Paul B. Jones
I am interested in studying how photochemistry is used in nature and in developing photochemical methods to generate synthetically and medically significant products. Because most organisms depend directly on photochemical processes for their survival, photochemistry is essential to life on earth. The chemistry of photosynthesis and vision are the best examples of this, but many species have more subtle life processes (e.g. circadian rhythms) that rely on photochemistry. In nature, photochemical pathways are well regulated and, like most biochemical pathways, generate specific products. In the laboratory, however, photochemical reactions often yield a variety of products with little or no regio- or stereoselectivity. Specificity in photochemical reactions is achieved in biochemical systems by steric and electronic control of the chromophore and of the molecules with which the excited chromophore interacts. Using these systems as a model, I intend to study how photochemistry can be used to control biological systems and how structure and reaction conditions can be used to direct photochemical reaction pathways. Several projects are summarized below.
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1. Photochemistry in Ionic Liquids Room temperature ionic liquids (RTILs) are salts that freeze below 25oC. Though such salts have been known for almost a century, they have recently gained popularity as “green” alternatives to conventional solvents. RTILs are generally immiscible with water and many organic solvents so that products and by-products can be easily removed via extraction. The ease of product removal allows easy re-use of the solvent. This property, coupled with an extremely low vapor pressure, makes RTILs much less environmentally intrusive than conventional solvents. A number of reactions have been shown to be compatible with ionic liquids, including: Diels-Alder cycloadditions, alkene halogenations, Stille couplings, and hydrogenation reactions. Investigations of reactions in RTILs continues to be an area of increasing activity.We became interested in studying how these unique solvents might affect photochemical reaction pathways. Because RTILs are highly conductive, we speculate that radical pairs formed in such solvents may undergo electron transfer to produce ion pairs. This type of radical pair – ion pair equilibrium has been described by Arnett. Our efforts in this area are know focussing on the amine mediated photoreduction of aryl ketones in a variety of RTILs.
2. Photoregulation of Biologically Active Molecules.
Over the past twenty years, technology for photoregulation of enzyme activity has been developed. This technology has been applied to the development of simple, artificial photosynthetic systems and to the in vivo clotting of blood. For therapeutic applications, however, these photoregulation systems suffer from several disadvantages. Two elements key to any enzyme photoregulation system that is to be used in vivo are: 1) complete enzyme inhibition prior to irradiation and 2) photoactivation with light that is benign to living tissue. One goal is to create an enzyme photoregulation strategy that combines both of these elements. I intend to pursue several strategies that utilize photoinduced electron transfer to appropriately substituted polyaromatic compounds while also serving as the enzyme inhibitor by covalently modifying the lysine amino groups on the enzyme surface. The ultimate goal of this project, however, is the development of chemistry that allows the delivery of a chemotherapeutic agent specifically to the desired location. This has the potential of greatly easing side effects in the treatment of ailments where the affected tissue is accessible to light sources, such as cancers of the throat, skin, and bladder, and disorders involving the eye.
In the example, an enzyme is inactivated by modification with a large blocking group – in this case, a saccharide conjugated to a protein – of the lysine groups on the enzyme outer surface. The blocking group carries a chromophore capable of photooxidizing the amino group to an imine. The imine will be readily hydrolyzed to give a lactone and free (active) enzyme.