The Effect of Bacteriochlorophyll <i>g</i> Oxidation on Energy and Electron Transfer in Reaction Centers from <i>Heliobacterium modesticaldum</i>
Bibliographic record
Abstract
The heliobacteria are a family of strictly anaerobic, Gram-positive, photoheterotrophs in the Firmicutes. They make use of a homodimeric type I reaction center (RC) that contains ∼20 antenna bacteriochlorophyll (BChl) g molecules, a special pair of BChl g' molecules (P800), two 8(1)-OH-Chl aF molecules (A0), a [4Fe-4S] iron-sulfur cluster (FX), and a carotenoid (4,4'-diaponeurosporene). It is known that in the presence of light and oxygen BChl g is converted to a species with an absorption spectrum identical to that of Chl a. Here, we show that main product of the conversion is 8(1)-OH-Chl aF. Smaller amounts of two other oxidized Chl aF species are also produced. In the presence of light and oxygen, the kinetics of the conversion are monophasic and temperature dependent, with an activation energy of 66 ± 2 kJ mol(-1). In the presence of oxygen in the dark, the conversion occurs in two temperature-dependent kinetic phases: a slow phase followed by a fast phase with an activation energy of 53 ± 1 kJ mol(-1). The loss of BChl g' occurs at the same rate as the loss of Bchl g; hence, the special pair converts at the same rate as the antenna Chl's. However, the loss of P800 photooxidiation and flavodoxin reduction is not linear with the loss of BChl g. In anaerobic RCs, the charge recombination between P800(+) and FX(-) at 80 K is monophasic with a lifetime of 4.2 ms, but after exposure to oxygen, an additional phase with a lifetime of 0.3 ms is observed. Transient EPR data show that the line width of P800(+) increases as BChl g is converted to Chl aF and the rate of electron transfer from A0 to FX, as estimated from the net polarization generated by singlet-triplet mixing during the lifetime of P800(+)A0(-), is unchanged. The transient EPR data also show that conversion of the BChl g results in increased formation of triplet states of both BChl g and Chl aF. The nonlinear loss of P800 photooxidiation and flavodoxin reduction, the biphasic backreaction kinetics, and the increased EPR line width of P800(+) are all consistent with a model in which the BChl g'/BChl g' and BChl g'/Chl aF' special pairs are functional but the Chl aF'/Chl aF' special pair is not.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".