Direct Activation of the C(sp3)-NH2 Bond of Primary Aliphatic Alkylamines by a High-Valent CoIII,IV2(µ-O)2 Diamond Core Complex.

Aliphatic alkylamines are abundant feedstock and versatile building blocks for many organic transformations. While remarkable progress has been made to construct C-N bonds on aliphatic and aromatic carbon centers, the activation and functionalization of C(sp3)-NH2 bonds in primary alkylamines remain a challenging process. In the present work, we discovered an unprecedented method to directly activate the C(sp3)-NH2 bond of primary alkylamines by a high-valent dinuclear CoIII,IV2(µ-O)2 diamond core complex. This reaction results in the installation of other functional groups such as halides and alkenes onto the α-carbon center concomitant with the 2-e- oxidation of the nitrogen atom on the amino group to form NH2OH. These results shed light on future development enabling versatile functionalization of primary alkylamines based on the dinuclear cobalt system. Moreover, our work suggests that a related high-valent copper-oxo intermediate is likely generated in the ammonia monooxygenase catalytic cycle to affect the oxidation of NH3 to NH2OH.

Unusual In-plane Aromaticity Facilitates Intramolecular Hydrogen Transfer in Long-Bonded cis-Isonitrosyl Methoxide.

The hydrogen-atom transfer from methoxy radical to nitric oxide, leading to the formation of formaldehyde and nitroxyl, represents a secondary reaction of photodissociation of methyl nitrite, which is used as rocket fuel. In this study, we explored the potential energy profile of the hydrogen-atom transfer using the electronic structure calculations at the DLPNO-CCSD(T)/aug-cc-pVTZ level of theory for two isomeric forms (cis and trans) of the pre-reaction complex. The cis-oriented pre-reaction complex has a weak elongated O─O bond, which gets further elongated in the hydrogen transfer transition state. This O─O bond stabilizes the pre-reaction complex by 32.9 kJ/mol. The O─O-induced stabilization is even greater for the transition state (48.2 kJ/mol), which was unexpected because of the larger O─O distance in the transition state structure. To address this paradox, we performed the electronic structure analysis of the reaction participants using the valence bond (VB) theory, natural resonance theory, topological analysis of the electron density and its derivatives, and analysis of the electron localization function distribution. This combined analysis led to the conclusion that the cis-transition state for hydrogen transfer, instead of being directly stabilized by the O─O interaction, gained substantial stabilization from the in-plane five-center six-electron aromaticity.

Structural insights into the mechanism of oxidative activation of heme-free H-NOX from Vibrio cholerae.

Bacterial heme nitric oxide/oxygen (H-NOX) domains are nitric oxide (NO) or oxygen sensors. This activity is mediated through binding of the ligand to a heme cofactor. However, H-NOX from Vibrio cholerae (Vc H-NOX) can be easily purified in a heme-free state that is capable of reversibly responding to oxidation, suggesting a heme-independent function as a redox sensor. This occurs by oxidation of Cys residues at a zinc-binding site conserved in a subset of H-NOX homologs. Remarkably, zinc is not lost from the protein upon oxidation, although its ligation environment is significantly altered. Using a combination of computational and experimental approaches, we have characterized localized structural changes that accompany the formation of specific disulfide bonds between Cys residues upon oxidation. Furthermore, the larger-scale structural changes accompanying oxidation appear to mimic those changes observed upon NO binding to the heme-bound form. Thus, Vc H-NOX and its homologs may act as both redox and NO sensors by completely separate mechanisms.

Opening the CoIII,IV2(µ-O)2 Diamond Core by Lewis Bases Leads to Enhanced C-H Bond Cleaving Reactivity.

The high-valent diiron(IV) intermediate Q is the key oxidant that cleaves strong C-H bonds of methane in the catalytic cycle of soluble methane monooxygenase (sMMO). sMMO-Q was previously reported as a bis-µ-oxo FeIV2(µ-O)2 diamond core but was recently described to have an open core with a long Fe···Fe distance. We recently reported a high-valent CoIII,IV2(µ-O)2 diamond core complex (1) that is highly reactive with sp3 C-H bonds. In this work, we demonstrated that the C-H bond cleaving reactivity of 1 can be further enhanced by introducing a Lewis base X, affording faster kinetic rate constants and the ability to cleave stronger C-H bonds compared to 1. We proposed that 1 first reacts with X in a fast equilibrium to form an open core species X-CoIII-O-CoIV-O (1-X). We were able to characterize 1-X using EPR spectroscopy and DFT calculations. 1-X exhibited an S = 1/2 EPR signal distinct from that of the parent complex 1. DFT calculations showed that 1-X has an open core with the spin density heavily delocalized in the CoIV-O unit. Moreover, 1-X has a more favorable thermodynamic driving force and a smaller activation barrier than 1 to carry out C-H bond activation reactions. Notably, 1-X is at least 4 orders of magnitude more reactive than its diiron open core analogues. Our findings indicate that the diamond core isomerization is likely a practical enzymatic strategy to unmask the strong oxidizing power of sMMO-Q necessary to attack the highly inert C-H bonds of methane.

