The fluorescent protein aptly named "Killer Red" (KRed) is capable of killing transfected cells and inactivating fused proteins upon exposure to visible light in the presence of oxygen. We have investigated the source of the bioactive species through a variety of photophysical and photochemical techniques. Our results indicate a Type I (electron transfer mediated) photosensitizing mechanism.

Red fluorescent proteins (RFPs) are indispensable tools for deep-tissue imaging, fluorescence resonance energy transfer applications, and super-resolution microscopy. Using time-resolved optical spectroscopy this study investigated photoinduced dynamics of three RFPs, KillerRed, mRFP, and DsRed. In all three RFPs, a new transient absorption intermediate was observed, which decays on a microsecond–millisecond time scale. This intermediate is characterized by red-shifted absorption at 1.68–1.72 eV (λmax = 720–740 nm). On the basis of electronic structure calculations, experimental evidence, and published literature, the chemical nature of the intermediate is assigned to an unusual open-shell dianionic chromophore (dianion-radical) formed via photoreduction. A doubly charged state that is not stable in the isolated (gas phase) chromophore is stabilized by the electrostatic field of the protein. Mechanistic implications for photobleaching, blinking, and phototoxicity are discussed.

Blue fluorescent proteins (BFPs) offer visualization of protein location and behavior, but often suffer from high autofluorescent background and poor signal discrimination. Through dual-laser excitation of bright and photoinduced dark states, mutations to the residues surrounding the BFP chromophore enable long-wavelength optical modulation of BFP emission. Such dark state engineering enables violet-excited blue emission to be increased upon lower energy, green co-illumination. Turning this green co-illumination on and off at a specific frequency dynamically modulates collected blue fluorescence without generating additional background. Interpreted as transient photoconversion between neutral cis- and anionic trans- chromophoric forms, mutations tune photoisomerization and ground state tautomerizations to enable long-wavelength depopulation of the millisecond-lived, spectrally shifted dark states. Single mutations to the tyrosine-based blue fluorescent protein T203V/S205V exhibit enhanced modulation depth and varied frequency. Importantly, analogous single point mutations in the non-modulatable BFP, mKalama1, creates a modulatable variant. Building modulatable BFPs offers opportunities for improved BFP signal discrimination vs. background, greatly enhancing their utility.

The effect of pH on the spectral and redox properties of model GFP synthetic chromophores (Cros) in a universal buffer system was investigated. The Cro hydroxy-derivatives demonstrated a Nernst-type electrochemical dependence of the anodic potential within the pH 2-8 range. Analogous studies on various fluorescent proteins were unsuccessful.

Understanding the photoinduced dynamics of fluorescent proteins is essential for their applications in bioimaging. Despite numerous studies on the ultrafast dynamics, the delayed response of these proteins, which often results in population of kinetically trapped dark states of various origins, is largely unexplored. Here, by using transient absorption spectroscopy spanning the time scale from picoseconds to seconds, we reveal a hidden reactivity of the bright blue-light emitting protein mKalama1 previously thought to be inert. This protein shows no excited-state proton transfer during its nanosecond excited-state lifetime; however, its tyrosine-based chromophore undergoes deprotonation coupled to non-radiative electronic relaxation. Such deprotonation causes distinct optical absorption changes in the broad UV-to-NIR spectral range (ca. 300-800 nm); the disappearance of the transient absorption signal has a complex nature and spans the whole microsecond-to-second time scale. The mechanisms underlying the relaxation kinetics are disclosed based on the X-ray structural analysis of mKalama1 and the high-level electronic structure calculations of proposed intermediates in the photocycle. We conclude that the non-radiative excited-state decay includes two major branches: internal conversion coupled to intraprotein proton transfer, where a conserved residue E222 serves as the proton acceptor; and ionization induced by two consecutive resonant absorption events, followed by deprotonation of the chromophore radical cation to bulk solvent through a novel water-mediated proton-wire pathway. Our findings open up new perspectives on the dynamics of fluorescent proteins as tracked by its optical transient absorption in the time domain extending up to seconds.