Paul S. Dobbin

Cytochrome bd confers nitric oxide resistance to Escherichia coli

The aerobic respiratory chain of Escherichia coli has two terminal quinol oxidases: cytochrome bo and cytochrome bd. Cytochrome bd was thought to function solely to facilitate micro-aerobic respiration. However, it has recently been shown to be overexpressed under conditions of nitric oxide (NO) stress; we show here that cytochrome bd is crucial for protecting E. coli cells from NO-induced growth inhibition by virtue of its fast NO dissociation rate.

Macromolecular delivery of 5-aminolaevulinic acid for photodynamic therapy using dendrimer conjugates

Intracellular porphyrin generation following administration of 5-aminolaevulinic acid (5-ALA) has been widely used in photodynamic therapy. However, cellular uptake of 5-ALA is limited by its hydrophilicity, and improved means of delivery are therefore being sought. Highly branched polymeric drug carriers known as dendrimers present a promising new approach to drug delivery because they have a well-defined structure capable of incorporating a high drug payload. In this work, a dendrimer conjugate was investigated, which incorporated 18 aminolaevulinic acid residues attached via ester linkages to a multipodent aromatic core. The ability of the dendrimer to deliver and release 5-ALA intracellularly for metabolism to the photosensitizer, protoporphyrin IX, was studied in the transformed PAM 212 murine keratinocyte and A431 human epidermoid carcinoma cell lines. Up to an optimum concentration of 0.1 mmol/L, the dendrimer was significantly more efficient compared with 5-ALA for porphyrin synthesis. The intracellular porphyrin fluorescence levels showed good correlation with cellular phototoxicity following light exposure, together with minimal dark toxicity. Cellular uptake of the dendrimer occurs through endocytic routes predominantly via a macropinocytosis pathway. In conclusion, macromolecular dendritic derivatives are capable of delivering 5-ALA efficiently to cells for sustained porphyrin synthesis. [Mol Cancer Ther 2007;6(3):876–85]

Sustained and efficient porphyrin generation in vivo using dendrimer conjugates of 5-ALA for photodynamic therapy

The use of endogenous protoporphyrin IX (PpIX) after administration of 5-aminolaevulinic acid (ALA) has led to many applications in photodynamic therapy (PDT). However the efficacy of ALA-PDT is sub-optimal for thicker tumours and improved ALA delivery and therapeutic response are required. We have investigated the conjugation of ALA to a second-generation dxcendrimer for enhancing porphyrin synthesis in vitro and in vivo in a murine tumour model using systemic i.p. administration. In vitro, the dendrimer was more efficient than ALA for porphyrin synthesis at low concentrations in good correlation with higher cellular ALA dendrimer accumulation. In vivo, the porphyrin kinetics from ALA exhibited an early peak between 3 and 4 h in most tissues, whereas the dendrimer induced sustained porphyrin production for over 24 h and basal values were not reached until 48 h after administration. Integrated porphyrin accumulation from the dendrimer and ALA, at equivalent molar ratios, was comparable showing that the majority of ALA residues were liberated from the dendrimer. The porphyrin kinetics appear to be governed by the rate of enzymatic cleavage of ALA from the dendrimer, which is consistent with in vitro results. ALA dendrimers may be useful for metronomic PDT, and multiple low-dose ALA-PDT treatments.

Structure–stability relationships of 3-hydroxypyridin-4-one complexes

The structural features of the ligand 1-ethyl-3-hydroxy-2-methylpyridin-4-one HL1 and its tris complexes [ML13]·3H2O (M = Fe 1, Ga 2 and Al 3) have been characterized by single-crystal X-ray diffraction. The crystal structures of complexes 1–3 are isomorphous. The comparisons among HL1 and 1–3 indicate that several resonance forms play an important role in determining their structures. It is concluded that the absolute stabilities of the complexes of Al3+ and Ga3+ are similar and both greater than that of the iron(III) complex. A structural comparison between ligand HL1 and the closely related 1-butyl-3-hydroxypyridin-2-one provides an explanation for the higher affinity of 3-hydroxypyridin-4-ones for Fe3+ over 3-hydroxypyridin-2-ones. The complexes of Al3+ and Ga3+ with HL1 can be crystallized both as tri- and dodeca-hydrates. It is shown by comparing the same complex in different environments that neither the interplanar angle nor the twist angle is a reliable structural parameter to characterize a metal complex.

3-[1-(Ethylamino)ethylidene]-6-methyl-3H-pyran-2,4-dione.