Efficient newton-raphson/singular value decomposition-based optimization scheme with dynamically updated critical condition number for rapid convergence of weighted histogram analysis method equations.

Selection of the successful optimization strategy is an essential part of solving numerous practical problems yet often is a nontrivial task, especially when a function to be optimized is multidimensional and involves statistical data. Here we propose a robust optimization scheme, referred to as NR/SVD-Cdyn, which is based on a combination of the Newton-Raphson (NR) method along with singular value decomposition (SVD), and demonstrate its performance by numerically solving a system of the weighted histogram analysis method equations. Our results show significant improvement over the direct iteration and conventional NR optimization methods. The proposed scheme is universal and could be used for solving various optimization problems in the field of computational chemistry such as parameter fitting for the methods of molecular mechanics and semiempirical quantum-mechanical methods. © 2019 Wiley Periodicals, Inc.

Symmetry Effects in Photoinduced Electron Transfer in Chlorin-Quinone Dyads: Adiabatic Suppression in the Marcus Inverted Region.

In donor-acceptor dyads undergoing photoinduced electron transfer (PET), a direction or pathway for electron movement is usually dictated by the redox properties and the separation distance between the donor and acceptor subunits, while the effect of symmetry is less recognized. We have designed and synthesized two isomeric donor-acceptor assemblies in which electronic coupling between donor and acceptor is altered by the orbital symmetry control with the reorganization energy and charge transfer exothermicity being kept unchanged. Analysis of the optical absorption and luminescence spectra, supported by the DFT and TD-DFT calculations, showed that PET in these assemblies corresponds to the Marcus inverted region (MIR) and has larger rate for isomer with weaker electronic coupling. This surprising observation provides the first experimental evidence for theoretically predicted adiabatic suppression of PET in MIR, which unambiguously controlled solely by symmetry.

Highly Reactive CoIII,IV2(µ-O)2 Diamond Core Complex That Cleaves C-H Bonds.

The selective activation of strong sp3 C-H bonds at mild conditions is a key step in many biological and synthetic transformations and an unsolved challenge for synthetic chemists. In nature, soluble methane monooxygenase (sMMO) is one representative example of nonheme dinuclear iron-dependent enzymes that activate strong sp3 C-H bonds by a high-valent diiron(IV) intermediate Q. To date, synthetic model complexes of sMMO-Q have shown limited abilities to oxidize strong C-H bonds. In this work, we generated a high-valent CoIII,IV2(µ-O)2 complex 3 supported by a tetradentate tris(2-pyridylmethyl)amine (TPA) ligand via one-electron oxidation of its CoIII2(µ-O)2 precursor 2. Characterization of 2 and 3 using X-ray absorption spectroscopy and DFT calculations showed that both species possess a diamond core structure with a short Co···Co distance of 2.78 Å. Furthermore, 3 is an EPR active species showing an S = 1/2 signal with clearly observable hyperfine splittings originated from the coupling of the 59Co nuclear spin with the electronic spin. Importantly, 3 is a highly reactive oxidant for sp3 C-H bonds, and an oxygenation reagent. 3 has the highest rate constant (1.5 M-1 s-1 at -60 °C) for oxidizing 9,10-dihydroanthracene (DHA) compared to diamond core complexes of other first-row transition metals including Mn, Fe and Cu reported previously. Specifically, 3 is about 4-5 orders of magnitude more reactive than the diiron analogs FeIII,IV2(µ-O)2 and FeIV2(µ-O)2 supported by TPA and related ligands. These findings shed light on future development of more reactive approaches for C-H bond activation by bioinspired dicobalt complexes.

Role of Conserved Histidine and Serine in the HCXXXXXRS Motif of Human Dual-Specificity Phosphatase 5.

