In oxygenic photosynthesis, solar energy drives the oxidation of water catalyzed by a Mn 4 Ca complex bound to the proteins of Photosystem II. Four protons are released during one turnover of the water oxidation cycle (S-state cycle), implying thermodynamic limitations at low pH. For proton concentrations ranging from 1 nM (pH 9) to 1 mM (pH 3), we have characterized the low-pH limitations using a new experimental approach: a specific pH-jump protocol combined with time-resolved measurement of the delayed chlorophyll fluorescence after nanosecond flash excitation. Effective pK values were determined for low-pH inhibition of the light-induced S-state transitions: pK 1 ؍ 3.3 ؎ 0.3, pK 2 ؍ 3.5 ؎ 0.2, and pK 3 ≈ pK 4 ؍ 4.6 ؎ 0.2. Alkaline inhibition was not observed. An extension of the classical Kok model facilitated assignment of these four pK values to specific deprotonation steps in the reaction cycle. Our results provide important support to the extended S-state cycle model and criteria needed for assessment of quantum chemical calculations of the mechanism of water oxidation. They also imply that, in intact organisms, the pH in the lumen compartment can hardly drop below 5, thereby limiting the ⌬pH contribution to the driving force of ATP synthesis.Light-driven water oxidation by plants, algae, and cyanobacteria is a pivotal process in biological solar energy conversion (1). Its evolutionary development, ϳ3.5 billion years ago, has boosted life on Earth by facilitating the efficient use of water as a source of electrons and protons for the synthesis of energystoring carbohydrates and biomass in general. The long-standing scientific interest in photosynthetic water oxidation has recently been invigorated by the vision of future technological systems that, akin to plants and cyanobacteria, use water as a substrate for light-driven formation of energy-rich and storable compounds (H 2 or other fuel materials) (2-5). We believe that insights into energetics and reaction mechanisms of the natural paragon could provide inspiration and guidelines for the development of new technologies (6 -8). However, the biological process is understood only insufficiently (9).In oxygenic photosynthesis, solar energy drives the oxidation of water and the accumulation of "energized" electrons by reduction of quinones (Fig.
We observe a substantial matrix-assisted laser desorption/ionization (MALDI) signal when irradiating femtosecond laser pulses in the near-infrared spectral range centered around 800 nm and using standard MALDI matrices with absorption bands in the ultraviolet (UV) regime. The laser pulse energy dependence of this novel phenomenon is investigated in comparison with MALDI with near-UV laser pulses. Our observations show that multiphoton absorption/ionization could be a major issue among the MALDI processes when the sample is irradiated with ultra-short laser pulses.
A detailed investigation is presented to simulate the reactive
flow field in a low-pressure flow reactor for
kinetic studies. This has been done to improve existing methods
for evaluating data from isothermal flow
kinetic measurements. The full Navier−Stokes equations for
compressible flows including transport phenomena
and chemical reactions have been solved numerically for the low Mach
number case. By using splitting
techniques for variables and spatial dimensions, the calculation time
could be reduced by more than 2 orders
of magnitude. This reduction allows the repeated application of
the solver to adjust parameters in the kinetic
model organized as an optimization problem and give best agreement
between experiment and calculation.
The model results for the nonreactive flow field have been
verified by comparison to imaging measurements
via two-dimensional laser-induced fluorescence of acetone tracer gases
for the visualization of diffusive mixing.
Numerical results of full reactive flow simulation have been
compared with the measurement of elementary
relaxation processes and vibrational energy transfer in collisions of
vibrationally excited hydrogen and deuterium
molecules. Spatially resolved axial and radial concentration
profiles of both species were obtained at room
temperature using coherent anti-Stokes Raman spectroscopy (CARS).
From the detailed numerical simulation
evaluated wall deactivation probabilities at 300 K for
H2(v=1) →(wall)
H2(v=0) (Ia) of γw = (1.5
± 0.3) ×
10-3 s and thermal rate constants for vibrational energy
transfer H2(v=1) +
D2(v=0) →
H2(v=0) +
D2(v=1)
(IIa) of k
vv = (6 ± 0.5) × 109
cm3 mol-1 s-1 were derived
using an optimization procedure specially adapted
to the present kinetic problem. They are larger, respectively, by
a factor of 2 (wall deactivation) and 1.4
(vibrational energy transfer) compared with values obtained from a
plug-flow evaluation. While the k
vv
data
from different experiments are now in excellent agreement, theoretical
results using recent ab initio potentials
still differ by a factor of 2.
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