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Group 1 elements exhibit the lowest electronegativity values in the Periodic Table. The chemical reduction of Group 1 metal cations M+ to M(0) is extremely challenging. Common tetraaryl borates demonstrate limited redox properties and are prone to decomposition upon oxidation. In this study, by employing simple yet versatile bipyridines as ligands, we synthesized a series of redox-active borate anions characterized by NMR and X-ray single-crystal diffraction. Notably, the borate anion can realize the reduction of Li+, generating elemental lithium metal and boron radical, thereby demonstrating its potent reducing ability. Furthermore, it can serve as a powerful two-electron-reducing reagent and be readily applied in various reductive homo-coupling reactions and Birch reduction of acridine. Additionally, this borate anion demonstrates its catalytic ability in the selective two-electron reduction of CO2 into CO. Chemical reduction of alkali cations to their metals is extremely challenging. Here, the authors synthesized a series of redox-active borate anions stabilized by bipyridine ligands which can reduce lithium ions generating elemental lithium metal and borate radicals. [ABSTRACT FROM AUTHOR]

Selective cleavages of N-[C.sub.α] and [C.sub.α]-C bonds of p-radical tautomers of amino acid residues in radical peptides have been examined theoretically by means of the density functional theory at the M06-2X/6-311++G(d,p) level. The majority of the bond cleavages are homolytic via β-scission. Their energy barriers depend largely on the ability of the radical being stabilized in the transition structures and the availability of a mobile proton in the vicinity of the p-radical center. The N-[C.sub.α] bond is less favorably cleaved than the [C.sub.α]-C bond (except Ser and Thr) for systems without a mobile proton. It is because, firstly, the homolytic cleavage is less favorable for the more polar N-[C.sub.α] bond than for the less polar [C.sub.α]-C bond. Secondly, a less stable σ-radical localized on the amide nitrogen atom of the incipient N-terminal fragment is formed for the former, while a more stable radical delocalized in a [π.sup.*](CO)-like orbital of the incipient C-terminal fragment is formed for the latter. In the presence of a mobile proton N-terminal to the β-radical center, some degrees of heterolytic cleavage character, as preferred by the polar N-[C.sub.α] bond, are observed. Consequently, its barrier is reduced. If the mobile proton is located at the C-terminal amide oxygen of the β-radical center, the [C.sub.α]-C bond cleavage will be significantly suppressed. It is because the radical in the incipient C-terminal fragment becomes more localized as a σ-radical on the carbon atom of its protonated amide group. With basic amino acid residues, the [C.sub.α]-C bond cleavage can be reactivated. Heterolytic cleavage of the polar N-[C.sub.α] bond can be largely facilitated if a mobile proton N-terminal to the β-radical center is available and the radical in the incipient C-terminal fragment is sufficiently stabilized, for instance, by the aromatic side chain of Trp and Tyr. Therefore, cleavages of the N-[C.sub.α] bond induced by the β-radical tautomer of Trp and Tyr are often preferred as compared with cleavages of the [C.sub.α]-C bond in peptide radical cations containing mobile protons. Key words: peptide dissociation, gas-phase ion, density functional theory, β-scission, radical chemistry. Nous avons modelise les clivages selectifs des liaisons N-[C.sub.α] et [C.sub.α]-C des tautomeres β-radicalaires de residus d'acides amines dans des peptides radicalaires au niveau M06-2X/6-311++G(d,p) de la theorie de la fonctionnelle de la densite. La majorite des clivages de liaison sont homolytiques et se produisent par scission β. Leurs barrieres energetiques dependent fortement de l'aptitude du radical a etre stabilise dans les structures de transition et de la presence d'un proton labile a proximite du centre β-radicalaire. Le clivage de liaison N-[C.sub.α] est moins favorable que celui de la liaison [C.sub.α]-C (sauf dans les residus Ser et Thr) dans les systemes sans proton labile. Ce phenomene s'explique premierement par le fait que le clivage homolytique des liaisons N-[C.sub.α] plus polaires est moins favorable que celui des liaisons [C.sub.α]-C moins polaires. Deuxiemement, un radical σ moins stable, localise sur l'atome d'azote de l'amide du fragment N-terminal naissant, se forme dans le premier cas, alors qu'un radical plus stable, delocalise dans une orbitale de type [π.sup.*](CO) du fragment C-terminal naissant, se forme dans le second cas. En presence d'un proton labile a la position N-terminale du centre β-radicalaire, on observe certains degres du caractere d'un clivage heterolytique, qui est favorise dans le cas de la liaison N [C.sub.α] polaire. Par consequent, sa barriere energetique est abaissee. Si le proton labile est localise sur l'oxygene de l'amide C-terminal du centre β-radicalaire, le clivage de la liaison [C.sub.α]-C sera ralenti de facon importante. Ce phenomene s'explique par le fait que le radical dans le fragment C-terminal naissant devient plus localise a la maniere d'un radical r sur l'atome de carbone de son groupement amide protone. Dans le cas des residus d'acides amines basiques, le clivage de la liaison [C.sub.α]-C peut etre reactive. Le clivage heterolytique de la liaison N-[C.sub.α] polaire peut etre grandement facilite si un proton labile en position N-terminale par rapport au centre β-radicalaire est present et que le radical du fragment C-terminal naissant est suffisamment stabilise, par exemple, par la chaine laterale aromatique du Trp et de la Tyr. En somme, les clivages de la liaison N-[C.sub.α] induits par le tautomere p-radicalaire du Trp et de la Tyr sont souvent favorises par rapport aux clivages de la liaison [C.sub.α]-C des cations de radicaux peptidiques comportant des protons labiles. [Traduit par la Redaction] Mots-cles : dissociation peptidique, ion en phase gazeuse, theorie de la fonctionnelle de la densite, scission p, chimie radicalaire., Introduction Gas-phase chemistry of odd electron peptide radical cations is of fundamental importance with respect to its analytical applications to mass spectrometry based protein identification. Numerous techniques for generation of [...]

