EPR optical detection of F centre pairs in alkali halides. - I : Pumping cycle kinetics and characteristics of the resonances

The EPR of F centres in the ground and excited states was optically detected in the following alkali halide crystals: NaCl, KF, KCl, KBr, KI, RbBr, and RbI. A decrease of the radiative quantum efficiency of the F centre luminescence was observed when microwave transitions were induced between the spin levels. The mechanism responsible for this effect was an electronic tunnelling through the crystal field potential; the electron in the relaxed excited state of an F centre (F~*) is transferred nonradiatively to another nearby F centre in its ground state (F0), and leads to the momentary formation of an ƒ¿ and an F\u27 centre. Such a process is a function of the total spin of the F~*-F0 pair. The role played by the paired centres was confirmed by measurements at different F centre concentration. Moreover, at high optical excitation pumping rates, the population of the intermediate complexes (F\u27-ƒ¿) is large enough to allow an estimation of the rate of the reverse process F\u27 + ƒ¿ \u27¨ F0 + F0

Highly potent bispecific sybodies neutralize SARS-CoV-2

The ongoing COVID-19 pandemic represents an unprecedented global health crisis. Here, we report the identification of a synthetic nanobody (sybody) pair (Sb#15 and Sb#68) that can bind simultaneously to the SARS-CoV-2 spike-RBD and efficiently neutralize pseudotyped and live-viruses by interfering with ACE2 interaction. Two spatially-discrete epitopes identified by cryo-EM translated into the rational design of bispecific and tri-bispecific fusions constructs, exhibiting up to 100- and 1000-fold increase in neutralization potency. Cryo-EM of the sybody-spike complex further revealed a novel up-out RBD conformation. While resistant viruses emerged rapidly in the presence of single binders, no escape variants were observed in presence of the bispecific sybody. The multivalent bispecific constructs further increased the neutralization potency against globally-circulating SARS-CoV-2 variants of concern. Our study illustrates the power of multivalency and biparatopic nanobody fusions for the development of clinically relevant therapeutic strategies that mitigate the emergence of new SARS-CoV-2 escape mutants

EPR optical detection of F centre pairs in alkali halides. - II : Spin-lattice relaxation of the F centre in its relaxed excited state

Like the EPR line of the fundamental state of the F centre in most of the alkali halide crystals, the line attributed to the relaxed excited state, detected optically for F-centre pairs is also Gaussian in shape. That is explained by an inhomogeneous broadening due to hyperfine interactions with the surrounding nuclei. The measurements of these resonances as a function of the microwave power allowed one to determine the saturation parameter and the broadening of the elementary line (homogeneous). On the other hand the measurement of the resonance signal as a function of temperature, the other parameters remaining constant, shows that it stays constant for T < 10°K and decreased for increasing temperature. This is attributed to the spin-lattice relaxation time T1 of the defect in its relaxed excited state. The relaxation mechanism is characterized by an Orbach process

Electron transfer by the tunnel effect and its influence on the F center luminescence in alkali halides

Concentration quenching plays an important role in luminescence phenomena, and is generally accounted for by pair interactions. In alkali halides containing F centers, whenever a member of an F center pair is optically excited at low temperature, it can return to its ground state either radiatively or by the formation of an intermediate F' center. In this case, the excited electron is transferred by fast tunnelling to a neighboring center. This process depends on the spin symmetry of the pair, which is determined by the inhomogeneity of the hyperfine field. Effects of applied magnetic field, spin lattice relaxation and spin-spin interaction on the tunnelling probability are investigated. Experimental confirmation of the theory is presented for the case of KCl