Substituent effects on the photophysics of the kaede chromophore

Kaede is the prototype of the optical highlighter proteins, which are an important subclass of the fluorescent proteins that can be permanently switched from green to red emitting forms by UV irradiation. This transformation has important applications in bioimaging. Optimising brightness, i.e. enhancing fluorescence characteristics, in these proteins is an important objective. At room temperature, the excited state dynamics of the red form of the kaede chromophore are dominated by a broad distribution of conformers with distinct excited state kinetics. Here, we investigate substituent effects on the photophysics of this form of the kaede chromophore. While an electron withdrawing substituent (nitro) red shifts the electronic spectra, the modified chromophores showed no significant solvatochromism. The lack of solvatochromism suggests small changes in permanent dipole moment between ground and excited electronic states, which is consistent with quantum chemical calculations. Ultrafast fluorescence and transient absorption spectroscopy reveal correlations between radiative and nonradiative decay rates of different conformers in the chromophores. The most significant effect of the substituents is to modify the distribution of conformers. The results are discussed in the context of enhancing brightness of optical highlighter proteins.

Targeting Ultrafast Spectroscopic Insights into Red Fluorescent Proteins

Red fluorescent proteins (RFPs) represent an increasingly popular class of genetically encodable bioprobes and biomarkers that can advance next-generation breakthroughs across the imaging and life sciences. Since the rational design of RFPs with improved functions or enhanced versatility requires a mechanistic understanding of their working mechanisms, while fluorescence is intrinsically an ultrafast event, a suitable toolset involving steady-state and time-resolved spectroscopic techniques has become powerful in delineating key structural features and dynamic steps which govern irreversible photoconverting or reversible photoswitching RFPs, and large Stokes shift (LSS)RFPs. The pertinent cis-trans isomerization and protonation state change of RFP chromophores in their local environments, involving key residues in protein matrices, lead to rich and complicated spectral features across multiple timescales. In particular, ultrafast excited-state proton transfer in various LSSRFPs showcases the resolving power of wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS) in mapping a photocycle with crucial knowledge about the red-emitting species. Moreover, recent progress in noncanonical RFPs with a site-specifically modified chromophore provides an appealing route for efficient engineering of redder and brighter RFPs, highly desirable for bioimaging. Such an effective feedback loop involving physical chemists, protein engineers, and biomedical microscopists will enable future successes to expand fundamental knowledge and improve human health.

Photophysics of the red-form Kaede chromophore

The chromophore responsible for colour switching in the optical highlighting protein Kaede has unexpectedly complicated excited state dynamics, which are measured and analysed here. This will inform the development of new imaging proteins.