Comparison of Thiyl, Alkoxyl, and Alkyl Radical Addition to Double Bonds: The Unusual Contrasting Behavior of Sulfur and Oxygen Radical Chemistry

The Chemical Reactivity of Membrane Lipids

It is well-known that aqueous dispersions of phospholipids spontaneously assemble into bilayer structures. These structures have numerous applications across chemistry and materials science and form the fundamental structural unit of the biological membrane. The particular environment of the lipid bilayer, with a water-poor low dielectric core surrounded by a more polar and better hydrated interfacial region, gives the membrane particular biophysical and physicochemical properties and presents a unique environment for chemical reactions to occur. Many different types of molecule spanning a range of sizes, from dissolved gases through small organics to proteins, are able to interact with membranes and promote chemical changes to lipids that subsequently affect the physicochemical properties of the bilayer. This Review describes the chemical reactivity exhibited by lipids in their membrane form, with an emphasis on conditions where the lipids are well hydrated in the form of bilayers. Key topics include the following: lytic reactions of glyceryl esters, including hydrolysis, aminolysis, and transesterification; oxidation reactions of alkenes in unsaturated fatty acids and sterols, including autoxidation and oxidation by singlet oxygen; reactivity of headgroups, particularly with reactive carbonyl species; and E/Z isomerization of alkenes. The consequences of reactivity for biological activity and biophysical properties are also discussed.

Lipid Peroxidation and Antioxidant Protection

Lipid peroxidation (LP) is the most important type of oxidative-radical damage in biological systems, owing to its interplay with ferroptosis and to its role in secondary damage to other biomolecules, such as proteins. The chemistry of LP and its biological consequences are reviewed with focus on the kinetics of the various processes, which helps understand the mechanisms and efficacy of antioxidant strategies. The main types of antioxidants are discussed in terms of structure-activity rationalization, with focus on mechanism and kinetics, as well as on their potential role in modulating ferroptosis. Phenols, pyri(mi)dinols, antioxidants based on heavy chalcogens (Se and Te), diarylamines, ascorbate and others are addressed, along with the latest unconventional antioxidant strategies based on the double-sided role of the superoxide/hydroperoxyl radical system.

Tuning Photoenolization-Driven Cycloadditions Using Theory and Spectroscopy

The first broad spectrum investigation into the photoenolization/Diels-Alder (PEDA) sequence was carried out using M06-2X/6-31+G(d,p) in conjunction with SMD solvation and supported by experimental UV-vis spectroscopy. A test set of 20 prodienes was chosen to examine the role of the H atom acceptor group (substituted and unsubstituted carbonyl, thiocarbonyl, and imine), the H atom donor group, and bystander ring substituents. As reaction partners for the photogenerated dienes, a diverse test set of 20 dienophiles was examined, comprising electron rich, electron poor, neutral, strain activated, hydrocarbon, and heteroatom-containing molecules including CO₂ and CO. A key finding of this work is the demonstration that the PEDA sequence of carbonyl based prodienes is tolerant of most substitution patterns. Another is that thiocarbonyl derivatives should behave analogously to the carbonyls but are likely to do so much more slowly, due to an inefficient intersystem crossing, an endothermic 1,5-hydrogen atom transfer (HAT) step, and a [1,5] sigmatropic H shift to regenerate the starting material that outcompetes the [4 + 2]cycloaddition. In contrast, the T₁ state of the ortho-alkyl imines displays the incorrect orbital symmetry for 1,5-HAT and is correspondingly accompanied by higher barriers, even in the excited state. However, provided these barriers can be overcome, the remaining steps in the PEDA sequence are predicted to be facile. The Diels-Alder reaction is predicted to be of much broader scope than reported synthetic literature: while electron poor dienophiles are expected to be the most reactive partners, ethylene and electron rich alkenes should react at a synthetically useful rate. CO is predicted to undergo a facile (4 + 1)cheletropic addition instead of the normal [4 + 2]cycloaddition pathway. This unique photoenolization/cheletropic addition (PECA) sequence could provide metal-free access to benzannelated cyclopentanones.

Electrosynthesis of 1,4-bis(diphenylphosphanyl) tetrasulfide via sulfur radical addition as cathode material for rechargeable lithium battery

Organic electrodes are promising as next generation energy storage materials originating from their enormous chemical diversity and electrochemical specificity. Although organic synthesis methods have been extended to a broad range, facile and selective methods are still needed to expose the corners of chemical space. Herein, we report the organopolysulfide, 1,4-bis(diphenylphosphanyl)tetrasulfide, which is synthesized by electrochemical oxidation of diphenyl dithiophosphinic acid featuring the cleavage of a P-S single bond and a sulfur radical addition reaction. Density functional theory proves that the external electric field triggers the intramolecular rearrangement of diphenyl dithiophosphinic acid through dehydrogenation and sulfur migration along the P-S bond axis. Impressively, the Li/bis(diphenylphosphanyl)tetrasulfide cell exhibits the high discharge voltage of 2.9 V and stable cycling performance of 500 cycles with the capacity retention of 74.8%. Detailed characterizations confirm the reversible lithiation/delithiation process. This work demonstrates that electrochemical synthesis offers the approach for the preparation of advanced functional materials.

