discs than in detached leaves, while in high-light-grown plants, there was little difference. Leaf discs showed lower rates of photosynthesis and ΔF/F _m′, and higher qN. The ΔF/F _m′ in leaf discs increased when leaf discs were exposed to high concentration of CO₂, suggesting that CO₂ diffusion to chloroplasts was limited in leaf discs floated on water. This revised version was published online in June 2006 with corrections to the Cover Date.     

Assessing photosynthetic efficiency in an experimental mangrove canopy using remote sensing and chlorophyll fluorescence

This study examined the ability of the photochemical reflectance index (PRI) to track changes in effective quantum yield (Δ F/F _m ′), non-photochemical quenching (NPQ), and the xanthophyll cycle de-epoxidation (DPS) in an experimental mangrove canopy. PRI was correlated with (Δ F/F _m ′) and NPQ over the 4-week measurement period and over the diurnal cycle. The normalised difference vegetation index (NDVI) was not correlated with any aspect of photochemical efficiency measured using chlorophyll fluorescence or xanthophyll pigments. This study demonstrated that photochemical adjustments were responsible for controlling the flow of energy through the photosynthetic apparatus in this mangrove forest canopy rather than canopy structural or chlorophyll adjustments.     

Up-regulation of cyclic electron flow and down-regulation of linear electron flow in antisense-<Emphasis Type="Italic">rca</Emphasis> mutant rice

To investigate how excess excitation energy is dissipated in a ribulose-1,5-bisphospate carboxylase/oxygenase activase antisense transgenic rice with net photosynthetic rate (P _N) half of that of wild type parent, we measured the response curve of P _N to intercellular CO₂ concentration (C _i), electron transport rate (ETR), quantum yield of open photosystem 2 (PS2) reaction centres under irradiation (F_v′/F_m′), efficiency of total PS2 centres (Φ_PS2), photochemical (q_P) and non-photochemical quenching (NPQ), post-irradiation transient increase in chlorophyll (Chl) fluorescence (PITICF), and P700⁺ re-reduction. Carboxylation efficiency dependence on C _i, ETR at saturation irradiance, and F_v′/F_m′, Φ_PS2, and q_P under the irradiation were significantly lower in the mutant. However, NPQ, energy-dependent quenching (q_E), PITICF, and P700⁺ re-reduction were significantly higher in the mutant. Hence the mutant down-regulates linear ETR and stimulates cyclic electron flow around PS1, which may generate the ΔpH to support NPQ and q_E for dissipation of excess excitation energy.     

A quantitative nature of somatic embryogenesis in soybean

Summary The objectives of this research were to investigate the genetic parameters associated with the in vitro formation of somatic embryos in soybean and to determine the effect of light intensity on the embryogenic capability of F₁, F₂, and backcross (RC₁P₁ and RC₁P₂) progenies derived from crosses between embryogenic (IAS-5 and Embrapa-1) and non-embryogenic (Paraná) cultivars. Immature cotyledons (4–6 mm in length) derived from the parental lines, F₁, F₂, RC₁P₁, and RC₁P₂ were grown for 90 d on the inductive N10 medium, after which the number of somatic embryos was recorded. Chi-square tests for goodness of fit showed that the genetic component of the somatic embryogenesis trait is controlled in a quantitative manner by approximately 10 genes. A normal distribution for somatic embryo formation in the F₂ generations was observed reinforcing the quantitative nature of the trait. Variation in light intensity (8–12 and 27–33 μmol m⁻²s⁻¹) had no effect on somatic embryo formation in the parental material tested.     

