Optimisation of chemical protein cleavage for erythropoietin semi-synthesis using native chemical ligation

Selective protein cleavage at methionine residues is a useful method for the production of bacterially derived protein fragments containing an N-terminal cysteine residue required for native chemical ligation. Here we describe an optimised procedure for cyanogen bromide-mediated protein cleavage, and ligation of the resulting fragments to afford biologically active proteins.     

Topical treatment with OGG1 enzyme affects UVB-induced skin carcinogenesis

Nonmelanoma skin cancer resulting from UVB exposure is a large and growing problem in the United States. Production of reactive oxygen species (ROS) during the UVB-induced inflammatory response results in the formation of oxidative DNA adducts such as 8-hydroxy-2-deoxyguanine (8-oxo-dG), which have been shown to contribute to the development of this cancer. The 8-oxoguanine DNA glycosylase (OGG1) enzyme repairs 8-oxo-dG adducts, suggesting that enhancing its activity in the skin might increase 8-oxo-dG repair thus preventing skin cancer development. We therefore used the SKH-1 murine model to examine the effect of topically applied OGG1 on UVB-induced skin cancer development. Mice were exposed three times weekly to UVB followed immediately by topical treatment with a formulation of liposome-encapsulated OGG1 enzyme for 25 weeks. While this treatment did not affect UVB-induced tumor multiplicity, it did reduce tumor size and dramatically reduced tumor progression, as indicated by tumor grade. These results suggest that oxidative DNA damage contributes to the progression of UVB-induced skin tumors and that a topical formulation containing OGG1, perhaps in conjunction with other DNA repair enzymes such as T4 endonuclease V, could be used in populations at high risk for skin cancer development.     

Synthesis and excited-state photodynamics of a chlorin-bacteriochlorin dyad--through-space versus through-bond energy transfer in tetrapyrrole arrays.

Understanding energy transfer among hydroporphyrins is of fundamental interest and essential for a wide variety of photochemical applications. Toward this goal, a synthetic free base ethynylphenylchlorin has been coupled with a synthetic free base bromobacteriochlorin to give a phenylethyne-linked chlorin-bacteriochlorin dyad (FbC-pe-FbB). The chlorin and bacteriochlorin are each stable toward adventitious oxidation because of the presence of a geminal dimethyl group in each reduced pyrrole ring. A combination of static and transient optical spectroscopic studies indicate that excitation into the Qy band of the chlorin constituent (675 nm) of FbC-pe-FbB in toluene results in rapid energy transfer to the bacteriochlorin constituent with a rate of approximately (5 ps)(-1) and efficiency of >99%. The excited bacteriochlorin resulting from the energy-transfer process in FbC-pe-FbB has essentially the same fluorescence characteristics as an isolated monomeric reference compound, namely a narrow (12 nm fwhm) fluorescence emission band at 760 nm and a long-lived (5.4 ns) Qy excited state that exhibits a significant fluorescence quantum yield (Phif=0.19). F?rster calculations are consistent with energy transfer in FbC-pe-FbB occurring predominantly by a through-space mechanism. The energy-transfer characteristics of FbC-pe-FbB are compared with those previously obtained for analogous phenylethyne-linked dyads consisting of two porphyrins or two oxochlorins. The comparisons among the sets of dyads are facilitated by density functional theory calculations that elucidate the molecular-orbital characteristics of the energy donor and acceptor constituents. The electron-density distributions in the frontier molecular orbitals provide insights into the through-bond electronic interactions that can also contribute to the energy-transfer process in the different types of dyads.     

New cyclic peptides via ring-closing metathesis reactions and their anti-bacterial activities

As part of a program investigating cyclic peptides with an internal aromatic hydrophobic scaffold as potential novel anti-bacterial agents, we explored the synthesis of simple tyrosine-based systems. These were prepared via key intermediates containing internal allylglycine and allyltyrosine residues for subsequent ring-closing metathesis reactions. Although the resulting anti-bacterial activity against Staphylococcus aureus was modest, this represents a novel and simple route to this class of compounds. One intermediate acyclic dipeptide precursor showed good activity against S. aureus with an MIC of 7.8 μg/mL.     

Intramolecular direct arylation in the synthesis of fluorinated carbazoles

The amination of 2-chloroanilines with aryl bromides and subsequent intramolecular direct arylation can be exploited in the synthesis of a range of fluorinated carbazoles, where the fluorine substituent can be introduced via the aniline, the aryl bromide or both substrates. Depending on substitution patterns, the two steps can either be performed in tandem in one-pot under microwave heating conditions or else require a two pot approach.     

