Quantum-mechanical insights into the anisotropic response of the cryptochrome radical pair to a weak magnetic field.

Cryptochrome photoreceptors contain a photochemically generated radical pair, which is thought to mediate sensing of the geomagnetic field direction in many living organisms. To gain insight into the response of the cryptochrome to a weak magnetic field, we have studied the quantum-mechanical hyperfine spin states of the radical pair. We identify quantum states responsible for the precise detection of the magnetic field direction, taking into account the strongly axial hyperfine interactions of each radical in the radical pair. The contribution of these states to the formation of the cryptochrome signaling state sharply increases when the magnetic field becomes orthogonal to the hyperfine axis of either radical. Due to such a response, the radical pair may be able to detect the particular field direction normal to the plane containing the hyperfine axes of the radicals.

Rational Control of Off-State Heterogeneity in a Photoswitchable Fluorescent Protein Provides Switching Contrast Enhancement.

Reversibly photoswitchable fluorescent proteins are essential markers for advanced biological imaging, and optimization of their photophysical properties underlies improved performance and novel applications. Here we establish a link between photoswitching contrast, one of the key parameters that dictate the achievable resolution in nanoscopy applications, and chromophore conformation in the non-fluorescent state of rsEGFP2, a widely employed label in REversible Saturable OpticaL Fluorescence Transitions (RESOLFT) microscopy. Upon illumination, the cis chromophore of rsEGFP2 isomerizes to two distinct off-state conformations, trans1 and trans2, located on either side of the V151 side chain. Reducing or enlarging the side chain at this position (V151A and V151L variants) leads to single off-state conformations that exhibit higher and lower switching contrast, respectively, compared to the rsEGFP2 parent. The combination of structural information obtained by serial femtosecond crystallography with high-level quantum chemical calculations and with spectroscopic and photophysical data determined in vitro suggests that the changes in switching contrast arise from blue- and red-shifts of the absorption bands associated to trans1 and trans2, respectively. Thus, due to elimination of trans2, the V151A variants of rsEGFP2 and its superfolding variant rsFolder2 display a more than two-fold higher switching contrast than their respective parent proteins, both in vitro and in E. coli cells. The application of the rsFolder2-V151A variant is demonstrated in RESOLFT nanoscopy. Our study rationalizes the connection between structural and photophysical chromophore properties and suggests a means to rationally improve fluorescent proteins for nanoscopy applications.

Anions of xenon clusters bound by long-range electron correlations.

In contrast with the single atom, atomic van der Waals clusters can form stable anions where the excess electron is bound due to long-range correlations with the electrons of the cluster. We report on extensive all-electron many-body ab initio studies on Xe clusters. Three-dimensional, planar, and linear structures of the clusters are investigated and compared. In particular, we find that the minimal number of Xe atoms in the cluster required to form a stable anion is 5 independently of the dimensionality of the cluster. We provide electron affinities for clusters made of 5, 6, and 7 atoms in all dimensions and find that the planar clusters form the most stable anions. The Dyson orbitals of the excess electrons are computed and analyzed.

Quantum states of magnetically induced anions.

In a magnetic field, an atom (or molecule) can attach an extra electron to form an unconventional anionic state which has no counterparts in field-free space. Assuming the atom to be infinitely heavy, these magnetically induced anionic states are known to constitute an infinite manifold of bound states. In reality, the species can move and its motion across the magnetic field couples to the motion of the attached electron. We treat this coupling, for the first time, quantum mechanically, and show that it makes the number of bound anionic states finite. Explicit numerical quantum results are presented and discussed.