Annette D. Allen

FRET Quenching of Photosensitizer Singlet Oxygen Generation

The development of activatable photodynamic therapy (PDT) has demonstrated a utility for effective photosensitizer quenchers. However, little is known quantitatively about Forster resonance energy transfer (FRET) quenching of photosensitizers, even though these quenchers are versatile and readily available. To characterize FRET deactivation of singlet oxygen generation, we attached various quenchers to the photosensitizer pyropheophorbide-alpha (Pyro) using a lysine linker. The linker did not induce major changes in the properties of the photosensitizer. Absorbance and emission wavelength maxima of the quenched constructs remained constant, suggesting that quenching by ground-state complex formation was minimal. All quenchers sharing moderate spectral overlap with the fluorescence emission of Pyro (J > or = 5.1 x 10(13) M(-1) cm(-1) nm4) quenched over 90% of the singlet oxygen, and quenchers with weaker spectral overlap displayed minimal quenching. A self-quenched double Pyro construct exhibited intermediate quenching. Consistent with a FRET deactivation mechanism, extension of the linker to a 10 residue polyproline peptide resulted in only the quenchers with spectral overlap almost 2 orders of magnitude higher (J > or = 3.7 x 10(15) M(-1) cm(-1) nm4) maintaining high quenching efficiency. Overall, there was good correlation (0.98) between fluorescence quenching and singlet oxygen quenching, implying that fluorescence intensity can be a convenient indicator for the singlet oxygen production status of activatable photosensitizers. Uniform singlet oxygen luminescence lifetimes of the compounds, along with minimal triplet state transient absorption were consistent with quenchers primarily deactivating the photosensitizer excited singlet state. In vitro, cells treated with well-quenched constructs demonstrated greatly reduced PDT induced toxicity, indicating that FRET-based quenchers can provide a level of quenching useful for future biological applications. The presented findings show that FRET-based quenchers can potently decrease singlet oxygen production and therefore be used to facilitate the rational design of activatable photosensitizers.

Solvolytic reactivity of 1-trifluoromethyl-1-phenylethyl tosylate. Correlation of substituent effects in the formation of highly destabilized carbonium ions

The solvolytic rate constants of 1-trifluoromethyl-1-phenylethyl toxylate (2) in solvents of widely different ionizing power and nucleophilicity are linearly related with slope mlt. slashsub OTslt. slash = 1.01 to the rates of 2-adamantyl tosylate in the same solvents. The rate ratio k(PhCHMeOTs)/k(2) is 2 x 10/sup 5/ in 100% EtOH. Added salts cause modest increases in the rate of solvolysis of 2 in 80% EtOH independent of the nucleophilicity or basicity of the salts. The isotope effect k(CH/sub 3/)/k(CD/sub 3/) on the rate of solvolysis of 2 ranges from values around 1.6 in the less ionizing solvents to values around 1.3 in more ionizing solvents. The product from 2 is mainly that of substitution in all solvents studied, with increasing amounts of elimination in the less ionizing solvents. These results are interpreted in terms of rate-limiting ionization of 2 to form a carbonium ion intermediate.

N-pyrrolylketene: a nonconjugated heteroarylketene.

N-Pyrrolylketene (5) is calculated to be destabilized and nonconjugated, with a preferred geometry with the pyrrolyl ring orthogonal to the ketenyl group. Ketene 5 is generated from N-pyrrolylacetic acid (7) with use of Mukaiyamas reagent, and reacts with imines forming β-lactams 10, with a product ratio correlation of log(cis/trans) with σ(+). Photolysis of N-diazoacetylpyrrole (14) in MeOH gives methyl N-pyrrolylacetate (15) from 5 and also ester 17, evidently by trapping of 2-(1-pyrrolylketene) (21), formed by a new vinylogous Wolff rearrangement.

TRIMETHYLSILYLKETENE : REACTIVITY AND STABILIZATION

Abstract Me 3 SiCHCO is stable thermally, and is less reactive than t BuCHCO toward H 2 O in neutral hydration, but in acid and base induced hydrations the reactivity is accelerated relative to alkylketenes. The contrasting reactivities are due to the relative degree of silicon stabilization of the ground state and the different transition states.

