Electron Transfer in <i>Rhodobacter sphaeroides</i> Reaction Centers Containing Zn-Bacteriochlorophylls: A Hole-Burning Study
Bibliographic record
Abstract
Nonresonant and resonant transient, photochemical hole-burned (HB) spectra are presented for primary electron donor states of a novel bacterial reaction center (Zn-RC) of Rhodobacter sphaeroides, containing six Zn-bacteriochlorophylls (Zn-BChls). A "Zn-β-RC" in which the Zn-BChl in the bacteriopheophytin (BPhe)-binding site on the A side (H(A)) has the Zn penta-coordinated, was also studied. The fifth ligand comes from a histidine introduced by site-directed mutagenesis. Formation of the P(+)Q(A)(-) state was observed in both types of RC, although under identical experimental conditions a significantly deeper P(-) band (corresponding to the lower-energy, special pair, excitonic component) was revealed in the Zn-RC. Assuming a similar lifetime of the P(+)Q(A)(-) state, the quantum yield of P(+)Q(A)(-) formation decreased by ~60% in the Zn-β-RC (compared to the Zn-RC), as was seen in a comparison of analogous (Mg) BChl-containing wild type and β-RCs of Rb. sphaeroides [Kirmaier et al. Science1991, 251, 922]. However, the average (weakly frequency-dependent) low-temperature electron transfer (ET) rates of the Zn-RC and Zn-β-RC (measured from zero phonon holes in resonant transient HB spectra) were both ~1 ps and similar to a rate previously measured in the Rb. sphaeroides native RC [Johnson et al. J. Phys. Chem. 1989, 93, 5953]. Electron transfer rates observed in this work on the Zn-RC yielded a P870* decay rate in good agreement with recent room-temperature, time-domain data [Lin et al. Proc. Natl. Acad. Sci. 2009, 106, 8537]. A lack of correlation observed between the holes near 810 and 883 nm, accounting for electrochromically induced shifts of the Zn-BChl transitions in the B(A,B) and H(A,B) binding sites, produced by formation of the P(+)BHQ(A)(-) state, indicates that the 810 nm bleach does not correspond to the P(+) (upper excitonic component of the dimer) band and is mostly contributed to by a shift of the B(B) absorption band. ZPH-action spectra indicated inhomogeneous broadening (Γ(inh)) of ~110 cm(-1) (Zn-RC) and ~130 cm(-1) (Zn-β-RC). Experimentally determined Γ(inh) decreased the number of variables in theoretical fits of the absorption and frequency-dependent shapes of resonant HB spectra, leading to more reliable Huang-Rhys factors for both low-frequency phonons and a pseudolocalized phonon, ω(SP), often referred to as the special pair marker mode.
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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) | 0.001 | 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".