Photostable, amino reactive and water-soluble fluorescent labels based on sulfonated rhodamine with a rigidized xanthene fragment

Highly water soluble fluorescent dyes were synthesized and transformed into new amino reactive fluorescent labels for biological microscopy. To this end, rhodamine 8 (prepared from 7-hydroxy-1,2,3,4-tetrahydroquinoline (7) and phthalic anhydride in 85% aq. H(3)PO(4)) was sulfonated with 30% SO(3) in H(2)SO(4) and afforded the water soluble disulfonic acid 3a (64%). Amidation of the carboxy group in 3a with 2-(methylamino)ethanol in the presence of O-(7-azabenzotriazol-1-yl)- N,N,N',N'-tetramethyluronium-PF(6)-(HATU) led to alcohol 3b (66%) which was transformed into the amino reactive mixed carbonate 3d with di(N-succinimidyl)carbonate and Et(3)N. Reaction of the carboxy group in 3a with MeNH(CH(2))(2)CO(2)Me and N,N,N',N'-tetramethyl-O-(N-succinimidyl)-uronium center dot BF(4)(-) (TSTU) yielded methyl ester 13. After saponification of the aliphatic carboxy group in 13, the compound was converted into NHS-ester 3e (using HATU and Et(3)N). Heating of 7 with trimellitic anhydride in H(3)PO(4) gave a mixture of dicarboxylic acids 14 and 15 (1:1). Regioisomer 15 was isolated, sulfonated with 30% SO(3) in H(2)SO(4), and disulfonic acid 3f was used for the synthesis of the mono NHS-ester 3g, in which the sterically unhindered carboxy group was selectively activated (with N-hydroxysuccinimide, HATU, and Et(3)N). The sulfonated rhodamines 3b, c and f are soluble in water (up to 0.1 M), have excellent photostabilities and large fluorescence quantum yields. Subdiffraction resolution images of tubulin filaments of mammalian cells stained with these dyes illustrate their applicability as labels for stimulated emission depletion microscopy and other fluorescence techniques.