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arxiv: 2405.14579 · v2 · submitted 2024-05-23 · ⚛️ physics.chem-ph

Assessment of S* in the Orange Carotenoid Protein

James P. Pidgeon , George A. Sutherland , Matthew S. Proctor , Shuangqing Wang , Dimitri Chekulaev , Sayantan Bhattacharya , Rahul Jayaprakash , Andrew Hitchcock
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Ravi Kumar Venkatraman Matthew P. Johnson C. Neil Hunter Jenny Clark
This is my paper

Pith reviewed 2026-05-24 01:12 UTC · model grok-4.3

classification ⚛️ physics.chem-ph
keywords orange carotenoid proteinphotoconversionS* statetransient absorptionground-state heterogeneitynon-photochemical quenchingcarotenoid excited states
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The pith

S* is not required for photoconversion in the orange carotenoid protein.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper tests whether the long-lived S* excited state of the carotenoid is necessary for the protein to switch from its inactive dark form to the active quenching form. By trapping OCP in a trehalose-sucrose glass film that blocks full conversion to the active state but still allows early photoproducts, they track how those products depend on the wavelength of the pump light. S* appears only below 495 nm, yet the initial products form across a broader wavelength range. This wavelength dependence matches the efficiency of full conversion observed in buffer solution, indicating that S* is not needed for the switch and instead reflects different starting structures already present in the dark-adapted protein.

Core claim

By comparison to the pump wavelength-dependence of the OCPo to OCPr conversion in buffer, we show that S* is not required for photoconversion, and that S* likely arises from ground-state heterogeneity within OCPo.

What carries the argument

Pump wavelength-dependent transient absorption in trehalose-sucrose glass films that trap initial photoproducts while blocking completion to OCPr.

If this is right

  • Initial photoproducts form even at pump wavelengths where the S* signature is absent.
  • The wavelength range that drives product formation in glass aligns with the range that drives full conversion in solution.
  • S* is visible only for pumps shorter than 495 nm and therefore traces a subpopulation of OCPo rather than an obligatory intermediate.
  • The photoconversion pathway can proceed from the excited-state manifold without passing through the long-lived S* state.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Measurements that separate subpopulations of OCPo could test whether specific ground-state conformers eliminate the S* signal while retaining conversion ability.
  • The same glass-trapping approach may clarify whether S*-like signals in other carotenoid-binding proteins also reflect heterogeneity rather than required intermediates.
  • If ground-state heterogeneity proves common, models of carotenoid photoprotection would need to incorporate multiple dark forms rather than a single uniform OCPo state.

Load-bearing premise

The trehalose-sucrose glass film prevents completion to OCPr while still permitting formation of initial OCP photoproducts.

What would settle it

If the wavelength dependence of initial photoproduct formation in the glass did not match the wavelength dependence of full OCPo-to-OCPr conversion efficiency in buffer, the conclusion that S* is unnecessary would not hold.

Figures

Figures reproduced from arXiv: 2405.14579 by Andrew Hitchcock, C. Neil Hunter, Dimitri Chekulaev, George A. Sutherland, James P. Pidgeon, Jenny Clark, Matthew P. Johnson, Matthew S. Proctor, Rahul Jayaprakash, Ravi Kumar Venkatraman, Sayantan Bhattacharya, Shuangqing Wang.

Figure 1
Figure 1. Figure 1: OCP conversion and absorbance change in solution (a,b), trapping in trehalose glass (c,d), and canthaxanthin structure (e). Absorbance spectra of OCPo (a,c) taken in 1 min intervals (uo to 1 hour) under constant white-light illumination (1600 µmol photon m−2 s −1 ), or OCPr (b,d) taken in 1 min intervals in darkness, both at 22 °C. For trehalose samples (c,d), the spectra at 15 hours are also displayed. No… view at source ↗
Figure 2
Figure 2. Figure 2: Picosecond (a,b) and ns–ms (c,d) transient absorption spectra of CAN￾binding OCP in trehalose glass (a–c) and in buffer (d) with pump wavelength 532 nm. Singlet decay occurs over ∼5 ps (a,b), and long-lived and static features are seen on the long-time (c,d), consistent with the literature on OCPo and OCPr in buffer. 35,53,54 Spectra have been averaged between the times indicated. In (d), buffer was consta… view at source ↗
Figure 3
Figure 3. Figure 3: Transient absorption spectra of CAN-binding OCPo in trehalose glass with pump wavelengths in the range 400 nm to 600 nm (specified on the right). Spectra have been averaged between the times specified on the top, with arrows denoting the size of the y-axis scale (∆A in mOD) for that column of spectra. Pump powers were tuned to give an initial ∼3 mOD peak GSB response at 1 ps. We observe that higher-energy … view at source ↗
Figure 4
Figure 4. Figure 4: Normalised dynamics and a triexponential global fit (equation inset bottom-left) in the long-lived GSB region for transient absorption experiments using different pump wavelengths on CAN-binding OCPo. The S* amplitude clearly increases as pump wavelength is reduced from 480 nm to 400 nm. Dynamics are taken as the 420–430 nm probe range average, and subsequently normalised to average one in the 0.35 ps to 0… view at source ↗
Figure 5
Figure 5. Figure 5: Fitted amplitudes from the global fit of transient absorption dynamics with absorbance of CAN-binding OCPo. Absorbance of OCPo in trehalose (line, left axis), with fitted amplitude α3 from a global triexponential fit of picosecond transient absorption dynamics (points, right axis). Error bars denote fit parameter standard errors. Dynamic fits are shown and described in [PITH_FULL_IMAGE:figures/full_fig_p0… view at source ↗
Figure 6
Figure 6. Figure 6: Experimental setup for absorbance measurements of CAN-binding OCP in buffer that is continually photoconverted by narrowband pump light (a), and average absorbance ratios against time (b). The pump power was tuned to give ap￾proximately the same initial OCPo excitation rate per-pump wavelength (except for 675 nm). All absorbed pump wavelengths give a conversion yield greater than the white-light probe alon… view at source ↗
read the original abstract

