Electron Transfer in Cyanobacterial Photosystem I
Bibliographic record
Abstract
The Photosystem I (PS I) reaction center contains two branches of nearly symmetric cofactors bound to the PsaA and PsaB heterodimer. From the x-ray crystal structure it is known that Trp697PsaA and Trp677PsaB are pi-stacked with the head group of the phylloquinones and are H-bonded to Ser692PsaA and Ser672PsaB, whereas Arg694PsaA and Arg674PsaB are involved in a H-bonded network of side groups that connects the binding environments of the phylloquinones and FX. The mutants W697FPsaA, W677FPsaB, S692CPsaA, S672CPsaB, R694APsaA, and R674APsaB were constructed and characterized. All mutants grew photoautotrophically, yet all showed diminished growth rates compared with the wild-type, especially at higher light intensities. EPR and electron nuclear double resonance (ENDOR) studies at both room temperature and in frozen solution showed that the PsaB mutants were virtually identical to the wild-type, whereas significant effects were observed in the PsaA mutants. Spin polarized transient EPR spectra of the P700+A1- radical pair show that none of the mutations causes a significant change in the orientation of the measured phylloquinone. Pulsed ENDOR spectra reveal that the W697FPsaA mutation leads to about a 5% increase in the hyperfine coupling of the methyl group on the phylloquinone ring, whereas the S692CPsaA mutation causes a similar decrease in this coupling. The changes in the methyl hyperfine coupling are also reflected in the transient EPR spectra of P700+A1- and the CW EPR spectra of photoaccumulated A1-. We conclude that: (i) the transient EPR spectra at room temperature are predominantly from radical pairs in the PsaA branch of cofactors; (ii) at low temperature the electron cycle involving P700 and A1 similarly occurs along the PsaA branch of cofactors; and (iii) mutation of amino acids in close contact with the PsaA side quinone leads to changes in the spin density distribution of the reduced quinone observed by EPR.
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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 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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".