Profile of Semiquinone Radicals, Phytohormones and Sugars in Pistacia vera L. cv. Kirmizi Development

The main aim of this study was to investigate metabolic responses of fruits, leaves, and shoots of pistachio trees (Pistacia vera L. cv. Kirmizi) during their development. Electron paramagnetic resonance spectroscopy revealed significant increase in generation of semiquinone radicals in fruits and leaves of pistachio, while the flower cluster thinning application was conducted in relation to the control; especially at the second term of the plant material collection. Moreover, flower abscission caused an increase in the level of phytohormones such as indole-3-acetic acid and abscisic acid in fruits at the first term of fruits sampling. In turn, high-performance liquid chromatography analysis revealed differences both in the profile as well as the contents of soluble sugars detected in pistachio organs. The highest total sugar content was found in fruits of pistachio where the flower cluster thinning application was made early. Total sugar levels were higher also in leaves and shoots of the above-mentioned pistachio plants in relation to the control until the third sampling time. The importance of high levels of fructose in the fruits following the flower abscission was observed. Additionally, analyses of mineral elements in organs showed that copper and phosphorus contents in fruits were higher after the flower abscission in relation to the control. To conclude, our findings signal on contribution of semiquinone radicals, paramagnetic manganese ions, phytohormones, nutrients, and sugars in pistachio organs development on the background of the flower cluster thinning which was applied before fructification.

Charge density studies of multicentre two-electron bonding of an anion radical at non-ambient temperature and pressure

The variation of charge density of two-electron multicentre bonding (pancake bonding) between semiquinone radicals with pressure and temperature was studied on a salt of 5,6-dichloro-2,3-dicyanosemiquinone radical anion (DDQ) with 4-cyano-N-methylpyridinium cation (4-CN) using the Transferable Aspheric Atom Model (TAAM) refinement. The pancake-bonded radical dimers are stacked by non-bonding π-interactions. With rising pressure, the covalent character of interactions between radicals increases, and above 2.55 GPa, the electron density indicates multicentric covalent interactions throughout the stack. The experimental charge densities were verified and corroborated by periodic DFT computations. The TAAM approach has been tested and validated for atomic resolution data measured at ambient pressure; this work shows this approach can also be applied to diffraction data obtained at pressures up to several gigapascals.

Differential Roles of a Family of Flavodoxin-Like Proteins That Promote Resistance to Quinone Mediated Oxidative Stress in Candida albicans

Survival of the fungal pathogen Candida albicans within a mammalian host relies on its ability to resist oxidative stress. The four flavodoxin-like proteins (PstI, PstII, Pst3, and Ycp4) that reside on the inner surface of the C. albicans plasma membrane represent a recently discovered antioxidant mechanism that is essential for virulence. Flavodoxin-like proteins combat oxidative stress by promoting a two-electron reduction of quinone molecules, which prevents formation of toxic semiquinone radicals. Previous studies indicated that Pst3 played a major role in promoting resistance to the small quinone molecules p-benzoquinone and menadione. Analysis of additional quinones confirmed this role for Pst3. To better define their function, antibodies were raised against each of the four flavodoxin-like proteins and used to quantify protein levels. Interestingly, the basal level of flavodoxin-like proteins differed, with Pst3 and Ycp4 being the most abundant. However, after induction with p-benzoquinone, PstI and Pst3 were the most highly induced, resulting in Pst3 becoming the most abundant. Constitutive expression of the flavodoxin-like protein genes from a TDH3 promoter resulted in similar protein levels and showed that PstI and Pst3 were better at protecting C. albicans against p-benzoquinone than PstII or Ycp4. In contrast, PstI and Ycp4 provided better protection against oxidative damage induced by tert-butyl hydroperoxide. Thus, both the functional properties and the relative abundance contribute to the distinct roles of the flavodoxin-like proteins in resisting oxidative stress. These results further define how C. albicans combats the host immune response and survives in an environment rich in oxidative stress.

o-Semiquinone radical anion isolated as an amorphous porous solid

The use of metal cations is a commonly applied strategy to create S > ½ stable molecular systems containing semiquinone radicals. Persistent mono-semiquinonato complexes of diamagnetic metal ions (S =...

