Rotational diffusion dynamics of Alexa flour dyes in aqueous organic environment

Abstract

The rotational reorientation times of three dye probes namely Alexa fluor-350 (neutral but polar), Alexa fluor-430 (monoanion) and Alexa fluor-488 (dianion) were determined in series of binary mixtures of DMSO-water, DMA-water, and 2-propanol-water. All the three series of mixtures show bivalued profile for viscosity as a function of mole fraction of organic solvent in water. The dielectric properties such as dielectric constant (ε) and Debye dielectric relaxation time (τD) of solutions change considerably across the composition range. The variation of rotational reorientation time (τr) with viscosity (η) for all the probes is studied. Alexa fluor 350 and alexa fluor 488 show hook type profile in all the three series of aqueous mixtures with rotational reorientation time (τr) being longer in organic solvent rich zone compared to water rich zone mainly due to dielectric friction and hydrogen bonding. Alexa fluor 430 shows hook type profile in 2-propanol-water mixtures similar to other two probes but in DMSO-water and DMA-water it has almost linear profile as rotational reorientation dynamics is almost similar in both organic solvent rich zone and water rich zone. The hook type profiles obtained suggest the importance of local molecular structure of binary mixture and its interaction with probe. Results obtained are discussed in light of SED (hydrodynamic) theory, dielectric friction theories and friction due to hydrogen bonding.