Quantitative applications of ¹H and ³¹P chemically induced dynamic nuclear polarization

Chemically Induced Dynamic Nuclear Polarization (CIDNP) spectroscopy has been widely used for many years for study of mechanisms of reactions that involve free radical pairs. Unlike Electron Spin Resonance (ESR) spectroscopy CIDNP provides an information on radicals that have been already reacted to paramagnetic species bringing the power of NMR into determination and identification of reaction pathways, yields and environmental effects. Since been discovered in year 1969 CIDNP was mostly applied in purely qualitative mechanistic studies where the simple rules developed by Kaptein allowed to deduce number of important reaction parameters. The quantitative application of CIDNP was rather difficult since there were sometimes no other experimental evidence to be compared especially within the cases where ESR failed. With the fast development of different theoretical calculation procedures which can provide rather precise knowledge on geometry, electron spin distribution and various magnetic properties of free radicals(g-factors, hyperfine coupling constants), CIDNP finds a new broad field of use especially where other physico-chemical methods are not successful by different reasons. Being a variety of Nuclear Magnetic Resonance (NMR) spectroscopy CIDNP is limited by the same drawbacks - bad time resolution (usually microseconds) and poor sensitivity. Despite that it can follow the radical processes that are by far too fast to be observed ESR spectroscopy - the most straightforward method to observe free radicals. The rate of fastest radical reactions that can be observed by CIDNP is determined by the rate of intersystem crossing which is about 10−10 −10−9s. The information on free radical properties and properties of the paramagnetic products of the reaction is then stored within the longitudinal relaxation time of the particular nuclei which is typically 1 - 10s. That very important information includes spin distribution, radical concentration, rotation diffusion and magnetic properties of radicals and rates of reactions. Within the certain conditions it can be extracted by the analysis of CIDNP spectra. The two different novel quantitative and semiquantitative applications of Chemically Induced Dynamic Nuclear Polarization combined with density functional quantum mechanical calculations and NMR studies are the subject of the present thesis. First application is of almost pure fundamental interest whereas the second finds its use in industry

Hydrogen Abstraction from the C15 Position of the Cholesterol Skeleton

[EN] Cholesterol (Ch) is an integral part of cell membrane, where it is prone to oxidation. In humans, oxidation of Ch is commonly linked to various pathologies like Alzheimer's disease, atherosclerosis, and even cancer, which proceed via mechanisms involving enzymatic and free radical pathways. The latter begin with hydrogen abstraction (HA) from Ch by a reactive free radical. It has been established that the most efficient HA from Ch occurs at C7, although HA from C4 by peroxyl radicals has recently been observed. Conversely, HA from Ch positions other than the thermodynamically preferred C7 or C4 has never been reported. We have designed a Ch derivative where a benzophenone moiety is linked to C7 by a covalent bond. This mirrors a specific orientation of Ch within a confined environment. Product analysis and time-resolved spectroscopic studies reveal an unprecedented HA from C15, which is a thermodynamically unfavorable position. This indicates that a specific topology of reactants is crucial for the reactivity of Ch. The relative orientation of the reactants can also be relevant in biological membranes, where Ch, polyunsaturated fatty acids, and numerous oxidizing species are confined in highly restricted and anisotropic environments.This work was supported by the Carlos III Institute of Health (Grants No. PII6/01877, "Miguel Servet fellowship" CPII16/00052 to I.A.), and by the Generalitat Valenciana (Prometeo 2017/075). We would like to thank Dr Fedora Grande for sending an exchange student (M.B.). D.N. and G.G. thank NAWI Graz for support.Palumbo, F.; Andreu Ros, MI.; Brunetti, M.; Schmallegger, M.; Gescheidt, G.; Neshchadin, D.; Miranda Alonso, MÁ. (2019). Hydrogen Abstraction from the C15 Position of the Cholesterol Skeleton. The Journal of Organic Chemistry. 84(23):15184-15191. https://doi.org/10.1021/acs.joc.9b02181S15184151918423Zerbinati, C., & Iuliano, L. (2017). 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Paramagnetic Molecular Grippers: The Elements of Six-State Redox Switches

