Redox-mediated carbon monoxide release from a manganese carbonyl—implications for physiological CO delivery by CO releasing moieties

The dynamics of hydrogen peroxide reactions with metal carbonyls have received little attention. Given reports that therapeutic levels of carbon monoxide are released in hypoxic tumour cells upon manganese carbonyls reactions with endogenous H2O2, it is critical to assess the underlying CO release mechanism(s). In this context, a quantitative mechanistic investigation of the H2O2 oxidation of the water-soluble model complex fac-[Mn(CO)3(Br)(bpCO2)]2–, (A, bpCO22– = 2,2′-bipyridine-4,4′-dicarboxylate dianion) was undertaken under physiologically relevant conditions. Characterizing such pathways is essential to evaluating the viability of redox-mediated CO release as an anti-cancer strategy. The present experimental studies demonstrate that approximately 2.5 equivalents of CO are released upon H2O2 oxidation of A via pH-dependent kinetics that are first-order both in [A] and in [H2O2]. Density functional calculations were used to evaluate the key intermediates in the proposed reaction mechanisms. These pathways are discussed in terms of their relevance to physiological CO delivery by carbon monoxide releasing moieties.


1.
The authors rightly highlight the species such as A are light-activated CO releasing molecules. It is not clear from the experimental section if the reaction kinetics and related mechanistic work were performed in the absence of light (e.g. using amberised glassware or using aluminium foil). In my view this is an important control to determine the role (or lack thereof) of light in the processes they are studying. For example, a kinetic experiment performed with and without the exclusion of light would enable this to be determined.

2.
In my view the DFT calculations require further explanation. From the experimental section and supporting information it is not evident if the authors are quoting the electronic energy of the states they have calculated, or if any thermal corrections from a vibrational spectrum calculation have been included. Indeed, they do not state if such a vibrational calculation has been performed. This is important to ensure firstly that the states are indeed minima on the potential energy surface but also, given the significant changes in molecularity during the calculated reactions, I would view determining the Gibbs energy changes as vital. I also note that the authors do not appear to have calculated any transition states for the processes shown in Figure 4 and 5. So I do agree with comments such as "Such calculations help in evaluating whether the respective intermediates are energetically accessible" but am less convinced by "Although DFT calculations suggest that reaction of A with either H2O2 or hydroperoxyl anion HOO-to give E is a reasonable first step" in the absence of a calculated barrier to this process. It is certainly thermodynamically accessible as shown by the authors data, but no insight is provided into the kinetics which makes it difficult to determine is this really is a reasonable first step. On a more trivial note, in a manuscript of this type I would expect the optimised coordinates for the calculated states to be presented in the Supporting Information so that others could repeat these calculations.

Decision letter (RSOS-211022.R0)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below. Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
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Once again, thank you for submitting your manuscript to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry. I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor Comments to the Author: I recommend that the manuscript be accepted, provided that the points noted by the reviewer(s) (in particular Reviewer 2) are addressed and a revised version is prepared on its basis.

RSC Subject Editor
Comments to the Author: (There are no comments.) ********************************************** Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) This manuscript reports studies of the reaction pathway of Mn(I)(CO)3Br(bpCO2) with H2O2. The studies reveal important insights into the factors (e.g., pH) that may influence the CO release reactivity. Overall, this manuscript is well written and provides understanding of the reactivity of a common family of CORMs. Differences in subcellular pH and H2O2 concentration may influence CO release. The insights into the relative reactivity of H2O2 and OOH-with the Mn(I) CORM are an interesting contribution to the literature. I support publication with modification.
Reviewer: 2 Comments to the Author(s) This manuscript by Barrett and co-workers describes a comprehensive mechanistic investigation in to the H2O2-induced CO release from a manganese-containing carbonyl compound. There has been considerable recent interest in using such molecules as potential therapeutic agents and detailed insight into the processes that underpin their activity in an aqueous environment are extremely important in rationalising the behaviour and informing the design of future targets. The experimental work is carefully performed, mixing detailed kinetic studies and also the identification of the Mn-based species arising from CO release and I recommend publication when the following issues have been addressed.
1. The authors rightly highlight the species such as A are light-activated CO releasing molecules. It is not clear from the experimental section if the reaction kinetics and related mechanistic work were performed in the absence of light (e.g. using amberised glassware or using aluminium foil). In my view this is an important control to determine the role (or lack thereof) of light in the processes they are studying. For example, a kinetic experiment performed with and without the exclusion of light would enable this to be determined. 2. In my view the DFT calculations require further explanation. From the experimental section and supporting information it is not evident if the authors are quoting the electronic energy of the states they have calculated, or if any thermal corrections from a vibrational spectrum calculation have been included. Indeed, they do not state if such a vibrational calculation has been performed. This is important to ensure firstly that the states are indeed minima on the potential energy surface but also, given the significant changes in molecularity during the calculated reactions, I would view determining the Gibbs energy changes as vital. I also note that the authors do not appear to have calculated any transition states for the processes shown in Figure 4 and 5. So I do agree with comments such as "Such calculations help in evaluating whether the respective intermediates are energetically accessible" but am less convinced by "Although DFT calculations suggest that reaction of A with either H2O2 or hydroperoxyl anion HOO-to give E is a reasonable first step" in the absence of a calculated barrier to this process. It is certainly thermodynamically accessible as shown by the authors data, but no insight is provided into the kinetics which makes it difficult to determine is this really is a reasonable first step. On a more trivial note, in a manuscript of this type I would expect the optimised coordinates for the calculated states to be presented in the Supporting Information so that others could repeat these calculations.

See Appendix A.
Decision letter (RSOS-211022.R1) We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.
Dear Dr Barrett: Title: Redox Mediated Carbon Monoxide Release from a Manganese Carbonyl-Implications for Physiological CO Delivery by CORMs. Manuscript ID: RSOS-211022.R1 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Thank you for your fine contribution. On behalf of the Editors of Royal Society Open Science and the Royal Society of Chemistry, I look forward to your continued contributions to the Journal. ******** RSC Associate Editor Comments to the Author: (There are no comments.) ********* Reviewer(s)' Comments to Author: