A series of mono-reactive, water-soluble, indodicarbocyanine dyes based on 2,3-dimethyl-3-(4-sulfobutyl)-3H-indole-5-sulfonic acid was synthesized. These dyes contain two aromatic sulfo groups and in addition to (up to 3) sulfoalkyl groups in the indolenine end groups. The impact of the number and position of sulfobutyl groups on the spectral properties and photostabilities of the dyes free in solutions and after binding to an antibody (IgG) was investigated. These dyes have absorption maxima between 649 and 653 nm, molar absorptivities between 240,000 and 256,000 M −1 cm −1 , emission maxima from 668 to 672 nm, and fluorescence quantum yields between 28 and 32% in free form and up to 43% when bound to IgG. Independent of the number of sulfo groups, these dyes exhibit no aggregation in water at concentrations up to 5 × 10 −5 M. The number of sulfoalkyl groups was found to directly correlate with the fluorescence quantum yields and photostabilities of the IgG conjugated dyes.
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