Enhancing the Viscosity-Sensitive Range of a BODIPY Molecular Rotor by Two Orders of Magnitude.

Molecular rotors are a class of fluorophores that enable convenient imaging of viscosity inside microscopic samples such as lipid vesicles or live cells. Currently, rotor compounds containing a boron-dipyrromethene (BODIPY) group are among the most promising viscosity probes. In this work, it is reported that by adding heavy-electron-withdrawing -NO2 groups, the viscosity-sensitive range of a BODIPY probe is drastically expanded from 5-1500 cP to 0.5-50 000 cP. The improved range makes it, to our knowledge, the first hydrophobic molecular rotor applicable not only at moderate viscosities but also for viscosity measurements in highly viscous samples. Furthermore, the photophysical mechanism of the BODIPY molecular rotors under study has been determined by performing quantum chemical calculations and transient absorption experiments. This mechanism demonstrates how BODIPY molecular rotors work in general, why the -NO2 group causes such an improvement, and why BODIPY molecular rotors suffer from undesirable sensitivity to temperature. Overall, besides reporting a viscosity probe with remarkable properties, the results obtained expand the general understanding of molecular rotors and show a way to use the knowledge of their molecular action mechanism for augmenting their viscosity-sensing properties.

Single-molecule microscopy studies of LHCII enriched in Vio or Zea.

Plants have developed multiple self-regulatory mechanisms to efficiently function under varying sunlight conditions. At high light intensities, non-photochemical quenching (NPQ) is activated on a molecular level, safely dissipating an excess excitation as heat. The exact molecular mechanism for NPQ is still under debate, but it is widely agreed that the direct participation of the carotenoid pigments is involved, one of the proposed candidate being the zeaxanthin. In this work, we performed fluorescence measurements of violaxanthin- and zeaxanthin-enriched major light-harvesting complexes (LHCII), in ensemble and at the single pigment-protein complex level, where aggregation is prevented by immobilization of LHCIIs onto a surface. We show that a selective enrichment of LHCII with violaxanthin or zeaxanthin affects neither the ability of LHCII to switch into a dissipative conformation nor the maximal level of induced quenching. However, the kinetics of the fluorescence decrease due to aggregation on the timescale of seconds are different, prompting towards a modulatory effect of zeaxanthin in the dynamics of quenching.