The structural results clearly indicate that 3-[1-(ethylamino)ethylidene]-6-methyl-3H-pyran-2,4-dione exists as a keto-enamine tautomer in the solid state. The H atom bonded to N(9) refines with a normal temperature factor and the bond distance of the keto group [C(4)--O(13)] of 1.263 (1) A is elongated due to resonance. This resonance is also indicated by an averaging of the single bonds [C(2)--C(3), C(3)--C(4), C(4)--C(5) and C(8)--N(9)] and double bonds [C(3)--C(8) and C(4)--O(13)] in the crystal structure and by the fact that not only is the pyran ring planar [r.m.s. deviation 0.0212 (9) A] but also there is planarity for the whole molecule [r.m.s. deviation 0.074 (1) A]. A strong intramolecular hydrogen bond is found between the 4-oxo group and the amine H atom.

Conformational control of the binding of diatomic gases to cytochrome c

The cytochromes c′ (CYTcp) are found in denitrifying, methanotrophic and photosynthetic bacteria. These proteins are able to form stable adducts with CO and NO but not with O2. The binding of NO to CYTcp currently provides the best structural model for the NO activation mechanism of soluble guanylate cyclase. Ligand binding in CYTcps has been shown to be highly dependent on residues in both the proximal and distal heme pockets. Group 1 CYTcps typically have a phenylalanine residue positioned close to the distal face of heme, while for group 2, this residue is typically leucine. We have structurally, spectroscopically and kinetically characterised the CYTcp from Shewanella frigidimarina (SFCP), a protein that has a distal phenylalanine residue and a lysine in the proximal pocket in place of the more common arginine. Each monomer of the SFCP dimer folds as a 4-alpha-helical bundle in a similar manner to CYTcps previously characterised. SFCP exhibits biphasic binding kinetics for both NO and CO as a result of the high level of steric hindrance from the aromatic side chain of residue Phe 16. The binding of distal ligands is thus controlled by the conformation of the phenylalanine ring. Only a proximal 5-coordinate NO adduct, confirmed by structural data, is observed with no detectable hexacoordinate distal NO adduct.

Hydroxypyridinone and 5-Aminolaevulinic Acid Conjugates for Photodynamic Therapy

Photodynamic therapy (PDT) is a promising treatment strategy for malignant and nonmalignant lesions. 5-Aminolaevulinic acid (ALA) is used as a precursor of the photosensitizer, protoporphyrin IX (PpIX), in dermatology and urology. However, the effectiveness of ALA-PDT is limited by the relatively poor bioavailability of ALA and rapid conversion of PpIX to haem. The main goal of this study was to prepare and investigate a library of single conjugates designed to coadminister the bioactive agents ALA and hydroxypyridinone (HPO) iron chelators. A significant increase in intracellular PpIX levels was observed in all cell lines tested when compared to the administration of ALA alone. The higher PpIX levels observed using the conjugates correlated well with the observed phototoxicity following exposure of cells to light. Passive diffusion appears to be the main mechanism for the majority of ALA-HPOs investigated. This study demonstrates that ALA-HPOs significantly enhance phototherapeutic metabolite formation and phototoxicity.

Study of hydrogen bonding in 1-ethyl-2-methyl-4-oxo-1,4-dihydro-pyridin-3-yloxyethanoic acid and 3-(1,2-diethyl-4-oxo-1,4-dihydropyridin-3-yloxy)propanoic acid by 1H NMR spectroscopy and X-ray crystallography

A series of 1,2-dialkyl-4-oxo-1,4-dihydropyridin-3-yloxyethanoic acids and 3-(1,2-dialkyl-4-oxo-1,4-dihydropyridin-3-yloxy)propanoic acids have been synthesized via the corresponding pyran-4-ones. Ring proton chemical shifts indicate hydrogen bonding between the side-chain carboxylic acid proton and the 4-oxo group of the heterocycle to be present only for the pyridinone species in DMSO solution. X-Ray crystallography demonstrates the hydrogen bonding in 1 -ethyl-2-methyl-4-oxo-1,4-dihydropyridin-3-yloxyethanoic acid to be very strong (O—O = 2.441 A) and intramolecular, giving a unique 8-membered chelate ring. The analogous hydrogen bonding in 3-(1,2-diethyl-4-oxo-1,4-dihydropyridin-3-yloxy)propanoic acid is however strong (O—O = 2.541 A) and inter-molecular, giving an infinite structure.

Structures of 3-hydroxy-1-(2-methoxyethyl)-2-methyl-4-pyridinone, its hydrochloride and 1-ethyl-3-hydroxy-2-methyl-4-pyridinone hydrochloride hydrate

3-Hydroxy-1-(2-methoxyethyl)-2-methyl-4(1H)-pyridinone (1) exists mainly in the quinoid form with a small contribution from the aromatic zwitterion form, while 3-hydroxy-1-(2-methoxyethyl)-2-methyl-4(1H)-pyridinone hydrochloride (2) and 1-ethyl-3-hydroxy-2-methyl-4(1H)-pyridinone hydrochloride hydrate (3) are observed to occur in the aromatic form with a minor contribution from the quinoid resonance form. Different substituents at the ring N position do not have significant geometric effects in either the neutral molecules, as a group, or in the hydrochloride salts.