Interaction of flavonoids within organized molecular assemblies of anionic surfactant

https://doi.org/10.1016/j.colsurfb.2004.01.012Get rights and content

Abstract

The interaction of various flavonoids (compounds having C6–C3–C6 configuration) with sodium dodecyl sulfate (SDS) an anionic surfactant was studied through absorption spectroscopy as a function of the concentration of surfactant above and below the critical micelle concentration (CMC) of the surfactant. A mechanism was proposed for the interaction between these flavonoids and anionic surfactants. The approximate number of additive molecules (flavonoids) incorporated per micelle was estimated at a particular concentration of SDS. Incorporation of additive in micelles shifts the UV absorption bands towards higher wavelengths of different magnitude. The spectral shift also depends upon the nature of the surfactant head group. The absorption spectra of the flavonoids in aqueous solution and in methanol are also reported.

Introduction

Surfactant micelles can be pictured as having a non-polar interior and a relatively polar interfacial region. The interior of the micelle is generally considered the locus of solubilization for non-polar solubilizate such as n-alkanes. Solubilizate molecules of relatively high polarity such as alcohols are believed to solubilize in the interfacial region of the micelle so that their polar functional groups (OH for example) could retain their contact with water. However, for molecules, such as aromatic hydrocarbons, which are nonpolar, conflicting suggestions have been presented in the literature [1], [2], [3], [4], [5], [6], [7], [8], [9] concerning their location in the micelle.

Micelles of ionic surfactants can interact electrostatically with highly polar solutes because the large surface charge densities of these aggregates lead to strong ion–dipole interactions [10], [11]. In addition, ionic micelles ordinarily have an extensive hydrophobic core region, which can interact strongly with hydrocarbons and halogenated hydrocarbons groups of solutes. Hydrophobic effects have often been considered to be dominant in determining the locus of solubilization [12], [13] but the effect of electrostatic interactions should also be considered in relation to the solubilization of organic solutes in the ionic micelles.

In the present work, four flavonoids Kaempferol (KFL), Kaempferide (KFD), Rhamnetin (RMT) and Apigenin (APG) are used as organic additives. Although the most significant function of flavonoids is their ability to impart color to various parts of plants, thereby making the plant survival possible. However, interest in their physiological action and their possible application to clinical therapy has long existed. The chemical study of such plants has been very productive and has led to the discovery of many therapeutically significant flavonoids. Wagner and coworkers have discussed the anti-hemorrhagic action of flavones in detail [14], [15]. To these results must be added the clinical investigations of Lietti and coworkers with anthocyanins [16], [17]. Similarly, flavonoids have shown to possess anti-inflammatory as well as analgesic activity [18]. Inhibition of HIV infection by baicalin, a flavone has been reported by Li et al. [19]. Flavone acetic acid (FAA) is a newly introduced synthetic flavonoid with successful clinical trials as anticancer agent [20]. Flavonoids also have the property to prevent coronary heart disease [21]. A number of cosmetic products appearing in the market show a marked trend of utilizing various flavonoids as effective radioprotectant creams [22].

The structures of these flavonoids are shown below:

Recently, we examined the interaction and solubilization of various hydrophobic additives in aqueous micellar anionic surfactant solutions [23], [24], [25]. In the present study the interaction of flavonoids within the aqueous solution of sodium dodecyl sulfate (SDS) is reported. The approximate numbers of additive molecules per micelle (n) have also been calculated from the absorbance data according to the method [25].

Section snippets

Materials

Sodium dodecyl sulfate (SDS), the product of Merck was of analytical grade and was used without further purification. The flavonoids; Kaempferol, Kaempferide, Rhamnetin and Apigenin obtained from Fluka, were 99.5% pure and were used without further purification. Water used was double distilled. Ethanol was also distilled before use.

Conductance measurements

The specific conductance of SDS solution was measured on Microprocessor Conductivity Meter of WTW Model LF-2000/C at 25.0±0.01. The critical micelle concentration of

Results and discussion

UV spectra of flavonoids exhibit two major absorption bands in the region 240–400 nm. Band-I (300–380 nm) is considered to be associated with absorption due to the ring-B cinnamoyl system and band-II (240–280 nm) is considered to be associated with absorption due to the ring-A benzoyl system as shown below.

On increasing oxygenation of ring-B in flavonoids, bathochromic shift of band-I occurs with each addition oxygen function. Increasing hydroxylation of ring-A in flavones and flavonols produces a

Acknowledgements

The financial support of QAU in form of URF is duly acknowledged. The help of Mr. Kashif Naeem in preparing this manuscript is highly appreciated.

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