Jasper H. M. van der Velde

Mechanism of Intramolecular Photostabilization in Self‐Healing Cyanine Fluorophores

Organic fluorophores, which are popular labels for microscopy applications, intrinsically suffer from transient and irreversible excursions to dark-states. An alternative to adding photostabilizers at high concentrations to the imaging buffer relies on the direct linkage to the fluorophore. However, the working principles of this approach are not yet fully understood. In this contribution, we investigate the mechanism of intramolecular photostabilization in self-healing cyanines, in which photodamage is automatically repaired. Experimental evidence is provided to demonstrate that a single photostabilizer, that is, the vitamin E derivative Trolox, efficiently heals the cyanine fluorophore Cy5 in the absence of any photostabilizers in solution. A plausible mechanism is that Trolox interacts with the fluorophore through intramolecular quenching of triplet-related dark-states, which is a mechanism that appears to be common for both triplet-state quenchers (cyclooctatetraene) and redox-active compounds (Trolox, ascorbic acid, methylviologen). Additionally, the influence of solution-additives, such as cysteamine and procatechuic acid, on the self-healing process are studied. The results suggest the potential applicability of self-healing fluorophores in stochastic optical reconstruction microscopy (STORM) with optical super-resolution. The presented data contributes to an improved understanding of the mechanism involved in intramolecular photostabilization and has high relevance for the future development of self-healing fluorophores, including their applications in various research fields.

A simple and versatile design concept for fluorophore derivatives with intramolecular photostabilization

Intramolecular photostabilization via triple-state quenching was recently revived as a tool to impart synthetic organic fluorophores with ‘self-healing’ properties. To date, utilization of such fluorophore derivatives is rare due to their elaborate multi-step synthesis. Here we present a general strategy to covalently link a synthetic organic fluorophore simultaneously to a photostabilizer and biomolecular target via unnatural amino acids. The modular approach uses commercially available starting materials and simple chemical transformations. The resulting photostabilizer–dye conjugates are based on rhodamines, carbopyronines and cyanines with excellent photophysical properties, that is, high photostability and minimal signal fluctuations. Their versatile use is demonstrated by single-step labelling of DNA, antibodies and proteins, as well as applications in single-molecule and super-resolution fluorescence microscopy. We are convinced that the presented scaffolding strategy and the improved characteristics of the conjugates in applications will trigger the broader use of intramolecular photostabilization and help to emerge this approach as a new gold standard.

On the impact of competing intra- and intermolecular triplet-state quenching on photobleaching and photoswitching kinetics of organic fluorophores

While buffer cocktails remain the gold-standard for photostabilization and photoswitching of fluorescent markers, intramolecular triplet-state quenchers emerge as an alternative strategy to impart fluorophores with ‘self-healing’ or even functional properties such as photoswitching. In this contribution, we evaluated various combinations of both approaches and show that inter- and intramolecular triplet-state quenching processes compete with each other rather than being additive or even synergistic. Often intramolecular processes dominate the photophysical situation for combinations of covalently-linked and solution-based photostabilizers and photoswitching agents. In this context we identified a new function of intramolecular photostabilizers, i.e., protection of fluorophores from reversible off-switching events caused by solution-additives, which were previously misinterpreted as photobleaching. Our studies also provide practical guidance for usage of photostabilizer-dye conjugates for STORM-type super-resolution microscopy permitting the exploitation of their improved photophysics for increased spatio-temporal resolution. Finally, we provide evidence that the biochemical environment, e.g., proximity of aromatic amino-acids such as tryptophan, reduces the photostabilization efficiency of commonly used buffer cocktails. Not only have our results important implications for a deeper mechanistic understanding of self-healing dyes, but they will provide a general framework to select label positions for optimal and reproducible photostability or photoswitching kinetics.

Self-healing dyes for super-resolution microscopy

In recent years optical microscopy techniques have emerged that allow optical imaging at unprecented resolution beyond the diffraction limit. Up to date, photostabilizing buffers are the method of choice to realize either photoswitching and/or to enhance the signal brightness and stability of the employed fluorescent probes. This strategy has, however, restricted applicability and is not suitable for live cell imaging. In this paper, we tested the performance of self-healing organic fluorophores with intramolecular photostabilization in super-resolution microscopy with targeted (STED) and stochastic readout (STORM). The overall goal of the study was to improve the spatial and temporal resolution of both techniques without the need for mixtures of photostabilizing agents in the imaging buffer. Due to its past superior performance we identified ATTO647N-photostabilizer conjugates as suitable candidates for STED microscopy. We characterize the photostability and resulting performance of NPA-ATTO647N oligonucleotide conjugates in STED microscopy. We find that the superior photophysical performance results in optimal STED imaging and demonstrate the possibility to obtain single-molecule fluorescent transients of individual fluorophores while illuminating with both the excitation- and STED-laser. Secondly, we show an analysis of photoswitching kinetics of self-healing Cy5 dyes (comprising TX, COT and NPA stabilizers) in the presence of TCEP- and cysteamine, which are typically used in STORM microscopy. In line with previous work, we find that intramolecular photostabilization strongly influences photoswitching kinetics and requires careful attention when designing STORM-experiments. In summary, this contribution explores the possibilities and limitations of self-healing dyes in super-resolution microscopy of differing modalities.

Author Correction: A simple and versatile design concept for fluorophore derivatives with intramolecular photostabilization.

This corrects the article DOI: 10.1038/ncomms10144.

Corrigendum: A simple and versatile design concept for fluorophore derivatives with intramolecular photostabilization

This corrects the article DOI: 10.1038/ncomms10144.