BACKGROUND: The mitogen-activated protein kinase (MAPK) pathway is functionally generic and critical in maintaining physiological homeostasis and normal tissue development. This pathway is under tight regulation, which is in part mediated by dual-specific phosphatases (DUSPs), which dephosphorylate serine, threonine, and tyrosine residues of the ERK family of proteins. DUSP5 is of high clinical interest because of mutations we identified in this protein in patients with vascular anomalies. Unlike other DUSPs, DUSP5 has unique specificity toward substrate pERK1/2. Using molecular docking and simulation strategies, we previously showed that DUSP5 has two pockets, which are utilized in a specific fashion to facilitate specificity toward catalysis of its substrate pERK1/2. Remarkably, most DUSPs share high similarity in their catalytic sites. Studying the catalytic domain of DUSP5 and identifying amino acid residues that are important for dephosphorylating pERK1/2 could be critical in developing small molecules for therapies targeting DUSP5. RESULTS: In this study, we utilized computational modeling to identify and predict the importance of two conserved amino acid residues, H262 and S270, in the DUSP5 catalytic site. Modeling studies predicted that catalytic activity of DUSP5 would be altered if these critical conserved residues were mutated. We next generated independent Glutathione-S-Transferase (GST)-tagged full-length DUSP5 mutant proteins carrying specific mutations H262F and S270A in the phosphatase domain. Biochemical analysis was performed on these purified proteins, and consistent with our computational prediction, we observed altered enzyme activity kinetic profiles for both mutants with a synthetic small molecule substrate (pNPP) and the physiological relevant substrate (pERK) when compared to wild type GST-DUSP5 protein. CONCLUSION: Our molecular modeling and biochemical studies combined demonstrate that enzymatic activity of phosphatases can be manipulated by mutating specific conserved amino acid residues in the catalytic site (phosphatase domain). This strategy could facilitate generation of small molecules that will serve as agonists/antagonists of DUSP5 activity.

Coexistence of structurally similar but electronically distinct isomers of delocalized cation radicals as a basis for the development of functional materials.

Certain low-symmetry aromatic molecules with a small HOMO/HOMO-1 energy gap might co-exist as electronic isomers in their cation radical states that differ only in bond lengths yet have distinct optical and electronic properties. These electronic isomers are sensitive to the environment and might be used for the development of novel functional materials.

Discovery and Characterization of Halogenated Xanthene Inhibitors of DUSP5 as Potential Photodynamic Therapeutics.

Dual specific phosphatases (DUSPs) are an important class of mitogen-activated protein kinase (MAPK) regulators, and are drug targets for treating vascular diseases. Previously we had shown that DUSP5 plays a role in embryonic vertebrate vascular patterning. Herein, we screened a library of FDA-approved drugs and related compounds, using a para-nitrophenylphosphate substrate (pNPP)-based assay. This assay identified merbromin (also known as mercurochrome) as targeting DUSP5; and, we subsequently identified xanthene-ring based merbromin analogs eosin Y, erythrosin B, and rose bengal, all of which inhibit DUSP5 in vitro. Inhibition was time-dependent for merbromin, eosin Y, 2',7'-dibromofluorescein, and 2',7'-dichlorofluorescein, with enzyme inhibition increasing over time. Reaction progress curve data fit best to a slow-binding model of irreversible enzyme inactivation. Potency of the time-dependent compounds, except for 2',7'-dichlorofluorescein, was diminished when dithiothreitol (DTT) was present, suggesting thiol reactivity. Two additional merbromin analogs, erythrosin B and rose bengal also inhibit DUSP5, but have the therapeutic advantage of being less sensitive to DTT and exhibiting little time dependence for inhibition. Inhibition potency is correlated with the xanthene dye's LUMO energy, which affects ability to form light-activated radical anions, a likely active inhibitor form. Consistent with this hypothesis, rose bengal inhibition is light-dependent and demonstrates the expected red shifted spectrum upon binding to DUSP5, with a Kd of 690 nM. These studies provide a mechanistic foundation for further development of xanthene dyes for treating vascular diseases that respond to DUSP5 inhibition, with the following relative potencies: rose bengal > merbromin > erythrosin B > eosin Y.

Acid-facilitated product release from a Mo(IV) center: relevance to oxygen atom transfer reactivity of molybdenum oxotransferases.