Interaction of human serum albumin with 10-hydroxycamptothecin: spectroscopic and molecular modeling studies

In this work, fluorescence spectroscopy in combination with circular dichroism spectroscopy and molecular modeling was employed to investigate the binding of 10-hydroxycamptothecin (HCPT) to human serum albumin (HSA) under simulative physiological conditions. The experiment results showed that the fluorescence quenching of HSA by HCPT was a result of the formation of HCPT–HSA complex. The corresponding association constants (K _a) between HCPT and HSA at four different temperatures were determined according to the modified Stern–Volmer equation. The results of thermodynamic parameters ΔG, ΔH, and ΔS indicated that hydrogen bonds and van der Waals forces played major roles for HCPT–HSA association. Site marker competitive displacement experiment indicated that the binding of HCPT to HSA primarily took place in sub-domain IIA (site I). Molecular docking study further confirmed the binding mode and the binding site obtained by fluorescence and site marker competitive experiments. The conformational investigation showed that the presence of HCPT decreased the α-helical content of HSA and induced the slight unfolding of the polypeptides of protein, which confirmed some micro-environmental and conformational changes of HSA molecules.     

Physiological state of phytoplankton communities in the Southwest Atlantic sector of the Southern Ocean, as measured by fast repetition rate fluorometry

The majority of the Southern Ocean is a high-nutrient low-chlorophyll (HNLC) ecosystem. Localized increases in chlorophyll concentration measured in the wake of bathymetric features near South Georgia demonstrate variations in the factors governing the HNLC condition. We explore the possibility that the contrast between these areas of high-chlorophyll and surrounding HNLC areas is associated with variations in phytoplankton photophysiology. Total dissolvable iron concentrations, phytoplankton photophysiology and community structure were investigated in late April 2003 on a transect along the North Scotia Ridge (53–54°S) between the Falkland Islands and South Georgia (58–33°W). Total dissolvable iron concentrations suggested a benthic source of iron near South Georgia. Bulk community measurements of dark-adapted photochemical quantum efficiency (F _v/F _m) exhibited a sharp increase to the east of 46°W coincident with a decrease in the functional absorption cross-section (σ_PSII). Phytoplankton populations east of 46°W thus displayed no physiological symptoms of iron or nitrate stress. Contrasting low F _v/F _m west of 46°W could not be explained by variations in the macronutrients nitrate and silicic acid and may be the result of taxon specific variability in photophysiology or iron stress. We hypothesize that increased F _v/F _m resulted from local relief from iron-stress near South Georgia, east of Aurora Bank, an area previously speculated to be a “pulse point” source of iron. Our measurements provide one of the first direct physiological confirmations that iron stress is alleviated in phytoplankton populations near South Georgia.     

Toward an accurate statistics of gapped alignments

Sequence alignment has been an invaluable tool for finding homologous sequences. The significance of the homology found is often quantified statistically by p-values. Theory for computing p-values exists for gapless alignments [Karlin, S., Altschul, S.F., 1990. Methods for assessing the statistical significance of molecular sequence features by using general scoring schemes. Proc. Natl. Acad. Sci. USA 87, 2264–2268; Karlin, S., Dembo A., 1992. Limit distributions of maximal segmental score among Markov-dependent partial sums. Adv. Appl. Probab. 24, 13–140], but a full generalization to alignments with gaps is not yet complete. We present a unified statistical analysis of two common sequence comparison algorithms: maximum-score (Smith-Waterman) alignments and their generalized probabilistic counterparts, including maximum-likelihood alignments and hidden Markov models. The most important statistical characteristic of these algorithms is the distribution function of the maximum score S _max, resp. the maximum free energy F _max, for mutually uncorrelated random sequences. This distribution is known empirically to be of the Gumbel form with an exponential tail P(S _max > x) ∼ exp(−λx) for maximum-score alignment and P(F _max > x) ∼ exp(−λx) for some classes of probabilistic alignment. We derive an exact expression for λ for particular probabilistic alignments. This result is then used to obtain accurate λ values for generic probabilistic and maximum-score alignments. Although the result demonstrated uses a simple match-mismatch scoring system, it is expected to be a good starting point for more general scoring functions.     