A Bio-Inspired Lanthanum-Ortho Quinone Catalyst for Aerobic Oxidation of Alcohol

<p>Oxidation reactions are fundamental transformations in organic synthesis and chemical industry. With oxygen or air as terminal oxidant, aerobic oxidation catalysis provides the most sustainable and economic oxidation processes. Most aerobic oxidation catalysis employs redox metal as its active center. While nature provides non-redox metal strategy as in pyrroloquinoline quinone (PQQ)-dependent methanol dehydrogenases (MDH), such an effective chemical version is unknown. Inspired by the recently discovered rare earth metal-dependent enzyme Ln-MDH, here we show that an open-shell semi-quinone anionic radical species in complexing with lanthanum could serve as a very efficient aerobic oxidation catalyst under ambient conditions. In this catalyst, the lanthanum metal serves only as a Lewis acid promoter and the redox process occurs exclusively on the semiquinone ligand. The catalysis is initiated by 1e^--reduction of lanthanum-activated <i>ortho</i>-quinone to a semiquinone-lanthanum complex La(<b>SQ^-.</b>)₂, which undergoes a coupled O-H/C-H dehydrogenation for aerobic oxidation of alcohols with up to 330 h^-1 TOF. This study suggests a possible functional mode of semiquinone radicals, widely observed with quinoproteins in Nature. Moreover, this unique reductive activation strategy as well as the resulted radical anion as redox ligand creates a new turning point in the development of efficient aerobic oxidation catalysis.</p>

A Bio-Inspired Lanthanum-Ortho Quinone Catalyst for Aerobic Oxidation of Alcohol

<p>Oxidation reactions are fundamental transformations in organic synthesis and chemical industry. With oxygen or air as terminal oxidant, aerobic oxidation catalysis provides the most sustainable and economic oxidation processes. Most aerobic oxidation catalysis employs redox metal as its active center. While nature provides non-redox metal strategy as in pyrroloquinoline quinone (PQQ)-dependent methanol dehydrogenases (MDH), such an effective chemical version is unknown. Inspired by the recently discovered rare earth metal-dependent enzyme Ln-MDH, here we show that an open-shell semi-quinone anionic radical species in complexing with lanthanum could serve as a very efficient aerobic oxidation catalyst under ambient conditions. In this catalyst, the lanthanum metal serves only as a Lewis acid promoter and the redox process occurs exclusively on the semiquinone ligand. The catalysis is initiated by 1e^--reduction of lanthanum-activated <i>ortho</i>-quinone to a semiquinone-lanthanum complex La(<b>SQ^-.</b>)₂, which undergoes a coupled O-H/C-H dehydrogenation for aerobic oxidation of alcohols with up to 330 h^-1 TOF. This study suggests a possible functional mode of semiquinone radicals, widely observed with quinoproteins in Nature. Moreover, this unique reductive activation strategy as well as the resulted radical anion as redox ligand creates a new turning point in the development of efficient aerobic oxidation catalysis.</p>

Pancake-bonding of semiquinone radicals under variable temperature and pressure conditions

The effects of temperature (100–370 K) and pressure (0–6 GPa) on the non-localized two-electron multicentric covalent bonds (`pancake bonding') in closely bound radical dimers were studied using single-crystal X-ray diffraction on a 4-cyano-N-methylpyridinium salt of 5,6-dichloro-2,3-dicyanosemiquinone radical anion (DDQ) as the sample compound. On cooling, the anisotropic structural compression was accompanied by continuous changes in molecular stacking; the discontinuities in the changes in volume and b and c cell parameters suggest that a phase transition occurs between 210 and 240 K. At a pressure of 2.55 GPa, distances between radical dimers shortened to 2.9 Å, which corresponds to distances observed in extended π-bonded polymers. Increasing pressure further to 6 GPa reduced the interplanar separation of the radicals to 2.75 Å. This may indicate that the covalent component of the interaction significantly increased, in accordance with the results of DFT calculations reported elsewhere [Molčanov et al. (2019), Cryst. Growth Des. 19, 391–402].

Strong antiferromagnetic coupling of the cobalt(ii)–semiquinone radical in a dinuclear complex with 2,2′-bipyrimidine ligands

A new dinuclear cobalt(ii)–semiquinone radical complex was synthesised and displayed strong antiferromagnetic exchange coupling, having −90.25 cm−1 of a J value, between the cobalt(ii) centres and semiquinone radicals.