The development of semiquinone-based resorcin[4]arene cavitands expands the toolbox of switchable molecular grippers by introducing the first paramagnetic representatives. The semiquinone (SQ) states were generated electrochemically, chemically, and photochemically. We analyzed their electronic, conformational, and binding properties by cyclic voltammetry, ultraviolet/visible (UV/vis) spectroelectrochemistry, electron paramagnetic resonance (EPR) and transient absorption spectroscopy, in conjunction with density functional theory (DFT) calculations. The utility of UV/vis spectroelectrochemistry and EPR spectroscopy in evaluating the conformational features of resorcin[4]arene cavitands is demonstrated. Guest binding properties were found to be enhanced in the SQ state as compared to the quinone (Q) or the hydroquinone (HQ) states of the cavitands. Thus, these paramagnetic SQ intermediates open the way to six-state redox switches provided by two conformations (open and closed) in three redox states (Q, SQ, and HQ) possessing distinct binding ability. The switchable magnetic properties of these molecular grippers and their responsiveness to electrical stimuli has the potential for development of efficient molecular devices

UV-triggered end group conversion of photo-initiated poly(methyl methacrylate)

The analysis of photo-initiated poly(methyl methacrylate) via electrospray ionization-mass spectrometry (ESI-MS) (synthesized by pulsed laser polymerization (PLP, at λ = 351 nm) of methyl methacrylate (MMA) and benzoin as photoinitiator at 6 mJ/pulse laser energy) evidences the presence of unidentified species. The determination of the origin of these species requires a detailed investigation via size exclusion chromatography-electrospray ionization-mass spectrometry (SEC/ESI-MS) and chemically induced dynamic nuclear polarization-nuclear magnetic resonance spectroscopy (CIDNP-NMR). It was found that post-irradiation of benzoin-initiated poly(methyl methacrylate) leads to α-cleavage of the benzoyl fragment leading to a sequence of cascade reactions, including the formation of an additional double bond within the polymer chain as evidenced via ESI-MS. Furthermore, the reaction products of the benzoyl radical post α-cleavage (e.g., benzaldehyde, phenyl methyl ketone, methyl formate, or methane) as well as the formed macroradical can be followed by CIDNP-NMR, which allows establishing a reaction mechanism for the UV-induced cleavage process. The study thus evidence that-if the integrity of UV initiated polymers is to be kept intact during their synthesis-very low irradiation energies need to be employed. © 2012 American Chemical Society

Time-Resolved EPR as a Tool to Investigate Oxygen Quenching in Photoinitiated Radical Polymerizations

It is challenging to obtain absolute rate constants for the quenching of organic radicals by molecular oxygen because they often do not present absorbance in the UV–vis range. Here, it is shown that time-resolved EPR (chemically induced dynamic electron polarization, or CIDEP) spectroscopy is useful in establishing rate constants for the addition of benzoyl radicals to molecular oxygen. It was found that benzoyl radicals are particularly reactive toward O₂ and can, therefore, act as oxygen scavengers in the initiating phase of radical polymerizations. Kinetic simulations underpin this reactivity

Proton-Coupled Electron Transfer from Hydrogen-Bonded Phenols to Benzophenone Triplets

Phenols with intramolecular hydrogen bond between a pendant base and the phenolic OH group react differently in polar and non-polar environments with electron/proton acceptors. This was demonstrated by using time resolved chemically induced dynamic nuclear polarization (TR CIDNP) and theoretical calculations. In benzene, those phenols undergo a concerted electron–proton transfer (EPT) that yields neutral ketyl and phenoxyl radicals. In polar acetonitrile, the reaction mechanism turns into an electron transfer from the phenol to the triplet ketone, accompanied by the shift of a proton from the phenolic OH group to the nitrogen atom of the pendant base to form a distonic radical cation. This behavior is similar to that of tyrosine H-bonded to basic residues in some radical enzymes. This solvent-induced mechanism switch in proton-coupled electron transfers is important in different biological systems, in which the same metabolites and intermediates can react differently depending on the specific local environments

