Rhodamines with a Chloronicotinic Acid Fragment for Live Cell Superresolution STED Microscopy

Formylation of 2,6‐dichloro‐5‐R‐nicotinic acids at C‐4 followed by condensation with 3‐hydroxy‐N,N‐dimethylaniline gave analogs of the popular TAMRA fluorescent dye with a 2,6‐dichloro‐5‐R‐nicotinic acid residues (R=H, F). The following reaction with thioglycolic acid is selective, involves only one chlorine atom at the carbon between pyridine nitrogen and the carboxylic acid group and affords new rhodamine dyes absorbing at 564/ 573 nm and emitting at 584/ 597 nm (R=H/ F, in aq. PBS). Conjugates of the dyes with “small molecules” provided specific labeling (covalent and non‐covalent) of organelles as well as of components of the cytoskeleton in living cells and were combined with fluorescent probes prepared from 610CP and SiR dyes and applied in two‐color STED microscopy with a 775 nm STED laser. Tetramethyl Rhodamine (TMR) is out of fashion? Fluorescent TMR‐clones containing a 6‐chloronicotinic acid residue were synthesized, attached to ligands and applied for selective and vital labelling of intracellular targets. Superresolution STED microscopy in one‐ and two‐colors demonstrated high optical resolution and target specificity of the good old TMR scaffold.