Twisted cyanines: a non-planar fluorogenic dye with superior photostability and its use in a protein-based fluoromodule

J Am Chem Soc. 2013 Jan 9;135(1):242-51. doi: 10.1021/ja308629w. Epub 2012 Dec 20.

Abstract

The cyanine dye thiazole orange (TO) is a well-known fluorogenic stain for DNA and RNA, but this property precludes its use as an intracellular fluorescent probe for non-nucleic acid biomolecules. Further, as is the case with many cyanines, the dye suffers from low photostability. Here, we report the synthesis of a bridge-substituted version of TO named α-CN-TO, where the central methine hydrogen of TO is replaced by an electron withdrawing cyano group, which was expected to decrease the susceptibility of the dye toward singlet oxygen-mediated degradation. An X-ray crystal structure shows that α-CN-TO is twisted drastically out of plane, in contrast to TO, which crystallizes in the planar conformation. α-CN-TO retains the fluorogenic behavior of the parent dye TO in viscous glycerol/water solvent, but direct irradiation and indirect bleaching studies showed that α-CN-TO is essentially inert to visible light and singlet oxygen. In addition, the twisted conformation of α-CN-TO mitigates nonspecific binding and fluorescence activation by DNA and a previously selected TO-binding protein and exhibits low background fluorescence in HeLa cell culture. α-CN-TO was then used to select a new protein that binds and activates fluorescence from the dye. The new α-CN-TO/protein fluoromodule exhibits superior photostability to an analogous TO/protein fluoromodule. These properties indicate that α-CN-TO will be a useful fluorogenic dye in combination with specific RNA and protein binding partners for both in vitro and cell-based applications. More broadly, structural features that promote nonplanar conformations can provide an effective method for reducing nonspecific binding of cationic dyes to nucleic acids and other biomolecules.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Benzothiazoles / chemical synthesis
  • Benzothiazoles / chemistry*
  • Cells, Cultured
  • Crystallography, X-Ray
  • DNA / chemistry
  • Fluorescent Dyes / chemistry*
  • HeLa Cells
  • Humans
  • Models, Molecular
  • Molecular Structure
  • Photochemical Processes
  • Quinolines / chemical synthesis
  • Quinolines / chemistry*
  • Single-Chain Antibodies / chemistry*

Substances

  • Benzothiazoles
  • Fluorescent Dyes
  • Quinolines
  • Single-Chain Antibodies
  • thiazole orange
  • DNA