Insight into the Thiol-yne Kinetics via a Computational Approach

Thiol-yne reactions have drawn attention because of the click nature as well as the regular step-growth network nature of their products, despite the radical-mediated reactant. However, the factors governing the reaction pathways have not been examined using quantum chemical tools in a comprehensive manner. Thereupon, we have systematically investigated the mechanism of thiol-yne reactions, focusing on the structural influences of thiol and alkyne functionalities. The reaction kinetics, structure-reactivity relations, and E/Z diastereoselectivity of the products have been enlightened for the first cycle of the thiol-yne polymerization reaction. For this reason, a diverse set of 11 thiol-yne reactions with four thiols and eight alkynes was modeled by means of density functional theory. We performed a benchmark study and determined the M06-2X/6-31+G(d,p) level of theory as the best cost-effective methodology to model such reactions. Results reveal that spin density, the stabilities of sulfur radicals for propagation, and the stability of alkenyl intermediate radicals for the chain transfer are the determining factors of each reaction rate. Intramolecular π-π stacking interactions at transition-state structures are found to be responsible for Z diastereoselectivity.

Photo-induced radical thiol-ene chemistry: a versatile toolbox for peptide-based drug design

While the global market for peptide/protein-based therapeutics is witnessing significant growth, the development of peptide drugs remains challenging due to their low oral bioavailability, poor membrane permeability, and reduced metabolic stability. However, a toolbox of chemical approaches has been explored for peptide modification to overcome these obstacles. In recent years, there has been a revival of interest in photoinduced radical thiol-ene chemistry as a powerful tool for the construction of therapeutic peptides.

Influence of early stages of triglyceride pyrolysis on the formation of PAHs as coke precursors

Molecular beam (MB) time-of-flight mass spectrometry has been used to investigate thermal decomposition of triolein, to reveal the mechanisms of low temperature soot/coke formation characteristic for triglycerides (TGs). Mass detected pyrolysis products were observed at incremented temperatures using both VUV single photon ionization (general product detection) and REMPI based selective detection of aromatic products. To augment the simple mass characterizations, we have employed stoichiometric considerations; we have supplemented the analysis further by using the detailed information available from product analysis of batch reactor TG cracking. Both the VUV photoionization and batch reactor studies indicated that formation of C7-sized stable products is a marker of significant triolein decomposition that is coupled with PAH formation. A significant fraction of the C7 species observed likely formed as a result of a C-C bond scission at the allylic position to the ω-9 double bond of oleic acid. REMPI detection indicated a high specificity for PAH formation at three distinct molecular weight values, 276, 352 and 444 amu (the latter being a fullerene precursor). The stoichiometric analysis has shown that these PAHs likely arise from condensation reactions of either C7- or C8-sized fragments (three, four and five, respectively). The C8-sized intermediate would become essential whenever the PAH product of C7 fragment condensation contained an odd number of carbon atoms, resulting in a less stable aromatic structure with an incomplete double bond conjugation. MB experiments involving either addition or in situ generation of hydrogen resulted in an enhancement of lower molecular weight PAH formation, i.e., a decrease in the effective number of condensing fragments. In contrast, an increase in temperature yielded the opposite effect.

Why Not Trans? Inhibited Radical Isomerization Cycles and Coupling Chains of Lipids and Alkenes with Alkane -thiols

Reversible addition of thiyl radicals to cis fatty acids converts them into trans fatty acids, L _Z + S^• ⇄ SL^• ⇄ L _E + S^•, in a cycle that, uninterrupted, would rapidly isomerize lipids exposed to radicals and thiols. One reason this does not happen in foods and organisms is because the cycle is interrupted, by exothermic allylic abstraction, L + S^• → L^• + SH. Autoinhibition limits the cis-trans cycle length to around 400-500 (L _E per S^•) in a MUFA model (methyl oleate) and just ∼13-15 in a PUFA lipid model (methyl linoleate). The weak C-H bonds in bisallylic groups in PUFAs thereby act as the first line of defense against thiyl cis-trans cycles in biolipid solutions (±O₂). With the intriguing exception of vitamin E in MUFA, thiyl-active antioxidants inhibit isomerization in much the same way as they protect against peroxidation. Applied to thiol-ene coupling (TEC), the allylic abstraction, degraded-chain paradigm resolved a raft of hitherto contradictory trends and findings in "click" TEC polymerization and organic synthesis methods.

Reactions of hydroxyl radicals with benzoic acid and benzoate

Density functional theory was used to study the mechanism and kinetics of benzoic acid with hydroxyl radicals in both gas and aqueous phases as well as benzoate with hydroxyl radicals in the aqueous phase at the M06-2X/6-311+G(d,p) level of theory.