Cyanine dyes are now recognized to have widespread application as fluorescent probes which can be conjugated to a variety of biological molecules, thus finding applications in DNA sequencing, flow cytometry and immunoassays. Cyanine dyes are characterized by strong spectral absorption bands with the absorption being tunable over a large spectral range by synthetic design.
In addition to the spectral characteristics of a cyanine dye, a practical dye is characterized by also being stable during isolation and purification, yet be reactive for covalent bonding with a target molecule under conditions which will not degrade the target molecule. Prior art cyanine dyes having reactive groups capable of covalently bonding a target molecule have all been limited in their utility either by structural stability or the reaction conditions required for coupling of the dye and target molecules.
An iodo acetyl moiety attached to a cyanine dye is capable of covalently reacting with a sulfhydryl group of a target molecule. The iodo acetyl group is limited in its utility by the sulfhydryl groups being present in only a small class of proteins. Other reactive moieties have included sulfonic acid, carboxylates, and sulfonates (for example see, Anal. Biochem. 243, 15-27: 1996; Bioconj. Chem. 4, 105-111: 1993; 8, 751-756: 1997; Cytometry 10, 3-10: 1989; 10, 11-19: 1989; U.S. Pat. No. 5,106,990; and J. Chem. Soc. Perkin Transactions 1, 143-7: 1998). Such moieties remain reactive during the course of subsequent chemical transformations and purification or are limited by the reaction conditions for bonding to a target molecule.
U.S. Pat. No. 5,627,027 is directed to a method for labeling proteins, cells, nucleic acid and DNA with a cyanine dye, as well as the reaction therebetween. The cyanine dye specified is limited in the position of a reactive group within the dye and its identity is limited to isothiocyanate, isocyanate, monochlorotriazine, dichlorotriazine, mono- or di-halogen substituted pyridine, mono- or di-halogen substituted diazine, aziridine, sulfonyl halide, acid halide, hydroxy succinimide ester, hydroxy sulfosuccinimide ester, imido ester, glyoxal and aldehyde. The method still further requires forming a covalent bond between the reactive group on the dye, and an amine or hydroxyl group on the material being labeled utilizing an electrophilic mechanism for coupling the dye and material being labeled.
U.S. Pat. No. 5,486,616 is directed to particular cyanine dye structures which are water soluble, and capable of reacting with amino, hydroxy or sulfhydryl groups through a reactive moiety. The dye structures including at least one sulfonic acid or sulfonate moiety attached directly to a benzyl portion of the dye utilizing an electrophilic mechanism for coupling the dye and material being labeled.
U.S. Pat. No. 5,268,486 discloses water soluble dyes which contain reactive moieties including isothiocyanate, isocyanate, monochlorotriazine, dichlorotriazine, mono- or di-halogen substituted pyridine, mono- or di-halogen substituted diazine, maleimide, aziridine, sulfonyl halide, acid halide, hydroxysuccinimide ester, hydroxysulfosuccinimide ester, imido ester, hydrazine, azidonitrophenyl, azide, 3-(2-pyridyl dithio)-proprionamide, glyoxal and aldehyde. These dyes are chosen to label target molecules containing amino-, hydroxy- and sulfhydryl groups.
In view of the prior art, it is apparent that there exists a need for a wider range of indole derivatives which will allow the facile synthesis of stable cyanine dyes with suitably positioned reactive groups for covalent attachment to target molecules. The cyanine dyes should allow covalent bonding to target molecules under conditions which will not degrade the target molecules. They should be stable, easily formed, purified and reactive under conditions of attachment to target molecules. This invention describes the facile synthesis of new, stable indole precursors derived from indoles. These precursors allow the facile synthesis of cyanine dyes with protected thiol, amine or hydroxyl groups. The protected groups can be deprotected to yield free thiol, amine or hydroxyl groups which can be covalently attached to target molecules via a nucleophilic mechanism.
In view of the prior art, it is apparent that there exists a need for a cyanine dye having a more generic utility. A need exists for a cyanine dye which is water soluble and contains at least one reactive moiety capable of labeling a range of substances in addition to compounds containing amino, hydroxy and sulfhydryls through covalent bonding with target molecules under conditions which will not degrade a target molecule.
Prior coupling reactions have been carried out by an electrophilic mechanism. Thus, there further exists a need for a generic class of cyanine dyes which are capable of coupling to a target molecule by a nucleophilic mechanism.