EFFECTS OF TRYPTOPHAN OXIDATION ON BACTERIORHODOPSIN STRUCTURE AND FUNCTION

Abstract— N -bromosuccinimide at low molar ratios specifically oxidizes tryptophan residues in purple membranes (bR). Loss of 1 mol tryptophan per mol bR (which corresponds to oxidation of Trp₁₀ or Trp₁₂ or part of both) produces slight changes in the absorption and CD spectra and in the photobleaching kinetics. The efficiency of reconstituting the retinylidene protein after bleaching and heptane extraction of these membranes was comparable to control values(–80%). Photocycling yield (M₄₁₂) with 265 nm or 530 nm excitation (15 ns pulse) was only slightly decreased. Another mol of tryptophan was reacted at higher NBS:bR molar ratio (10:1). Partial oxidation of several residues rather than titration of a specific residue occurred: HPLC indicated no reaction with the chromophore. A considerable loss of extinction at 570 nm including enhanced red and blue shifts of LD_max at low and high pH respectively, was observed. Also the photobleaching rate was faster and functional retinylidene membranes could not be reconstituted from heptane-extracted, apomembranes. Exogenous retinal could still locate the attachment site, based on formation of the fluorescent NaBH₄-reduced retinoyl adduct. Perturbation in the near UV and visible CD infer changes in helical conformation, trimer dissociation and decreased asymmetry of the chromophore. Photocycling efficiency was greatly decreased. The relative decrease was greater for 265 nm rather than 530 nm excitation. These results are consistent with co-operative destabilization of the protein conformation by oxidized tryptophan residues, which leads to a decrease in the hydrophobicity of chromophoric site.     

The computation of Karplus equation coefficients and their components using self-consistent field and second-order polarization propagator methods

The Karplus equation has been investigated by ab initio computation of the spin-spin coupling constants for a series of rotated ethane geometries. The couplings have been calculated at the self-consistent field (SCF) level as well as using the second-order polarization propagator approximation (SOPPA) and the second-order polarization propagator approximation with coupled cluster singles and doubles amplitudes (SOPPA(CCSD)) and have been compared with results of previous calculations. The four principal components of the coupling constants rather than just the Fermi-contact have been calculated, and the common supposition that the Fermi-contact term is totally dominant has been confirmed. The derivatives of the orbital paramagnetic and orbital diamagnetic terms are significant but opposite in sign for the case of this rotation in ethane. It is found that the coefficients in the Karplus equation are largely overestimated at the SCF level, whereas the SOPPA(CCSD) results are in good agreement with coefficients derived from experimental coupling constant data or the results of multi-configurational self-consistent field (MCSCF) calculations. It is further observed that extending the Fourier series in the Karplus equation to include cos(3θ) and cos(4θ) terms neither significantly improves the quality of the fit nor significantly changes the values of the other coefficients. In order to simulate the Abraham and Pachler equation, calculations varying the nuclear charge on hydrogen have been performed. These will allow an abstract but flexible prediction of the effect of electronegative substituents.     

Molecular layering and local order in thin films of 1-alkyl-3-methylimidazolium ionic liquids using X-ray reflectivity

X-ray reflectivity measurements in air of thin films of 1-alkyl-3-methylimidazolium salts in the liquid, liquid crystalline and solid states supported on Si(111) are described. The films show Bragg features in both liquid crystalline and solid phases, but only after an initial annealing cycle. Kiessig fringes are observed only for the 1-octadecyl-3-methyl-imidazolium hexafluorophosphate films and, following analysis using Parratt32, a bi-layer model is proposed whereby the molecules are orientated with ionic groups at both salt-air and salt-silicon interfaces.     

PHOSPHORESCENCE MAXIMA AND TRIPLET STATE LIFETIMES OF NAD + AND ε-NAD+ IN TERNARY COMPLEXES WITH HORSE LIVER ALCOHOL DEHYDROGENASE

This paper describes the phosphorescence emission and decay times of NAD+ and its fluorescent etheno derivative, epsilon-NAD+, in the pyrazole ternary complex with horse liver alcohol dehydrogenase (ADH). We show that the epsilon-NAD+ triplet state, as well as the tryptophan triplet state, can be utilized to monitor the coenzyme-enzyme interaction. The decays of NAD+ and AMP are single exponential, and the lifetimes are the same within experimental error. The phosphorescence lifetimes, evaluated as single exponentials, are slightly shorter in epsilon-NAD+ than they are in epsilon-AMP. Whereas the decay of epsilon-AMP was adequately fit by a single exponential with a time constant of very close to 0.5 s, it was necessary to fit the decay of epsilon-NAD+ to a double exponential. Ternary complexes with NAD+ excited at 297 nm exhibit decay kinetics nearly identical to those of ADH by itself. On the other hand, when excitation of the epsilon-NAD+ ternary complex is provided at 313 nm, where there is very little absorption by either tryptophan residue, the decay law of the ternary complex is similar to that of epsilon-NAD+ in solution. Our results demonstrate that NAD+ and epsilon-NAD+ quench tryptophan phosphorescence in ADH. Normalizing the phosphorescence intensity to the 0-0 vibronic band assigned to Trp-15 (blue-edge), we calculate a 21% decrease in the phosphorescence associated with Trp-314 at stoichiometric saturation of the coenzyme binding sites with NAD+ in the ternary complex. When the active sites are saturated with epsilon-NAD+, the relative phosphorescence due to Trp-314 decreases by 63%.(ABSTRACT TRUNCATED AT 250 WORDS)     

