Applicability of Transition State Theory to the (Proton-Coupled) Electron Transfer in Photosynthetic Water Oxidation with Emphasis on the Entropy of Activation
Round 1
Reviewer 1 Report
In this work, the authors mainly discussed the central conclusion that during the tyrosine radical (TyrZ) oxidizes the Mn4Ca cluster bound oxygen in treatment of S3->S0 transition state of PSII, there is a pronounced entropic slowdown of the oxygen-evolution transition. Although no more additional experimental data is available, the author's explanation in combination of Eyring’s transition state theory and Marcus’ electron transfer theory, is both reasonable and interesting, with textbook quality. This is obviously an extremely important work I've ever read. Thus, I recommend that it be published directly.
Author Response
We thank the reviewer for the kind words and endorsement.
Kind regards,
Paul Greife on behalf of all the authors
Reviewer 2 Report
The manuscript by Dau and Greife is an excellent piece of work discussing the activation factors in the S3->S0 ET step, recognized as the rate limiting step in photosynthetic oxygen evolution. I definitely recommend the manuscript for publication in Inorganics. I have only some minor notes:
I suggest the authors to specify (maybe in brackets) how the tunneling distance of 2.5 A is estimated for the ET S3-> (S4)-> S0 (page 4 line 36).
Even if it is work in progress, the authors could give some short details on the experiments aimed at proving the entropy/enthalpy compensation to support the identical rate constant values at isokinetic temperatures.
Abstract: use the subscripts for the numbers associated to the Si states. The same in page 6 line 203.
Page 2 line 58 the symbol of the transmission probability coefficient does not appear in the main text (also later on in the manuscript).
Author Response
We thank the reviewer for the constructive criticism and endorsement. Regarding the points raised by the reviewer:
I suggest the authors to specify (maybe in brackets) how the tunneling distance of 2.5 Å is estimated for the ET S3-> (S4)-> S0 (page 4 line 36).
We have modified the original sentence to now include the relevant information to estimate the tunneling distance in brackets:
‘This corresponds to a Van-der-Waals tunneling distance of only about 2.5 Å (estimate obtained by subtracting from the internuclear (TyrZ-)O—O6 distance a Van-der-Waals radius of 1.6 Å [30] for each oxygen atom).’
Even if it is work in progress, the authors could give some short details on the experiments aimed at proving the entropy/enthalpy compensation to support the identical rate constant values at isokinetic temperatures.
We have clarified the language of the original statement and added several sentences on planned and in-progress work,
‘We propose that compensation of the decreased activation enthalpy by an increased activation entropy explains identical rate constant values for differing organisms at similar temperatures in the physiological temperature range. This entropy-enthalpy compensation in the activation energy of the oxygen-evolution step is still insufficiently understood and requires further investigation (work in progress). To this end, the experimental determination of the entropy and enthalpy of activation for various organisms as well as for PSII with genetically modified H-bond networks could address both, (i) a possible physiological role in genetic adaptation of various organisms to different environmental temperatures and (ii) elucidation the molecular determinants (in terms of differing amino acid environments) of the experimentally observable shifts between the enthalpy and entropy of activation, especially if complemented by informative computational investigation. In general, entropy-enthalpy compensation is well known, albeit somewhat controversial, for the free energy of activation relating to various catalytic processes in biological and non-biological systems [33-39]. Irrespective of the explanation of entropy-enthalpy compensation at the molecular level, ..‘
Abstract: use the subscripts for the numbers associated to the Si states. The same in page 6 line 203.
We apologize for these oversights and have fixed the relevant text.
Page 2 line 58 the symbol of the transmission probability coefficient does not appear in the main text (also later on in the manuscript).
During reformatting the paper to fit the journal template the character for k (kappa) was replaced by another causing confusion. We apologize for the error and thank the reviewer for their helpful comment.
Kind regards,
Paul Greife on behalf of all the authors
