Functional modification of agarose: A facile synthesis of a fluorescent agarose–guanine derivative

Abstract

A new fluorescent polymeric material was synthesized by grafting the nucleobase guanine on to the backbone of agarose. The synthesis involved a rapid water based method under microwave irradiation using potassium persulphate (KPS) as an initiator. The emission spectrum of the modified agarose recorded in 0.1 M aqueous NaOH (5 × 10⁻⁵ M) solution exhibited emission maxima (λ_em,max) at 340 nm by excitation at 274 nm. The emission intensity was enhanced by ca. 85% compared to that of pure guanine solution of the same concentration. When the concentration of the pure guanine solution is made equivalent to the concentration of the guanine molar component (3.63 × 10⁻⁵) present in 5 × 10⁻⁵ M solution of modified agarose, then ca. 105% enhancement in emission intensity was observed. The remarkable fluorescent activity of the agarose–guanine derivative may have potential uses as sensor in various applications.

Introduction

Fluorescence phenomenon was harnessed to study agarose gelling system by Hayashi, Kinoshita, and Yasueda (1980). Polysaccharide conjugates were prepared with fluorescein to distinguish underivatized polysaccharides as well as for localizing and quantifying cell surface proteins in cell biology research (Glabe, Harty, & Rosen, 1983). Other fluorescent polysaccharides and their conjugates were prepared with an eye to identifying biomolecules, sensing pH as well as preparing cellulose based organic light emitting diode (Karakawa et al., 2007, Kobayashi et al., 1990, Qiu et al., 2005, Schulz et al., 2009, Suizhou et al., 2003). Urreaga and De la Orden (2007), have reported modification of cellulose with amino compounds and their fluorescence properties. Synthesis and fluorescent properties of pyrene-lableled guanine base was reported for studying the secondary structures of G-rich DNA (Okamoto, Kanatani, Ochi, Saitob, & Saito, 2004). There exist numerous reports in the literature on the modification of polysaccharides employing various strategies, e.g. grafting, cross linking, etc.

In an ongoing program of our laboratory on modification of seaweed polysaccharides for preparing new materials with improved functional properties (Meena et al., 2006a, Meena et al., 2006b, Meena et al., 2007a, Meena et al., 2008, Prasad et al., 2005a, Prasad et al., 2005b, Prasad et al., 2006a, Prasad, Meena, & Siddhanta, 2006), we report herein functional modification of agarose (Fig. 1) by grafting guanine (Fig. 1) on to agarose by a water based method. This guanine modified agarose exhibited exceptionally strong fluorescent properties. Guanine is 2-amino-6-hydroxypurine, which is one of the four nitrogenous bases found in nucleic acids (Finar, 2004). Agarose is a hydrophilic polymer and is widely used in biomedical applications and bioengineering. The basic disaccharide repeating units of agarose consists of (1,3) linked β-d-galactose (G) and (1,4) linked α-l-3,6-anhydrogalactose (A) (Fig. 1) (Rochas & Lahaye, 1989). To our knowledge, this agarose derivative and its effects are being reported for the first time.