Spiro-Aziridine and Bislactam Formation from Bisketene−Imine Cycloadditions

1,2-Bisketenes 6 react with imines PhCHNAr, (E)-2, forming spiro-aziridines 7. DFT computations indicate that this occurs by nucleophilic attack of the imine on the carbonyl carbon of the more reactive arylketene moiety, followed by cyclization, and not by prior cyclization of the 1,2-bisketene forming a carbene lactone intermediate. Computations also indicate that the previously studied bisketene 10 from benzocyclobutadiene 9 is 4.0 kcal/mol less stable than carbene lactone 12 that would result from cyclization but that the failure to observe 12 results from a lower barrier for 10 to instead revert to 9. 1,2-, 1,3-, and 1,4-Bisketenylbenzenes 16, 19, and 22 react with imines forming bis(β-lactams), with a preference for formation of mixtures of trans, trans chiral (±) and achiral diastereomeric products.

The unusual hydration reactivity of acylketenes: Theoretical and experimental studies

Ab initio MO calculations indicate that the hydration of acylketenes may be assisted by complexation of the nucleophilic H2O molecule to the oxygen of the acyl substituent. Experimental studies for hydration of EtO2CC(Bu-t)=C=O (4) support this pathway.

Pyridylketenes: Structure reactivity effects in nucleophilic and radical addition

2-, 3-, and 4-Pyridylketenes have been generated in CH3CN by photochemical Wolff rearrangements and identified by their ketenyl absorption in the infrared at 2127, 2125, and 2128 cm–1, respectively. Reaction of these pyridylketenes with n-BuNH2 results in the formation of intermediate amide enols from the 3- and 4-pyridylketenes, which are then converted to the corresponding pyridylacetamides. However , 2-pyridylketene forms a long-lived 1,2-dihydropyridine intermediate stabilized by an intramolecular hydrogen bond, and this is converted to the 2-pyridylacetamide with a rate constant 107 less than those for the conversion of the amide enols from the 3- and 4-pyridylketenes to amides. Hydration of the pyridylketenes results in the formation of an acid enol intermediate in the case of the 3-isomer, while the 2- and 4-isomers form longer-lived dihydropyridines. The pyridylketenes react with the stable free radical tetramethylpiperidinyloxyl (TEMPO,TO) forming 1,2-diaddition products ArCH(OT)CO2T.

Protonation of 1-aryl-3,3,3-trifluoropropynes

1-Aryl-3,3,3-trifluoropropynes (1) react in aqueous acid with ρ+ = −6.5, and the gas phase basicity of 4-CH3C6H4CCCF3 is 7.8 kcal/mol less favorable than that of 4-CH3C6H4CCH, showing high electron demand and major destabilization in ArC+CHCF3.

Ketene reactions with tertiary amines.

Tertiary amines react rapidly and reversibly with arylketenes in acetonitrile forming observable zwitterions, and these undergo amine catalyzed dealkylation forming N,N-disubstituted amides. Reactions of N-methyldialkylamines show a strong preference for methyl group loss by displacement, as predicted by computational studies. Loss of ethyl groups in reactions with triethylamine also occur by displacement, but preferential loss of isopropyl groups in the phenylketene reaction with diisopropylethylamine evidently involves elimination. Quinuclidine rapidly forms long-lived zwitterions with arylketenes, providing a model for catalysis by cinchona and related alkaloids in stereoselective additions to ketenes.

The bisketene radical cation and its formation by oxidative ring-Opening of cyclobutenedione

Parent cyclobutenedione 1 was photolyzed and ionized in an Ar matrix at 10K. The bisketene 2 that results in both cases (in the form of its radical cation after ionization) was characterized by its IR spectrum and by high-level quantum chemical calculations. Experiment and theory show that the neutral bisketene has only a single conformation where the two ketene moieties are nearly perpendicular, whereas the radical cation is present in two stable planar conformations. The mechanism of the ring-opening reaction, both in the neutral and in the radical cation, is discussed on the basis of calculations. In the latter case it is a nonsynchronous process that involves an avoided crossing of states.