We examine the effect of temperature on three viscosity-sensitive fluorophores termed ‘molecular rotors’. In the case of the conjugated porphyrin dimer, it can be used for measuring both viscosity and temperature concurrently.

Oxidation of cellular structures is typically an undesirable process that can be a hallmark of certain diseases. On the other hand, photooxidation is a necessary step of photodynamic therapy (PDT), a cancer treatment causing cell death upon light irradiation. Here, the effect of photooxidation on the microscopic viscosity of model lipid bilayers constructed of 1,2‐dioleoyl‐sn‐glycero‐3‐phosphocholine has been studied. A molecular rotor has been employed that displays a viscosity‐dependent fluorescence lifetime as a quantitative probe of the bilayer's viscosity. Thus, spatially‐resolved viscosity maps of lipid photooxidation in giant unilamellar vesicles (GUVs) were obtained, testing the effect of the positioning of the oxidant relative to the rotor in the bilayer. It was found that PDT has a strong impact on viscoelastic properties of lipid bilayers, which ‘travels’ through the bilayer to areas that have not been irradiated directly. A dramatic difference in viscoelastic properties of oxidized GUVs by Type I (electron transfer) and Type II (singlet oxygen‐based) photosensitisers was also detected.

SummaryCentrioles are small barrel-shaped structures that form centrosomes and cilia [1]. Centrioles assemble around a central cartwheel comprising the Sas-6 and Ana2/STIL proteins. The amino termini of nine Sas-6 dimers form a central hub of ∼12 nm radius from which nine dimer spokes radiate, placing the Sas-6 carboxyl termini at the outer edge of the ∼60 nm radius cartwheel [2]. Several centriole proteins are distributed in a toroid around the cartwheel, and super-resolution light microscopy studies have measured the average radii of these ∼100–200 nm radius toroids with a ‘precision’ — or standard deviation (s.d. or 1σ) — of ±∼10–40 nm. The organization of Ana2/STIL within the cartwheel, however, has not been resolvable. Here, we develop methods to calculate the average toroidal radius of centriolar proteins in the ∼20–60 nm range with a s.d. of just ±∼4–5 nm, revealing that the amino and carboxyl termini of Ana2 are located in the outer cartwheel region.

Conjugated porphyrin dimers have emerged as versatile viscosity‐sensitive fluorophores that are suitable for quantitative measurements of microscopic viscosity by ratiometric and fluorescence lifetime‐based methods, in a concentration‐independent manner. Here, we investigate the effect of extended conjugation in a porphyrin‐dimer structure on their ability to sense viscosity and temperature. We show that the sensitivity of the fluorescence lifetime to temperature is a unique property of only a few porphyrin dimers.