the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
24 Feb 2021
24 Feb 2021
Non-classical disproportionation revealed by photo-CIDNP NMR
- 1Institute of Physical Chemistry, Albert-Ludwigs-Universität Freiburg, Freiburg, 79104, Germany
- 2Department of Chemistry, Technical University of Munich, Garching, 85748, Germany
- 1Institute of Physical Chemistry, Albert-Ludwigs-Universität Freiburg, Freiburg, 79104, Germany
- 2Department of Chemistry, Technical University of Munich, Garching, 85748, Germany
Abstract. Photo-chemically induced dynamic nuclear polarization (photo-CIDNP) was used to observe the light-induced disproportionation reaction of 6,7,8-trimethyllumazine starting out from its triplet state to generate a pair of radicals comprising a one-electron-reduced and a one-electron oxidized species. Our evidence is based on the measurement of two marker proton hyperfine couplings, Aiso(H(6α)) and Aiso(H(8α)), which we correlated to predictions from density functional theory. The ratio of these two hyperfine couplings is reversed in the oxidized and the reduced radical species. Observation of the dismutation reaction is facilitated by the exceptional C–H acidity of the methyl group at position 7 of 6,7,8-trimethyllumazine and the slow proton exchange associated with it, which leads to NMR-distinguishable anionic (TML–) and neutral (TMLH) protonation forms.
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Jakob Wörner et al.
Status: final response (author comments only)
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RC1: 'Comment on mr-2021-22', Anonymous Referee #1, 03 Mar 2021
Research on photolyases and cryptochromes is presently a hot field. Cryptochromes are even considered to be involved in “animal detected magnetic resonance”. Therefore, an update on flavin-related compounds, their capacity to form radical pairs and to build up hyperpolarization is highly desirable. The manuscript revealed the existence of a transient high redox state of 6,7,8-trimethyllumazine by means of photo-CIDNP NMR. The compound is able to form upon illumination radical pairs with its two different protonation states. The experimental isotropic hyperfine coupling constants are obtained and correlate well with DFT data. Combining DFT and CIDNP, the careful analysis of the data on the basis of Kaptein’s sign rule is convincing and allows a remarkable reconstruction of a reaction dynamics involving photochemistry, spin-chemistry as well as proton transfer.
As for the title, I feel that the authors are unnecessarily opening a discussion that is not really helpful. Why should a disproportionation (2A = A+ + A-) not also be able to be triggered photochemically (A* + A = A+ + A-)? In the textbook by Klessinger and Michel, photochemically triggered disproportionations are indeed discussed (for example, as a result of Norrish I). One could also ask whether a system A + AH really “disproportionates”, since A and AH are different species. Nevertheless, it seems to me that a very special CIDNP-capable donor-acceptor system is present here, perhaps deserving the term “non-classical”.
Two questions:
- From the Figure 6B, it seems like the H(7α) also enjoys very high amplitude of electron spin density, but in the photo-CIDNP spectrum, there is no polarization at H (7α). How to understand such a contradiction?
- In the supplementary information, Table S2, Table S3, there are three times H(6α), three times H(7α), and three times H(8α). What do they refer to?
Minor comments:
- The “R.” of “Rhodobacter sphaeroides” needs to be introduced (line 35).
- “it absorbs at shorter wavelengths” (line 41): Should an antenna not exactly do that? Might be helpful to mention the absorption maxima.
- “operating at 14.1 T and a 1H freq of 600 MHz” (line 66): Sounds to be two different things.
- “fibre” (line 70) = fiber.
- Fig Caption Fig 3: Dashed lines show laser excitation.
- Line 212: radical pair formation from a triplet precursor: Is that surprising considering flavin photo-chemistry?
- Line 244: To what refers the “this”?
- Line 253: The use of the asterisk in a text of photochemical relevance might be misleading.
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RC2: 'Comment on mr-2021-22', Anonymous Referee #2, 25 Mar 2021
The comment was uploaded in the form of a supplement: https://mr.copernicus.org/preprints/mr-2021-22/mr-2021-22-RC2-supplement.pdf
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AC1: 'Author Comment on mr-2021-22', Stefan Weber, 31 Mar 2021
The comment was uploaded in the form of a supplement: https://mr.copernicus.org/preprints/mr-2021-22/mr-2021-22-AC1-supplement.pdf
Jakob Wörner et al.
Jakob Wörner et al.
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