Micellization of Alkyltriphenylphosphonium Bromides in Ethylene Glycol and Diethylene Glycol + Water Mixtures: Thermodynamic and Kinetic Investigation

The thermodynamics of micellization and other micellar properties of alkyl- (C₁₀-, C₁₂-, C₁₄- and C₁₆-) triphenylphosphonium bromides in water + ethylene glycol (EG) (0 to 30% v/v) mixtures over a temperature range of 298 to 318 K and cetyltriphenylphosphonium bromide in water + diethylene glycol (DEG) mixtures (0 to 30% v/v) at 298 K have been studied conductometrically. In all cases, an increase in the percentage of co-solvent results in an increase in the cmc values. On the basis of these results, the thermodynamic parameters, the Gibbs energy (ΔG _m^o), enthalpy (ΔH _m^o) and entropy (ΔS _m^o) of micellization have been evaluated. In addition to the conductivity measurements, kinetic experiments have also been done to determine the dependence of observed rate constant for the nucleophilic substitution reaction of p-nitrophenyl acetate and benzohydroxamate ions in the presence of the surfactant cetyltriphenylphosphonium bromide with a varying concentration of EG and DEG ranging from 0 to 50% v/v at pH=7.9 and 300 K. All of the reactions followed pseudo-first-order kinetics. An increase in the surfactant concentration results in an increase in the reaction rate and for a given surfactant concentration, the rate constant decreases as the concentration of co-solvent in the mixture increases. The kinetic micellar effects have been explained by using the pseudophase model. The thermodynamic and structural changes originating from the presence of solvents control the micellar kinetic effects.     

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2006 is provided. As with earlier installments in this series, the emphasis is on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure confirmation or stereochemical assignments.     

Gradient elution isotachophoresis with direct ultraviolet absorption detection for sensitive amino acid analysis

This work demonstrates coupling of the newly described electrophoretic enrichment technique of gradient elution isotachophoresis (GEITP) to a low-cost, conventional ultraviolet absorbance detector to realize sensitive measurements with a universal detector, eliminating the need for fluorescent analytes or derivatization. The effects of various parameters on enrichment were studied, including current density varied by leading electrolyte concentration, current density varied by applied electric field, and counter-flow acceleration across varying capillary inner diameters. Optimized parameters were applied to the enrichment and separation of the amino acids tryptophan (Trp) and tyrosine (Tyr). Limits of detection for Trp and Tyr were 51 and 215 nM, respectively, reflecting sensitivity enhancements of 860- and 1900-fold. Analysis times were less than 6 min, and peak height RSDs were less than 4%. A demonstration of enrichment and separation of these amino acids from artificial cerebrospinal fluid is additionally shown as a first step to realizing biochemical monitoring by GEITP.     

Stereostructure assignment of flexible five-membered rings by GIAO 13C NMR calculations: prediction of the stereochemistry of elatenyne

The stereochemistry of conformationally mobile five-membered rings is often hard to assign from NMR data, and [2,2']bifuranyl systems are even more challenging. GIAO (13)C NMR chemical shifts have been calculated for a series of [2,2']bifuranyl and pyranopyran species, taking into account their conformational flexibility using weighted averages of the data for all low energy conformers. We show that calculation of (13)C NMR chemical shifts using the geometries obtained using molecular mechanics greatly reduces the computational expense without a significant loss of accuracy, even in this demanding system. The results were sufficiently accurate to distinguish not only the pyran and furanyl isomers but also between all the diastereoisomeric forms. As a result of this validation, we predict the stereochemistry for the recently proposed revised structure of the natural product elatenyne, which contains a [2,2']bifuranyl core.     

N,N'-diarylureas: a new family of atropisomers exhibiting highly diastereoselective reactivity

2,6-disubstituted N-aryl ureas rotate slowly about their Ar-N bonds and can exist as separable atropisomers. They also react remarkably diastereoselectively, with the urea axis controlling new stereogenic centers with high fidelity in a variety of nucleophilic and electrophilic addition reactions. The sense of diastereoselectivity in lateral lithiation-electrophilic quench reactions is electrophile-dependent and appears to be the result of stereospecific reaction with one of two interconvertible diastereoisomeric organolithiums.