THE TRIPLET AND RADICAL SPECIES OF HAEMATOPORPHYRIN AND SOME OF ITS DERIVATIVES

Abstract— Nanosecond laser flash photolysis and pulse radiolysis have been used to generate and characterise the triplet state, and semioxidised and semireduced radicals of haematoporphyrin, and three 0 -acyl compounds derived from it (the monoacetate, the diacetate and the disuccinate).
After 347 nm irradiation in water containing 2% Triton X-100, haematoporphyrin forms the triplet state (φ_T= 0.92) and photoionises monophotonically (φ_I= 0.03). For the O -acyl derivatives, φ_T approaches unity and photoionisation is reduced. In acetone the triplet yield of all four compounds are close to unity. The difference and corrected spectra for the triplet species are presented and decay rates ( k ₁˜10⁴s^-1) and oxygen quenching constants ( k _Q˜1.5times10⁹ M ^-1s^-1) for the triplet state have been measured. The difference and corrected spectra for the semi-reduced species in methanol and semi-oxidised species in aqueous Triton X-100 are presented.
The photophysical characteristics in fluid solution of haematoporphyrin and its 0 -acyl derivatives are rather similar to those previously recorded for other photosensitising porphyrins.     

Acyl-CoA subunit selectivity in the pikromycin polyketide synthase PikAIV: steady-state kinetics and active-site occupancy analysis by FTICR-MS

Polyketide natural products generated by type I modular polyketide synthases (PKSs) are vital components in our drug repertoire. To reprogram these biosynthetic assembly lines, we must first understand the steps that occur within the modular "black boxes." Herein, key steps of acyl-CoA extender unit selection are explored by in?vitro biochemical analysis of the PikAIV PKS model system. Two complementary approaches are employed: a fluorescent-probe assay for steady-state kinetic analysis, and Fourier Transform Ion Cyclotron Resonance-mass spectrometry (FTICR-MS) to monitor active-site occupancy. Findings from five enzyme variants and four model substrates have enabled a model to be proposed involving catalysis based upon acyl-CoA substrate loading followed by differential rates of hydrolysis. These efforts suggest a strategy for future pathway engineering efforts using unnatural extender units with slow rates of hydrolytic off-loading from the acyltransferase domain.     

Detection of transient laser magnetic resonance signals in pulsed CO2 laser irradiation of SF6/H2O and SF6/NO mixtures

A technique for detecting transient far-infrared laser magnetic resonance (LMR) signals, induced by pulsed CO₂ laser photolysis, has been developed. The method is illustrated with preliminary data from photolysis of SF₆ containing H₂O or NO. In the former mixture. OH(X ²Π) formation and decay is observed, while in the latter, a transient decrease in NO(X ²Π) concentration followed by a return to the original value is observed. Possible physical and chemical mechanisms are discussed.     

Structural and Stereochemical Analysis of a Modular Polyketide Synthase Ketoreductase Domain Required for the Generation of a cis-Alkene

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Porphyrin sensitizers in tumour phototherapy. Novel sensitizers of the chlorin and bacteriochlorin class with amphiphilic properties

The requirements for activity in a tumour-photosensitizing drug are outlined. A series of metallo tetrasulphonatophthalocyanines are shown to be inactive in an in vivo assay of tumour photosensitization; however, some less water-soluble compounds (hydroxylated derivatives of octaethylchlorin and octaethylbacteriochlorin) are shown to possess promising activity.     

Chemistry of Vanadyl Porphyrins

The chemistry of vanadyl porphyrins has been explored using vanadyl octaethylporphyrin as the type substance, and with special reference to the occurrence of such substances in petroporphyrins. Spectroscopic properties (electronic spectra, IR spectra, mass spectra) are described, and electronic spectra are employed to detect the occupation of the vacant axial coordination site. Direct substitution of vanadyl octaethylporphyrin furnishes meso-substituted products: various nitro, chloro and benzoyloxy derivatives are characterised. Trans alkylation is observed when vanadyl octaethylporphyrin is heated in vacuo on various supports, including clay minerals: the reaction provides a model for the generation of the homologous series of vanadyl polyalkylporphyrins found in crude oils and bitumens.     

The hydroxylation of aromatic compounds with hypofluorous acid

The reaction of hypofluorous acid with aromatic substrates (benzene, p-xylene, naphthalene, and various monosubstituted benzenes) leads to phenolic products. Isomer distribution studies suggest that the reagent has the characteristics of an electrophilic species which is not the OH radical, and which may be regarded as HO^δ+?F^δ-. The p-methoxyphenol isolated from the p-deuterioanisole/hypofluorous acid reaction shows a marked NIH effect (77% incorporation of deuterium). The bearing of these results on microsomal hydroxylation is discussed briefly.     

Shear-stress measurement in aerodynamic testing using cholesteric liquid crystals

The change in the selective reflection spectra of cholesteric liquid crystals with shear is described, and the results indicate that, although it appears to be a promising technique for flow visualization in aerodynamic testing, the method is only suitable for approximate measurement of shear. An improved method for visualising flow is proposed that utilizes a shear-induced texture change from the uncoloured focal-conic state to the selectively reflecting Grandjean state. The sensitivity of the transition to shear is investigated and the effects of adding silica particles to modify the viscosity is examined.