Thienoguanosine, a unique non-perturbing reporter for investigating rotational dynamics of DNA duplexes and their complexes with proteins.

Time-resolved fluorescence anisotropy (TRFA) provides key information on the dynamics of biomolecules and their interaction with ligands. However, since natural nucleosides are almost non-fluorescent, its application to DNA duplexes (dsDNA) requires fluorescent labels, which can alter dsDNA stability, hinder protein binding, and complicate interpretation of TRFA experiments due to their local motion. As shown here, thienoguanosine (G), a fluorescent analogue of guanosine, overcomes all these limitations. We recorded the TRFA decays of G-labelled dsDNA of different lengths. G behaved as a rigid, non-perturbing reporter, since no fast correlation time was recorded for any tested dsDNA. Due to its perfect stacking, only two correlation times, instead of the typical three, describe G-labelled dsDNA rotational dynamics. Thanks to these features, we provided a complete description of the tumbling of the different dsDNA and their complexes with the Set and Ring Associated (SRA) domain of UHRF1, a key epigenetic regulator, obtaining values in excellent agreement with theoretical predictions. Moreover, G was also found sensitive to SRA-induced base flipping of neighboring nucleobases. In the DNA label toolbox, G thus stands out as a unique reporter for investigating the rotational dynamics of dsDNA and protein/dsDNA complexes.