We report that pyridinium ions (HPyr+) accelerate the conversion of [Tp*MoIVOCl(OPMe3)] (1) to [Tp*MoIVOCl(NCCH3)] (2) by 103-fold, affording 2 in near-quantitative yield; Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate. This novel reactivity and the mechanism of this reaction were investigated in detail. The formation of 2 followed pseudo-first-order kinetics, with the observed pseudo-first-order rate constant (k obs) linearly correlated with [HPyr+]. An Eyring plot revealed that this HPyr+-facilitated reaction has a small positive value of ∆S ‡ indicative of a dissociative interchange (Id) mechanism, different from the slower associative interchange (Ia) mechanism in the absence of HPyr+ marked with a negative ∆S ‡. Interestingly, log(k obs) was found to be linearly correlated to the acidity of substituted pyridinium ions. This novel reactivity is further investigated using combined DFT and ab initio coupled cluster methods. Different reaction pathways, including Id, Ia, and possible alternative routes in the absence or presence of HPyr+, were considered, and enthalpy and free energies were calculated for each pathway. Our computational results further underscored that the Id route is energetically favored in the presence of HPyr+, in contrast with the preferred Ia-NNO pathway in the absence of HPyr+. Our computational results also revealed molecular-level details for the HPyr+-facilitated Id route. Specifically, HPyr+ initially becomes hydrogen-bonded to the oxygen atom of the Mo(IV)-OPMe3 moiety, which lowers the activation barrier for the Mo-OPMe3 bond cleavage in a rate-limiting step to dissociate the OPMe3 product. The implications of our results were discussed in the context of molybdoenzymes, particularly the reductive half-reaction of sulfite oxidase.

Ab Initio Study Predicts That Enigmatic Isonitrosyl Fluoride Should Be Stable at Low Temperatures yet Unnoticeable Due to Its Photoreactivity.

Isonitrosyl fluoride F-ON remains an undetected molecule despite multiple attempts to generate it and successful identification of other isonitrosyl halides (X-ONs) via phototransformations of corresponding X-NOs. We investigated this problem using ab initio methods and found no evidence of instability of F-ON at low temperatures of 8-10 K. Instead, experimental observation of F-ON is likely challenged by the (1) different nature of photoexcitation of F-NO and its quantum yield being lower than those of other X-NOs and (2) the presence of a bright charge-transfer transition in the F-ON spectrum that likely overlaps with the weak band of F-NO used for photoexcitation. Formation of F-ON via symmetry-prohibited photoexcitation of F-NO is followed by its immediate photodecomposition to the charge-transfer excited state and its conversion to F-NO upon de-excitation. Thus, F-ON should be readily observable using non-photochemistry methods such as microwave spectroscopy.

Poly-p-hydroquinone Ethers: Isoenergetic Molecular Wires with Length-Invariant Oxidation Potentials and Cation Radical Excitation Energies.

Typical poly-p-phenylene wires are characterized by strong interchromophoric electronic coupling with redox and optical properties being highly length-dependent. Herein we show that an incorporation of a pair of para-methoxy groups at each p-phenylene unit in poly-p-phenylene wires (i.e., PHEn) changes the nodal structure of HOMO that leads to length-invariant oxidation potentials and cation radical excitation energies. As such, PHEn represents a unique class of isoenergetic wires where hole delocalization mainly occurs via dynamic hopping and thus may serve as an efficient medium for long-range charge transfer. Availability of these wires will allow demonstration of long-range electron transfer via incoherent hopping using donor-bridge-acceptor systems with isoenergetic PHEn-based wires as bridges.

Modeling of S-Nitrosothiol-Thiol Reactions of Biological Significance: HNO Production by S-Thiolation Requires a Proton Shuttle and Stabilization of Polar Intermediates.

Nitroxyl (HNO), a reduced form of the important gasotransmitter nitric oxide, exhibits its own unique biological activity. A possible biological pathway of HNO formation is the S-thiolation reaction between thiols and S-nitrosothiols (RSNOs). Our density functional theory (DFT) calculations suggested that S-thiolation proceeds through a proton transfer from the thiol to the RSNO nitrogen atom, which increases electrophilicity of the RSNO sulfur, followed by nucleophilic attack by thiol, yielding a charge-separated zwitterionic intermediate structure RSS+ (R)N(H)O- (Zi), which decomposes to yield HNO and disulfide RSSR. In the gas phase, the proton transfer and the S-S bond formation are asynchronous, resulting in a high activation barrier (>40 kcal mol-1 ), making the reaction infeasible. However, the barrier can decrease below the S-N bond dissociation energy in RSNOs (≈30 kcal mol-1 ) upon transition into an aqueous environment that stabilizes Zi and provides a proton shuttle to synchronize the proton transfer and the S-S bond formation. These mechanistic features suggest that S-thiolation can easily lend itself to enzymatic catalysis and thus can be a possible route of endogenous HNO production.

