Photonic Control of Reversible Binding of Small Molecules to Metal Ions and DNA

Pal, Suman (2014) Photonic Control of Reversible Binding of Small Molecules to Metal Ions and DNA. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Development of novel receptors for heavy metal ions is crucial for the detection and imaging of the ions in biological samples. The first part of this thesis focuses on the development of novel receptors for metal ions and their use in detection of the ions in biological and contaminated water samples based on optical methods. The second half of the thesis deals with optical modulation of a metal receptor and control of the binding affinity of the receptor using light (chapter 4). In continuation of the study on optical modulation, chapter 5 of this dissertation deals with reversible light-gated of binding of a small molecule to DNA. The first chapter offers a general introduction to chemosensing and chemodosimetric detection processes of ions using fluorescence techniques. The later part of the chapter also provides an introduction to photochromic systems, their properties and applications. The second chapter discusses the design and properties of a novel naphthalenedimide (NDI)-dithiocarbamate based fluorescence probe. The probe is selective towards Hg2+ ion and senses the ion in a chemodosimetric mode. The probe with Hg2+ ion affords an irreversible color change and a fourfold enhancement of the fluorescence intensity along with a distinct 43 nm blue shift of the emission wavelength. Although there are many chemodosimeters for Hg2+ ions, our system offers a highly selective, fast, easy, and quantitative means for the detection of Hg2+ in aqueous medium at a physiological pH range. In cell detection was also achieved with this chemodosimeter. In the third chapter detection of another heavy metal ion Ag+ has been achieved with a naphthalene based chemosensor having thiocarbamate appendages. This chemosensor was also found to detect Hg2+. However, differential detection of Ag+ and Hg2+ has also been achieved. In the aqueous medium it forms organic nanoaggregates, which undergo further aggregation and grow in size in the presence of Ag+ ions. We wondered if a metal ion receptor can be conjugated to a photochromic system and the binding of a metal ion can be controlled by means of light. Thus the fourth chapter deals with a pyridine based Cu2+ receptor connected to a benzodimethyldihydropyrene based photochromic system. It was observed that the binding affinity of the ligand to a Cu2+ ion alters by tenfolds upon photoisomerization of the photochromic unit. This is the first demonstration of a reversible photomodulation of the binding of a metal ion using the dimethyldihydropyrene based system. In the fifth and the final chapter of this thesis, studies on interaction between a charged photochromic system with the calf thymus DNA has been discussed. The dicationic benzodihydropyrene photochromic system binds to the negatively charged phosphodieaster backbone of the DNA. The open form of the dicationic benzodihydropyrene binds four times stronger to the DNA compared to the closed form. The binding is reversible in nature.

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