A triple-quadrupole mass spectrometer has been modified for bond-dissociation energy measurements via threshold collision-induced dissociations (TCIDs) by replacing the conventional collision cell with a ring ion guide. Optimal operating conditions for the ring ion guide were determined or derived, and validated using a set of complexes for which bond dissociation energies are known. A comparison with reference data (within a range of 16-57 kcal/mol) indicates an accuracy approaching that of TCID determined on a guided ion-beam mass spectrometer. Complexes for which bond-dissociation energies were measured include metal ion complexes of simple ligands, amino acids and peptides, as well as of carbonic acid. There is excellent agreement between our experimental data and literature data, as well as theoretical data determined using a high-level computational method.

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In this study, dissociative one‐electron transfer dissociation of [CuII(dien)Y(G/A)W]•2+ [dien = diethylenetriamine; Y(G/A)W = tyrosyl (glycyl/alanyl)tryptophan] was used to generate the tripeptide radical cations [Y(G/A)W]•+; subsequent loss of the Tyr side chain formed [Gα•(G/A)W]+. The π‐centered species [YGWπ•]+ generated the α‐centered species [Gα•GW]+ through Cα–Cβ bond cleavage, as revealed using infrared multiple photon dissociation (IRMPD) measurements and density functional theory (DFT) calculations. Comparisons of experimental and theoretical IR spectra confirmed that both the charge and spin densities of [Y(G/A)Wπ•]+ were delocalized initially at the tryptophan indolyl ring; subsequent formation of the final [Gα•(G/A)W]+ structure gave the highest spin density at the α‐carbon atom of the N‐terminal glycine residue, with a proton solvated by the first amide oxygen atom. The IRMPD mass spectra and action spectra of the [Gα•(G/A)W]+ species were all distinctly different from those of their isomeric [G(G/A)Wπ•]+ species. The mechanism of formation of the captodative [Gα•(G/A)W]+ species—with the charge site separated from the radical site—from [Y(G/A)Wπ•]+ has been elucidated. DFT calculations suggested that the Cα–Cβ bond cleavage of the tyrosine residue in the radical cationic [Y(G/A)Wπ•]+ precursor involves (a) through‐space electron transfer between the indolyl and phenolic groups; (b) formation of proton‐bound dimers through Cα–Cβ cleavage of the tyrosine residue; and (c) a concerted proton rearrangement from the phenolic OH group to the carboxyl group and formation of the α‐carbon‐centered product [Gα•(G/A)W]+ through hydrogen bond cleavage. The barriers for the electron transfer (a), the Cα–Cβ cleavage (b), and the protonation rearrangement (c) were 12.8, 26.5, and 10.3 kcal mol−1, respectively. [ABSTRACT FROM AUTHOR]