Serendipitous discovery of light-induced (In Situ) formation of an Azo-bridged dimeric sulfonated naphthol as a potent PTP1B inhibitor.

BACKGROUND: Protein tyrosine phosphatases (PTPs) like dual specificity phosphatase 5 (DUSP5) and protein tyrosine phosphatase 1B (PTP1B) are drug targets for diseases that include cancer, diabetes, and vascular disorders such as hemangiomas. The PTPs are also known to be notoriously difficult targets for designing inihibitors that become viable drug leads. Therefore, the pipeline for approved drugs in this class is minimal. Furthermore, drug screening for targets like PTPs often produce false positive and false negative results. RESULTS: Studies presented herein provide important insights into: (a) how to detect such artifacts, (b) the importance of compound re-synthesis and verification, and (c) how in situ chemical reactivity of compounds, when diagnosed and characterized, can actually lead to serendipitous discovery of valuable new lead molecules. Initial docking of compounds from the National Cancer Institute (NCI), followed by experimental testing in enzyme inhibition assays, identified an inhibitor of DUSP5. Subsequent control experiments revealed that this compound demonstrated time-dependent inhibition, and also a time-dependent change in color of the inhibitor that correlated with potency of inhibition. In addition, the compound activity varied depending on vendor source. We hypothesized, and then confirmed by synthesis of the compound, that the actual inhibitor of DUSP5 was a dimeric form of the original inhibitor compound, formed upon exposure to light and oxygen. This compound has an IC50 of 36 µM for DUSP5, and is a competitive inhibitor. Testing against PTP1B, for selectivity, demonstrated the dimeric compound was actually a more potent inhibitor of PTP1B, with an IC50 of 2.1 µM. The compound, an azo-bridged dimer of sulfonated naphthol rings, resembles previously reported PTP inhibitors, but with 18-fold selectivity for PTP1B versus DUSP5. CONCLUSION: We report the identification of a potent PTP1B inhibitor that was initially identified in a screen for DUSP5, implying common mechanism of inhibitory action for these scaffolds.

Dihedral-Angle-Controlled Crossover from Static Hole Delocalization to Dynamic Hopping in Biaryl Cation Radicals.

In cases of coherent charge-transfer mechanism in biaryl compounds the rates follow a squared cosine trend with varying dihedral angle. Herein we demonstrate using a series of biaryl cation radicals with varying dihedral angles that the hole stabilization shows two different regimes where the mechanism of the hole stabilization switches over from (static) delocalization over both aryl rings to (dynamic) hopping. The experimental data and DFT calculations of biaryls with different dihedral angles unequivocally support that a crossover from delocalization to hopping occurs at a unique dihedral angle where the electronic coupling (Hab ) is one half of reorganization (λ), that is, Hab =λ/2. The implication of this finding in non-coherent charge-transfer rates is being investigated.

Critical Role of the Secondary Binding Pocket in Modulating the Enzymatic Activity of DUSP5 toward Phosphorylated ERKs.

DUSP5 is an inducible nuclear dual-specificity phosphatase that specifically interacts with and deactivates extracellular signal-regulated kinases ERK1 and ERK2, which are responsible for cell proliferation, differentiation, and survival. The phosphatase domain (PD) of DUSP5 has unique structural features absent from other nuclear DUSPs, such as the presence of a secondary anion-binding site in the proximity of the reaction center and a glutamic acid E264 positioned next to the catalytic cysteine C263, as well as a remote intramolecular disulfide linkage. The overall 400 ns molecular dynamics simulations indicate that the secondary binding site of DUSP5 PD acts as an allosteric regulator of the phosphatase activity of DUSP5. Our studies have identified E264 as a critical constituent of the dual binding pocket, which regulates the catalytic activity of DUSP5 by forming a salt bridge with arginine R269. Molecular dynamics studies showed that initial occupation of the secondary binding pocket leads to the breakage of the salt bridge, which then allows the occupation of the active site. Indeed, biochemical analysis using the pERK assay on mutant E264Q demonstrated that mutation of glutamic acid E264 leads to an increase in the DUSP5 catalytic activity. The role of the secondary binding site in assembling the DUSP5-pERK pre-reactive complex was further demonstrated by molecular dynamics simulations that showed that the remote C197-C219 disulfide linkage controls the structure of the secondary binding pocket based on its redox state (i.e., disulfide/dithiol) and, in turn, the enzymatic activity of DUSP5.

