Beyond O6-Benzylguanine: O6-(5-Pyridylmethyl)guanine as a Substrate for the Self-Labeling Enzyme SNAP-Tag.

SNAP-tag is a genetically encoded label for tracking proteins of interest that is known to utilize O6-benzylguanine derivatives as substrates. In this work, mass spectrometric analysis revealed that SNAP-tag also accepts O6-(5-pyridylmethyl)guanine derivatives as substrates. A fluorescently conjugated O6-(5-pyridylmethyl)guanine was synthesized and used to selectively label intracellular compartments. This compound also acts as a zinc(II) ion-sensitive fluorescent indicator and therefore can be used in organelle-specific zinc(II) imaging.