Elucidating the mechanism of ultrafast reactions requires molecules which are activated upon photoexcitation. For photoinduced electron transfer, such molecules are readily available. For photoinduced proton transfer, molecules are required which become strong acids (photoacids) in the excited state. Although molecules which rearrange to generate permanent acids, i.e., photoacid generators, are important in the lithographic industry, molecules which are transient acids are more elusive. This program has sought to develop such molecules--of general interest to a variety of investigators in the field--and is unique in using the principles of theoretical organic chemistry and organic synthesis to provide such reagents. The group, in the process, is producing photoacids with acidities approaching that of strong mineral acids. More recently, these methods have been extended to photoacids in biology, taking advantage of the unique mechanism of the green fluorescent protein (GFP), which exhibits a proton transfer cascade through a relay mechanism. By taking GFP apart and putting it back together, the group is elucidating the details of the photophysics of this revolutionary protein and applying them to a number of related systems. In addition to continuing these studies, they propose to apply these principles to the development of GFP chromophores, stripped of their protective protein barrel,and to a new category of sensors which use both fluorescence off-on switching and proton transfer to give two variables in the sensing process. They further propose to continue studies on new photoacids which reveal details of ion-pair dynamics and effects of diastereoselectivity on acid dissociation. A wide array of objects including supercritical media and molecular jets will be studied.

The intellectual merit of the proposed activity lies in the design and synthesis of new "super" photoacids, the validation of models for the molecular details of ultrafast proton transfer, and the use of those models in proton transfer in biology. The broader impacts of the proposed activity encompass the development of new spectroscopic and mechanistic tools for the larger community of spectroscopists and photobiologists, the transfer of this technology and knowledge base through dissemination of these results at conferences and the literature, the collaboration with scientists in a number of laboratories, and the development of a set of future scientists skilled in organic synthesis, spectroscopy, mechanistic chemistry, and collaborative research. As part of this effort, Prof. Tolbert will visit 10 historically black universities the first year of the grant to compare their efforts in building undergraduates for graduate school in chemistry and compare those with his experience at Clark Atlanta University in order to improve his effectiveness in recruitment of African-Americans to Clark Atlanta.

Project Report

Excited-state proton transfer is the result of the ability of certain molecules to become strong acids when they absorb light. This property of such molecules, called "photoacids", enables the study of very fast proton-transfer reactions, one of the most fundamental reactions in all of chemistry. Despite this general utility, it was not known that photoacids existed in nature until the discovery of the green fluorescent protein (GFP). GFP forms as the result of a cyclization among three amino acids that produces a hydroxybenzylideneimidazolone (HBI), in which the tyrosine residue is a photoacid. As a result, the green emission produced upon irradiation of GFP is the product of an excited-state proton transfer from the HBI chromophore. We have studied many properties of this chromophore, as well as over 160 of its derivatives, to learn the details of its photochemistry and photophysics. The intellectual merit of this work is that we have defined a number of decay processes that take place in such systems, e.g., double-bond twisting, that lead to fast decay and inefficient fluorescence. Since the HBI chromophore, within the protein coat, is very sensitive to its environment, we have also been developing this "fluorescent engine" to identify other environments that can modify or activate its fluorescence. This includes other proteins, other capsule-like molecules, and many biological materials such as bile acids. These results suggest wider impacts for these discovery in the development of new and easily-used agents for identification of target proteins that are markers for disease or other biological dysfunctions.

National Science Foundation (NSF)
Division of Chemistry (CHE)
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Tyrone D. Mitchell
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Georgia Tech Research Corporation
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