Effect of 24-Epibrassinolide on Chlorophyll Fluorescence and Photosynthetic CO<Subscript>2</Subscript> Assimilation in <Emphasis Type="Italic">Vicia faba</Emphasis> Plants Treated with the Photosynthesis-Inhibiting Herbicide Terbutryn

Simultaneous measurements of chlorophyll (Chl) fluorescence and CO₂ assimilation (A) in Vicia faba leaves were taken during the first weeks of growth to evaluate the protective effect of 24-epibrassinolide (EBR) against damage caused by the application of the herbicide terbutryn (Terb) at pre-emergence. V. faba seeds were incubated for 24 h in EBR solutions (2 × 10⁻⁶ or 2 × 10⁻⁵ mM) and immediately sown. Terb was applied at recommended doses (1.47 or 1.96 kg ha⁻¹) at pre-emergence. The highest dose of Terb strongly decreased CO₂ assimilation, the maximum quantum yield of PSII photochemistry in the dark-adapted state (F _V/F _M), the nonphotochemical quenching (NPQ), and the effective quantum yield (ΔF/F′_M) during the first 3–4 weeks after plant emergence. Moreover, Terb increased the basal quantum yield of nonphotochemical processes (F ₀/F _M)_, the degree of reaction center closure (1 − q _p), and the fraction of light absorbed in PSII antennae that was dissipated via thermal energy dissipation in the antennae (1 − F′_V/F′_M). The herbicide also significantly reduced plant growth at the end of the experiment as well as plant length, dry weight, and number of leaves. The application of EBR to V. faba seeds before sowing strongly diminished the effect of Terb on fluorescence parameters and CO₂ assimilation, which recovered 13 days after plant emergence and showed values similar to those of control plants. The protective effect of EBR on CO₂ assimilation was detected at a photosynthetic photon flux density (PFD) of 650 μmol m⁻² s⁻¹ and the effect on ΔF/F′_M and photosynthetic electron transport (J) was detected under actinic lightings up to 1750 μmol m⁻² s⁻¹. The highest dose of EBR also counteracted the decrease in plant growth caused by Terb, and plants registered the same growth values as controls.     

Kinetic and thermodynamic analysis of dimerization inhibitors binding to HIV protease monomers by surface plasmon resonance

The analysis of kinetic and thermodynamic parameters of binding of peptide and nonpeptide dimerization inhibitors of HIV protease (HIVp) to the enzyme monomers immobilized on an optical chip has been studied by surface plasmon resonance. The molecular interactions were investigated at different inhibitor concentrations (0–80 μM) and temperatures (15–35°C). Determination of kinetic (k _on, k _off), equilibrium (K _d), and thermodynamic (ΔG, ΔH, and -TΔS) has shown that both inhibitors are characterized by similar interaction parameters and the entropic term (-TΔS) of about −20 kcal/mol is the main driving force for the HIVp complex formation with the inhibitors, while the positive value (14 kcal/mol) of the enthalpic term (ΔH) counteracted the complex formation.     

Variation in carbon isotope discrimination within and among Sphagnum species in a temperate wetland

Field samples of bryophytes are highly variable in carbon isotope discrimination values (Δ, a measure of ¹³CO₂ uptake relative to ¹²CO₂), but it is unknown what affects Δ under field conditions, or how variation in Δ relates to bryophyte performance. This study employed field and greenhouse common garden studies to evaluate the influence of microsite, seasonal, and genetic variation on Δ in peatmosses. Three species of Sphagnum that occupy hollow (S. recurvum), carpet (S. palustre), and hummock (S. tenerum) habitats were sampled for relative growth rates (RGR), C:N ratio, and Δ throughout a growing season. Values of Δ ranged from 19.0 to 27.1‰. This variation was unrelated to species (P=0.61). However, Δ varied seasonally (P<0.001), with lower discrimination in the spring (mean 22.5‰), followed by summer (23.8‰) and winter (24.7‰). There was also significant microsite variation (P=0.015) which disappeared when plants were grown in a common garden. In both spring and summer, microsite variation in Δ was inversely related to RGR (P<0.001), but unrelated to C:N ratios (P>0.08). These results suggest that environmental, not genetic, variation at microsites affects Δ in non-vascular plants. However, environmental control of Δ is unlike that in vascular plants where water limitation lowers chloroplastic demand and increases resistance to carbon uptake. In non-vascular plants, water limitation lowers chloroplastic demand and decreases resistance to carbon uptake. These processes have additive effects and generate high spatial and seasonal variability in Δ. Received: 29 April 1999 / Accepted: 8 November 1999