Photophysics and Rotational Dynamics of a β-Carboline Analogue in Nonionic Micelles: Effect of Variation of Length of the Headgroup and the Tail of the Surfactant

Abstract

Effect of variation of length of nonionic surfactants in terms of the headgroup as well as the tail part on the photophysical and rotational dynamical properties of a beta-carboline analogue, 3-acetyl-4-oxo-6,7-dihydro-12H-indolo-[2,3-a]quinolizine (AODIQ) has been investigated. Steady-state and time-resolved fluorescence and fluorescence anisotropy have been exploited for the purpose. The experiments revealed modification of the photophysics of AODIQ by the conjugate effect of polarity and rigidity of the micellar environments with varying poly(ethylene oxide) chain length in the case of Triton X series and the alkyl chain length in the case of Tween series surfactants. Fluorometric studies suggest that the fluorophore resides at the micelle-water interface in all these systems. The enhancements in the steady-state anisotropy in all the micellar media compared to those in pure aqueous solution reflect that the fluorophore is located in motionally restricted regions introduced by the nonionic micelles. Contrary to the single exponential nature of the fluorescence anisotropy decay of AODIQ in aqueous medium, they were found to be biexponential in the micellar environments. The rotational relaxation of AODIQ in the micellar environments has been discussed in light of the two-step and wobbling in a cone model. The model helps to evaluate different rotational parameters and to ascertain the location of the fluorophore in the micellar media. The significant feature is that the motional restriction decreases with an increase in the poly(ethylene oxide) chain length while it increases with an increase in the alkyl chain length. The difference in the extent of water penetration due to variation in the thickness of the palisade layer and therefore a variation in the micellar polarity with a variation of the length of poly(ethylene oxide) and alkyl chain has been argued to be responsible.

Binding interaction of differently charged fluorescent probes with egg yolk phosphatidylcholine and the effect of β-cyclodextrin on the lipid–probe complexes: A fluorometric investigation

“…Local polarity around a probe in biomimetic microheterogeneous environments can be estimated from a comparison of the spectral properties of the fluorophore in the environment with those of the probe in pure solvents or solvent mixtures of known polarities [48,49]. In the present study we have endeavored to estimate the micropolarities of the different environments around the fluorophores keeping in mind the fact that there are only few reports on the prospective use of polarity sensitive fluorophore molecules for the determination of micropolarity in the context of the present study.…”

“…We have monitored the fluorescence behavior of the three probes in water-dioxane mixtures of varying composition to determine the polarities around the probes in different microenvironments. Choice of water/dioxane mixture over water/alcohol is emphasized from the fact that the former covers a wider range of polarity [48,49].…”

Studies of Triton X-165–β-cyclodextrin interactions using both extrinsic and intrinsic fluorescence

“…1 shows the variation of the fluorescence intensity as a function of the surfactant concentration and allows one to determine the critical micellar concentration (CMC) of the surfactant [48,49]. The blue shift of the fluorescence maximum indicates that the polarity of the micellar environment is less than the polarity of the bulk water.…”

“…where S represents the surfactant concentration and N is the aggregation number of a micellar system (here N = 82) [5,48,49]. The binding constant has been determined to be 2.4 Â 10 4 l mole À1 from the plot of 1/DI against [L] À1 (see Supporting material, S2).…”

“…This is consistent with the inference drawn from the I À induced quenching study discussed above as well as the micropolarity measurements described in the Supporting material, S3. [48,49].…”