Lumichrome (7,8-dimethylalloxazine) undergoes excited state double proton transfer in two solvents: acetic acid and pyridine. This research extends the mechanistic understanding of this process. Evidence is presented that points to two different mechanisms in the two solvent systems. In acetic acid, the one-step, concerted double proton transfer mechanism is supported. In pyridine, a two-step mechanisms is proposed. In the first step, dissociation occurs in the ground state, with the formation of the lumichrome anion and the pyridinium cation. In the second step, excited state single proton transfer takes place, from the pyridinium cation to the lumichrome anion, forming the tautomer species. / In other solvents, e.g., alcohols, ethers, excited state proton transfer is not observed. In order to explain this it is proposed that the first ground state pK(,a) (defined as the dissociation constant in aprotic solvents) of lumichrome is highly dependent on the hydrogen-bond strength of the solvent, becoming more acidic in weakly hydrogen bonding solvents. If this is the case, then lumichrome exists in neutral and anionic forms in equilibirum in these solvents, and the cyclic hydrogen bonding required for proton transfer cannot occur. / 7-azaindole has long been known to undergo excited state proton transfer in hydrocarbon and alcohol solutions. We have demonstrated the occurrence of an analogous phototautomerization in the purine bases adenine and guanine, which are structurally similar to 7-azaindole. However, this proton transfer does not occur in hydrocarbon and alcohol solvents. Rather, as in the case of lumichrome, acetic acid and pyridine catalyze tautomerization in the purine bases. From these results we have drawn certain interesting conclusions as to the role of the solvent in excited state proton transfer. In hydroxyl-type solvents, such as alcohols and acetic acid, the pK(,a) of the solvent must match as closely as possible the pK(,a) of the molecule in question in order for proton transfer to occur. In pyridine, the more acidic the molecule, the more efficient is phototautomerization. / Source: Dissertation Abstracts International, Volume: 45-09, Section: B, page: 2784. / Thesis (Ph.D.)--The Florida State University, 1984.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_75426 |
| Contributors | BEZVINER, J. LAWRENCE., Florida State University |
| Source Sets | Florida State University |
| Detected Language | English |
| Type | Text |
| Format | 175 p. |
| Rights | On campus use only. |
| Relation | Dissertation Abstracts International |
Page generated in 0.0022 seconds