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. Red fluorescent proteins (RFPs) have powered bioimaging advances due to their long-wavelength emissions and reduced phototoxicity. We present a comprehensive review of major categories of RFPs including the photoconvertible, photoswitchable, and large Stokes shift (LSS) FPs that achieve red emissions. We critically correlate the available crystallographic and spectroscopic results for ultrafast and functional structural dynamics insights into their redding mechanisms.+image

Automated summary

Red fluorescent proteins (RFPs) are a popular class of genetically encodable bioprobes and biomarkers that can advance breakthroughs in imaging and life sciences. Understanding their working mechanisms requires suitable spectroscopic techniques to delineate key structural features and dynamic steps. Progress has been made in designing RFPs with improved functions and versatility.