Dalluge, J. J. | bioRxiv Science

bioRxiv2024

Red Light Mediated Photo-Conversion of Silicon Rhodamines to Oxygen Rhodamines for Single-Molecule Microscopy

The rhodamine motif has been modified in myriad ways to produce probes with specific fluorescent and chemical properties optimal for a variety of microscopy experiments. Recently, far-red emitting silicon rhodamines have become popular in single-molecule localization microscopy (SMLM), since these dyes are membrane-permeable and can be used alongside red fluorophores for two-color imaging. While this has expanded multi-color SMLM imaging capabilities, we demonstrate that silicon rhodamines can create previously unreported photoproducts with significantly blueshifted emissions, which appear as bright single-molecule crosstalk in the red emission channel. We show that this fluorescence is caused by the replacement of the central silicon group with oxygen after 640 nm illumination, turning far-red silicon rhodamines (JFX650, JF669, etc.) into their red oxygen rhodamine counterparts (JFX554, JF571, etc.). While this blueshifted population can cause artifacts in two-color SMLM data, we demonstrate up to 16-fold reduction in crosstalk using oxygen-scavenging systems. We also leverage this far-red photoconversion to demonstrate UV-free photoactivated localization microscopy (PALM) without the need for additives, and with 5-fold higher efficiency than the Cy5 to Cy3 conversion. Finally, we demonstrate multiplexed pseudo two-color PALM in a single emission channel by separating localizations by their photo-activation wavelengths instead of their emission wavelengths. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=82 SRC="FIGDIR/small/595223v1_ufig1.gif" ALT="Figure 1"> View larger version (32K): org.highwire.dtl.DTLVardef@141fe66org.highwire.dtl.DTLVardef@1a4f9f5org.highwire.dtl.DTLVardef@16605dorg.highwire.dtl.DTLVardef@1109255_HPS_FORMAT_FIGEXP M_FIG C_FIG

biophysics↗

bioRxiv2023

Electrostatically regulated active site assembly governs reactivity in non-heme iron halogenases

Non-heme iron halogenases (NHFe-Hals) catalyze the direct insertion of a chloride/bromide ion at an unactivated carbon position using a high-valent haloferryl intermediate. Despite more than a decade of structural and mechanistic characterization, how NHFe-Hals preferentially bind specific anions and substrates for C-H functionalization remains unknown. Herein, using lysine halogenating BesD and HalB enzymes as model systems, we demonstrate strong positive cooperativity between anion and substrate binding to the catalytic pocket. Detailed computational investigations indicate that a negatively charged glutamate hydrogen-bonded to irons equatorial-aqua ligand acts as an electrostatic lock preventing both lysine and anion binding in the absence of the other. Using a combination of UV-Vis spectroscopy, binding affinity studies, stopped-flow kinetics investigations, and biochemical assays, we explore the implication of such active site assembly towards chlorination, bromination, and azidation reactivities. Overall, our work highlights previously unknown features regarding how anion-substrate pair binding govern reactivity of iron halogenases that are crucial for engineering next-generation C-H functionalization biocatalysts.

biochemistry↗