Long-range electron transfer in proteins can be rationalized as a sequential short-distance electron-hopping processes via amino acid residues having low ionization energy as relay stations. Tyrosine residues can serve as such redox-active intermediates through one-electron oxidation to form a π-radical cation at its phenol side chain. An electron transfer from a vicinal functional group to this π-electron hole completes an elementary step of charge migration. However, transient oxidized/reduced intermediates formed at those relay stations during electron transfer processes have not been observed. In this study, formation of analog reactive intermediates via electron donor–acceptor coupling is observed by using IRMPD action spectroscopy. An elementary charge migration at the molecular level in model tyrosine-containing peptide radical cations [M]˙+ in the gas phase is revealed with its unusual Cα–Cβ bond cleavage at the side chain of the N-terminal residue. This reaction is induced by the radical character of the N-terminal amino group (–NH2˙+) resulting from an n → π+ interaction between the nonbonding electron pair of NH2 (n) and the π-electron hole at the Tyr side chain (π+). The formation of –NH2˙+ is supported by the IRMPD spectrum showing a characteristic NH2 scissor vibration coupled with Tyr side-chain stretches at 1577 cm−1. This n → π+ interaction facilitates a dissociative electron transfer with NH2 as the relay station. The occurrence of this side-chain cleavage may be an indicator of the formation of reactive conformers featuring the n → π+ interaction. [ABSTRACT FROM AUTHOR]

We report herein the first detailed study of the mechanism of redox reactions occurring during the gas-phase dissociative electron transfer of prototypical ternary [CuII(dien)M]˙2+ complexes (M, peptide). The two final products are (i) the oxidized non-zwitterionic π-centered [M]˙+ species with both the charge and spin densities delocalized over the indole ring of the tryptophan residue and with a C-terminal COOH group intact, and (ii) the complementary ion [CuI(dien)]+. Infrared multiple photon dissociation (IRMPD) action spectroscopy and low-energy collision-induced dissociation (CID) experiments, in conjunction with density functional theory (DFT) calculations, revealed the structural details of the mass-isolated precursor and product cations. Our experimental and theoretical results indicate that the doubly positively charged precursor [CuII(dien)M]˙2+ features electrostatic coordination through the anionic carboxylate end of the zwitterionic M moiety. An additional interaction exists between the indole ring of the tryptophan residue and one of the primary amino groups of the dien ligand; the DFT calculations provided the structures of the precursor ion, intermediates, and products, and enabled us to keep track of the locations of the charge and unpaired electron. The dissociative one-electron transfer reaction is initiated by a gradual transition of the M tripeptide from the zwitterionic form in [CuII(dien)M]˙2+ to the non-zwitterionic M intermediate, through a cascade of conformational changes and proton transfers. In the next step, the highest energy intermediate is formed; here, the copper center is 5-coordinate with coordination from both the carboxylic acid group and the indole ring. A subsequent switch back to 4-coordination to an intermediate IM1, where attachment to GGW occurs through the indole ring only, creates the structure that ultimately undergoes dissociation. [ABSTRACT FROM AUTHOR]

The elementary mechanism of radical‐mediated peptide tyrosine nitration, which is a hallmark of post‐translational modification of proteins under nitrative stress in vivo, has been elucidated in detail by using an integrated approach that combines the gas‐phase synthesis of prototypical molecular tyrosine‐containing peptide radical cations, ion–molecule reactions, and isotopic labeling experiments with DFT calculations. This reaction first involves the radical recombination of.NO2 towards the prerequisite phenoxyl radical tautomer of a tyrosine residue, followed by proton rearrangements, finally yielding the stable and regioselective 3‐nitrotyrosyl residue product. In contrast, nitration with the π‐phenolic radical cation tautomer is inefficient. This first direct experimental evidence for the elementary steps of the radical‐mediated tyrosine nitration mechanism in the gas phase provides a fundamental insight into the regioselectivity of biological tyrosine ortho‐nitration. [ABSTRACT FROM AUTHOR]