Isothiocyanate conjugates, preparation

The preparation and reactions of V-thiocarbonyl carbohydrate derivatives, such as sugar isothiocyanates, thioamides, thioureas, thiocarbamates and their conjugates have been the subject of valuable comprehensive as well as specialized accounts, which should be consulted for details.198... 

These compounds are readily prepared by the reaction of fluorescein isothiocyanate with the polypeptide in a DMSO/water mixture and fluorescein-polyacrylate conjugate (6) is prepared by the reaction of fluoresceinamine with polyacrylic acid by using the mixed anhydride method [150]. In all cases, the absorption and emission maxima were similar at 490 and 525 nm, respectively, and reflect the photophysical properties of fluorescein itself. 

Scheme 11.4. Preparation of PNA-neomycin conjugates.Reaction conditions (a) (i) neomycin isothiocyanate, pyridine, cat. DMAP (ii) IM HCPdioxane, 1,2-ethanedithiol.

More often, carbamates of dextran are prepared by one-step conversion of the polysaccharide with isocyanates or isothiocyanates. This path is especially useful for the conjugation of fluorescent dyes. Fluorescent-marked dextrans are commercial products today. Fluorescent dextran derivatives with different molecular weights and substituents are available as invaluable tracers for studies on microcirculation and vascular permeability in health and disease [8]. Fluorescein isothiocyanate (FITC) dextran (actually the thiocarba-mate) has been well established in this area of research since the 1970s [358]. The isothiocyanate of fluorescein is covalently bound to dextran leading to... 

An alternative method for the synthesis of different dextran conjugates with fluorescent dyes is the reaction of a water-soluble amino dextran with the succinimidyl ester of the corresponding dye [360]. The amide linkage is more stable than the corresponding thiocarbamate formed with isothiocyanates. This method is displayed schematically in Fig. 54 for the preparation of 5- (and 6-) carboxynaphthofluorescein-labelled dextran from amino dextran (Fig. 47, [361,362]). 

A fluorescent-labelled conjugate has been prepared by reaction of fluorescein isothiocyanate with the oestradiol 6-carboxymethyloxime-BSA conjugate,for use in the fluorescence-immunoassay of oestradiol. 

The reaction of isothiocyanates with thiols is, in general, impractical for the preparation of dithiocarbamates. since A -monosubstitutcd derivatives readily decompose into the starting materials. Nevertheless, the method is synthetically useful in the case of aryl isothiocyanates. Thus, cellulose-aryl isothiocyanatc conjugates exhibited a substantial thiol binding capacity which was absent for nonaromatic analogs. 42 143... 

Fluorescent dye-polypeptide conjugates were prepared by labeling with either fluorescein isothiocyanate adsorbed onto Celite (51) or with 1-dimethylaminonaphthalene sulfonyl chloride (DNS) dissolved in ethanol (65). The degree of labeling was determined from the amount of dye on the polypeptide estimated by fluorescent intensity or ultraviolet absorption measurements and from the concentration of the polypeptide determined by Kjeldahl nitrogen. There were generally two to four dye residues per polypeptide molecule. 

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