Methionine-Based Radicals: Time Scales and Species

International audienceAbstract l -Methionine is an amino acid, which provides anti-oxidative properties. We report on radicals and radical cations being likely (short-lived) intermediates formed upon photo-oxidation reactions of methionine. In this context, we present photo-CIDNP experiments indicating that the character of the photooxidants is decisive for the observation of CIDNP effects based on methionine. Based on calculated hyperfine data and pK a values and on our experimental observations, we suggest that CIDNP polarizations are produced by an overlay of at least three geminal radical pairs, i.e., two α -thio carbon-centered radicals D · and G ·, aminyl radical N ·, and, possibly, 2c–3e radical cation SN . + as short-lived reaction intermediates

Examples for biological reactivity involving free radicals followed by CIDNP

It is shown how chemically induced dynamic nuclear polarisation (CIDNP) spectroscopy is able to efficiently complement electron paramagnetic resonance (EPR), when molecular transformations of free radical pairs are investigated. This is demonstrated in three examples of modelling biologically relevant phenomena, particularly oxidative stress and antioxidant activity. Lipid peroxidation, topological control in the oxidation of cholesterol, and a mechanistic study of antioxidant activity of natural tea and wine polyphenols are presented.Andreu Ros, MI.; Neshchadin, D.; Batchelor, SN.; Miranda Alonso, MÁ.; Gescheidt, G. (2013). Examples for biological reactivity involving free radicals followed by CIDNP. Molecular Physics. 111(18-19):2992-2998. doi:10.1080/00268976.2013.809805S2992299811118-1

UV-Triggered End Group Conversion of Photo-Initiated Poly(methyl methacrylate)

The analysis of photo-initiated poly­(methyl methacrylate) via electrospray ionization-mass spectrometry (ESI–MS) (synthesized by pulsed laser polymerization (PLP, at λ = 351 nm) of methyl methacrylate (MMA) and benzoin as photoinitiator at 6 mJ/pulse laser energy) evidences the presence of unidentified species. The determination of the origin of these species requires a detailed investigation via size exclusion chromatography-electrospray ionization-mass spectrometry (SEC/ESI–MS) and chemically induced dynamic nuclear polarization-nuclear magnetic resonance spectroscopy (CIDNP–NMR). It was found that post-irradiation of benzoin-initiated poly­(methyl methacrylate) leads to α-cleavage of the benzoyl fragment leading to a sequence of cascade reactions, including the formation of an additional double bond within the polymer chain as evidenced via ESI–MS. Furthermore, the reaction products of the benzoyl radical post α-cleavage (e.g., benzaldehyde, phenyl methyl ketone, methyl formate, or methane) as well as the formed macroradical can be followed by CIDNP–NMR, which allows establishing a reaction mechanism for the UV-induced cleavage process. The study thus evidence thatif the integrity of UV initiated polymers is to be kept intact during their synthesisvery low irradiation energies need to be employed

Photoinduced Reactivity of the Soft Hydrotris(6-<i>tert</i>-butyl-3-thiopyridazinyl)borate Scorpionate Ligand in Sodium, Potassium, and Thallium Salts

The soft scorpionate ligand hydrotris­(6-<i>tert</i>-butyl-3-thiopyridazinyl)­borate (<b>Tn</b>) was found to exhibit pronounced photoreactivity. Full elucidation of this process revealed the formation of 6-<i>tert</i>-butylpyridazine-3-thione (<b>PnH</b>) and 4,5-dihydro-6-<i>tert</i>-butylpyridazine-3-thione (<b>H</b>_<b>2</b><b>PnH</b>). Under exclusion of light, no solvolytic reactions occur, allowing the development of high-yield preparation protocols for the sodium, potassium, and thallium salts and improving the yield for their derived copper boratrane complex. The photoreactivity is relevant for all future studies with electron-deficient scorpionate ligands