Energy Gap between the Poly-p-phenylene Bridge and Donor Groups Controls the Hole Delocalization in Donor-Bridge-Donor Wires.

Poly-p-phenylene wires are critically important as charge-transfer materials in photovoltaics. A comparative analysis of a series of poly-p-phenylene (RPPn) wires, capped with isoalkyl (iAPPn), alkoxy (ROPPn), and dialkylamino (R2NPPn) groups, shows unexpected evolution of oxidation potentials, i.e., decrease (-260 mV) for iAPPn, while increase for ROPPn (+100 mV) and R2NPPn (+350 mV) with increasing number of p-phenylenes. Moreover, redox/optical properties and DFT calculations of R2NPPn/R2NPPn+• further show that the symmetric bell-shaped hole distribution distorts and shifts toward one end of the molecule with only 4 p-phenylenes in R2NPPn+•, while shifting of the hole occurs with 6 and 8 p-phenylenes in ROPPn+• and iAPPn+•, respectively. Availability of accurate experimental data on highly electron-rich dialkylamino-capped R2NPPn together with ROPPn and iAPPn allowed us to demonstrate, using our recently developed Marcus-based multistate model (MSM), that an increase of oxidation potentials in R2NPPn arises due to an interplay between the electronic coupling (Hab) and energy difference between the end-capped groups and bridging phenylenes (Δε). A comparison of the three series of RPPn with varied Δε further demonstrates that decrease/increase/no change in oxidation energies of RPPn can be predicted based on the energy gap Δε and coupling Hab, i.e., decrease if Δε < Hab (i.e., iAPPn), increase if Δε > Hab (i.e., R2NPPn), and minimal change if Δε ≈ Hab (i.e., ROPPn). MSM also reproduces the switching of the nature of electronic transition in higher homologues of R2NPPn+• (n ≥ 4). These findings will aid in the development of improved models for charge-transfer dynamics in donor-bridge-acceptor systems.

From Wires to Cables: Attempted Synthesis of 1,3,5-Trifluorenylcyclohexane as a Platform for Molecular Cables.

Multiple molecular wires braided together in a single assembly, termed as molecular cable, are promising next-generation materials for effective long-range charge transport. As an example of the platform for constructing molecular cables, 1,3,5-trifluorenylcyclohexane (TFC) and its difluorenyl analogues (DFCs) were systematically investigated both experimentally (X-ray crystallography) and theoretically (DFT calculations). Although the syntheses of DFCs were successfully achieved, the synthesis of TFC, which involved a similar intramolecular Friedel-Crafts cyclization as the last step, was unsuccessful. An exhaustive study of the conformational landscape of cyclohexane ring of TFC and DFCs revealed that TFC is a moderately strained molecule (∼17 kcal/mol), and computational studies of the reaction profile show that this steric strain, present in the transition state, is responsible for the unusually high (∼5 years) reaction half-life. A successful synthesis of TFC will require that the steric strain is introduced in multiple steps, and such alternative strategies are being currently explored.

A search for blues brothers: X-ray crystallographic/spectroscopic characterization of the tetraarylbenzidine cation radical as a product of aging of solid magic blue.

Magic blue (MB+˙ SbCl6− salt), i.e. tris-4-bromophenylamminium cation radical, is a routinely employed one-electron oxidant that slowly decomposes in the solid state upon storage to form so called 'blues brothers', which often complicate the quantitative analyses of the oxidation processes. Herein, we disclose the identity of the main 'blues brother' as the cation radical and dication of tetrakis-(4-bromophenyl)benzidine (TAB) by a combined DFT and experimental approach, including isolation of TAB+˙ SbCl6− and its X-ray crystallography characterization. The formation of TAB in aged magic blue samples occurs by a Scholl-type coupling of a pair of MB followed by a loss of molecular bromine. The recognition of this fact led us to the rational design and synthesis of tris(2-bromo-4-tert-butylphenyl)amine, referred to as 'blues cousin' (BC: Eox1 = 0.78 V vs. Fc/Fc+, λmax(BC+˙) = 805 nm, εmax = 9930 cm−1 M−1), whose oxidative dimerization is significantly hampered by positioning the sterically demanding tert-butyl groups at the para-positions of the aryl rings. A ready two-step synthesis of BC from triphenylamine and the high stability of its cation radical (BC+˙) promise that BC will serve as a ready replacement for MB and an oxidant of choice for mechanistic investigations of one-electron transfer processes in organic, inorganic, and organometallic transformations.