The orange carotenoid protein (OCP) is the water-soluble mediator of non-photochemical quenching in cyanobacteria, a crucial photoprotective mechanism in response to excess illumination. OCP converts from a dark-adapted inactive state (OCPo) to an active quenching conformation (OCPr) under high-light conditions, resulting in a concomitant redshift in the absorption of the bound carotenoid. Here, we test whether a long-lived carotenoid singlet excited state (S*) is required for this photoconversion. We measured pump wavelength-dependent transient absorption of OCPo trapped in trehalose-sucrose glass films. We found that initial OCP photoproducts are still formed despite the glass preventing completion to OCPr, and that S* is only apparent for <495 nm pumps. By comparison to the pump wavelength-dependence of the OCPo to OCPr conversion in buffer, we show that S* is not required for photoconversion, and that S* likely arises from ground-state heterogeneity within OCPo.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The paper claims that the long-lived carotenoid singlet excited state S* is not required for photoconversion of OCP from its inactive OCPo to active OCPr form. Pump wavelength-dependent transient absorption measurements on OCPo in trehalose-sucrose glass films show that initial photoproducts still form for excitation wavelengths >495 nm (where S* is absent), and this threshold matches the pump dependence of full OCPo to OCPr conversion measured in buffer; the authors conclude that S* likely originates from ground-state heterogeneity within OCPo.

Significance. If the central comparison holds, the result would be significant for carotenoid photophysics and photoprotection mechanisms in cyanobacteria, as it would indicate that photoconversion proceeds from the Franck-Condon region without needing the long-lived S* state and would redirect attention to ground-state conformational distributions. The use of a glass matrix to trap early intermediates while comparing wavelength dependences is a direct experimental test that strengthens the argument when matrix effects are controlled.

major comments (1)
  1. [Abstract and glass-film experiments description] The load-bearing assumption that the trehalose-sucrose glass prevents completion to OCPr while leaving the initial photoproduct pathways and ground-state heterogeneity unchanged (allowing direct comparison of wavelength thresholds to buffer data) is not supported by an explicit check that early-time (<200 fs) transient spectra or effective absorption cross-sections are matrix-independent. Without this, the absence of S* in glass for >495 nm pumps does not rigorously demonstrate that S* is dispensable in buffer.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for highlighting this important point regarding the glass-matrix experiments. We respond to the major comment below.

read point-by-point responses

  1. Referee: The load-bearing assumption that the trehalose-sucrose glass prevents completion to OCPr while leaving the initial photoproduct pathways and ground-state heterogeneity unchanged (allowing direct comparison of wavelength thresholds to buffer data) is not supported by an explicit check that early-time (<200 fs) transient spectra or effective absorption cross-sections are matrix-independent. Without this, the absence of S* in glass for >495 nm pumps does not rigorously demonstrate that S* is dispensable in buffer.

    Authors: We agree that an explicit side-by-side comparison of early-time (<200 fs) transient spectra (or absorption cross-sections) between the trehalose-sucrose glass and aqueous buffer would provide stronger support for the assumption that the glass leaves the initial photoproduct pathways and ground-state heterogeneity unaltered. The current manuscript relies on the observed match between the pump-wavelength threshold for photoproduct formation in glass and the threshold for full OCPo-to-OCPr conversion in buffer, together with the known ability of the glass to trap early intermediates. In the revised version we will add a dedicated paragraph (and, if space permits, a supplementary figure) that (i) cites literature on the matrix effects of trehalose-sucrose glasses on carotenoid photophysics and (ii) explicitly notes the lack of a direct <200 fs matrix-independence test as a limitation while arguing that the wavelength-threshold agreement still supports the central conclusion. We will also state that future work could include such a direct comparison. revision: yes

Circularity Check

0 steps flagged

No circularity; direct experimental comparison of wavelength dependences

full rationale

The paper reports transient absorption measurements of OCPo in trehalose-sucrose glass films as a function of pump wavelength, observes that initial photoproducts form for pumps >495 nm (where S* is absent), and directly compares this threshold to the pump-wavelength dependence of OCPo-to-OCPr conversion measured separately in buffer. No equations, fitted parameters, or derivations are present that reduce the conclusion to inputs by construction. No self-citations are invoked as load-bearing premises. The result follows from the observed experimental wavelength thresholds without renaming, ansatz smuggling, or uniqueness theorems.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Experimental spectroscopy paper; no free parameters, invented entities, or non-standard axioms are introduced. Relies on standard assumptions of transient absorption interpretation.

axioms (1)
  • domain assumption Pump-wavelength dependence of transient absorption signals can be used to identify or rule out specific excited-state contributions.
    Standard assumption in ultrafast spectroscopy of carotenoids.

pith-pipeline@v0.9.0 · 5751 in / 1096 out tokens · 22543 ms · 2026-05-24T01:12:40.963255+00:00 · methodology

discussion (0)

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Reference graph

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7 extracted references · 7 canonical work pages · 1 internal anchor

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