Reversible intramolecular triplet-triplet energy transfer in benzophenone-N-methylphthalimide dyad.

In the present paper, we synthesized a series of benzophenone (BP)-N-methylphthalimide (MePI) dyads (Cn, n = 3, 6, and 9, where n denotes the number of methylene in the linker) and investigated the photochemical properties and intramolecular triplet-triplet energy transfer from BP(T1) to MePI. Formation of two different intramolecular complexes was found, that is, a ground-state complex and a singlet exciplex. The formation of the triplet-equilibrium between MeBP and MePI was observed. The triplet-equilibrium constant (1.0 and 1.1 for C6 and C9, respectively) and forward ((3.8 +/- 1.3) x 107 and (3.9 +/- 1.2) x 107 s-1 for C6 and C9, respectively) and back ((3.8 +/- 1.3) x 107 and (3.6 +/- 1.2) x 107 s-1 for C6 and C9, respectively) energy transfer rates were estimated from the result of transient absorption measurements. From the van't Hoff plots, enthalpy and entropy change for the equilibrium formation were estimated.

Chiral photocages based on phthalimide photochemistry.

A new class of photoremovable protecting groups, based on a photoinduced decarboxylation reaction coupled with the elimination of the caged molecule, is described for 2-phthalimido-3-hydroxy-propionate derivatives. When derived from enantiopure N-phthaloyl- serine or threonine, the chirality of the starting amino acid is transmitted to the protected (caged) molecule. These photocages possess good properties for their use in biological systems, and the introduction of chirality opens new possibilities for the study of diastereoselective photochemistry and stereodifferentiation processes involving the release of the caged molecule.

Synthesis and application of derivatizable oligonucleotides.

To investigate protein-DNA interactions, we have synthesized a versatile phthalimide-protected 5-(3-aminopropyl)-2'-deoxyuridine nucleoside probe. The modified residue was incorporated into deoxyoligonucleotides by automated synthesis. The standard oligonucleotide workup also exposed the pendent amino group, which was found to react with either fluorescent labelling agents or, as detailed below, a photoactivatable cross-linking agent. In the dark, a strand with a photolabile group adjacent to the 3' end served as a primer for synthetic template-directed DNA synthesis by the Klenow fragment of E. coli DNA polymerase I, by bacteriophage T4 DNA polymerase, and by avian myeloblastosis virus (AMV) reverse transcriptase. Brief illumination with 302 nm light afforded covalent complexes between DNA and the polymerases; labelling of AMV reverse transcriptase was predominantly in the beta subunit.