CLIMATE CHANGE AND OPTIMAL ROTATION IN A FLAMMABLE FOREST

ABSTRACT. This paper builds a Faustmann‐based model to study the effects of increased climate‐induced fire risk on the optimal forest rotation period. Simulations using species prevalent in North American forests indicate that both the commercial and socially optimal rotation ages decline as the risk increases. The reduced carbon absorbed by the standing timber can then create a positive feedback effect. This has potentially important policy implications. The Kyoto ratification agreement reached in the autumn of 2001 was dependent on allowing the ‘Umbrella Group’ of countries to use their forests' carbon‐absorbing ability to offset their need for fossil fuel emission reductions. This carbon‐absorbing ability will decline if rotation ages decrease with increased fire risk, weakening the force of the argument for allowing these countries to use their carbon ‘sinks’ to avoid reducing anthropomorphic emissions.     

ACTIVATION KINETICS OF PHOTOSYNTHETIC OXYGEN EVOLUTION UNDER 20–40 NANOSECOND LASER FLASHES

Abstract— Chlorella samples, incubated for varying periods in darkness, were exposed to a series of 20–40 nsec flashes, spaced 15 sec apart, from a Q-switched ruby laser. A stationary Teflon-covered platinum electrode measured the microjet of oxygen produced by each flash. After a dark preincubation exceeding 3 min, at 23°C, little or no oxygen is evolved until the third flash in a sequence. The yields from subsequent flashes increase monotonically until a constant value is reached. If low levels of background light are supplied, or if the interval between series of flashes is decreased, oxygen is evolved on the second or even the first flash. Very similar results were obtained from analogous experiments with tailless 28 μsec flashes from a xenon flashtube. In particular, very little oxygen was evolved from the second flash following a long dark period whatever the spacing between flashes. This means that there are no systematic differences between the effects of saturating 20 nsec and 28 μsec light flashes on the activation processes during the first few flashes following a long dark period. No oscillations of flash yield with successive flashes were observed because of the long interval between flashes. These results are consistent with the idea that the reaction center of Photosystem II must undergo a dark process lasting considerably longer than 28 μsec before it can absorb a second photon.     

THE COSTATE VARIABLE IN NATURAL RESOURCE OPTIMAL CONTROL PROBLEMS

ABSTRACT. In this paper we discuss the role of the costate variable (shadow value) for the resource stock in both nonrenewable and renewable resource problems. We separate the information in this variable into a scarcity and a cost effect. The scarcity effect is the portion of the shadow value that is due just to the scarcity of the resource relative to its demand, while the cost effect is a measure of the impact of the marginal unit upon future extraction costs. It is shown that in the nonrenewable resource, mining, problem both can exist simultaneously, but in the renewable resource, fisheries, model the two effects are mutually exclusive. In our analysis of the fisheries model we develop an expression for the time path of the marginal unit of fish stock. We do this using the theorem of Continuous Dependence on Initial Conditions. This result is then used to generate the conclusion that g(x) is the biological own rate of interest, where g(x) is the growth function for the resource stock, x.     

MODELING THE SPATIAL DISTRIBUTION OF THE ECONOMIC COSTS AND BENEFITS OF ILLEGAL GAME MEAT HUNTING IN THE SERENGETI

ABSTRACT. Illegal game meat hunting in the Serengeti National Park, Tanzania, and adjacent game reserves provides an important source of protein and cash income to local communities. We construct a profitability model that describes the spatial distribution of the economic costs and benefits of illegal hunting in the Serengeti during the late 1980s and early 1990s. Costs included capital investment in hunting weapons, W_R, and the opportunity cost of hunting, W_O, both held to be constants; and two spatially variable components, the logistic effort of traveling to hunting areas, W_L, and the penalties incurred if arrested, W_P. Benefit was the expected income from the sale of meat from resident wildlife species. The model suggests: (1) W_R is the most important cost. (2) W_L is the second most important cost and likely to determine the spatial distribution of hunting activity if hunters seekto minimize costs. (3) W_O and W_P are of minor importance, the former because alternative sources of income provide low pay, the latter because the overall chance of being arrested is low. (4) W_P exceeds W_L only in areas close to the boundary of protected areas. (5) Although resident wildlife contributes only a minor share of illegal offtake compared to the migratory herds, hunting resident wildlife is profitable in 68% of the area. This suggests that hunting of resident and migratory wildlife is highly profitable and may explain why the utilization of the target populations has become increasingly unsustainable.