Double-helical dinuclear bis(dipyrromethene) complexes formed by self-assembly

Bis(dipyrromethene) ligands linked by an alkyl spacer between beta and beta' positions are shown to give helical dimers or monomers, dependent upon the length of the alkyl linker, upon complexation. Ligands consisting of methylene, ethylene, and propylene linkers -(CH(2))(n)()- (n = 1, 2, and 3) give helical dimers, while longer linking chains (n = 4, 5, or 6) give monomers or mixtures of dimers and monomers. X-ray crystal structures of the dimeric zinc complexes (n = 1, 2, and 3) reveal that the angles between dipyrromethene planes and the extent of helicity in the complexes differ as the length of the linker varies. The extent of helicity was assessed and found to be dependent upon the length and, specifically, the conformational preferences of the alkyl spacer unit. The presence of an ethylene linker gave complexes of greatest helicity. The use of a methylene spacer gave less helical structures upon complexation, while propylene spacers gave only slightly helical complexes. Our studies identify the crucial importance that the conformational preferences of the beta-beta' alkyl spacer group plays in the coordination algorithm of self-assembly to form dipyrromethene based complexes.     

PHOTOCHEMISTRY OF MELANIN PRECURSORS: DOPA, 5-S-CYSTEINYLDOPA AND 2,5-S,S'-DICYSTEINYLDOPA

The photochemistries of the melanin precursors dopa, 5-S-cysteinyldopa (5-SCD) and 2.5-S,S'-dicysteinyldopa (2.5-SCD) were investigated by 265-nm laser flash photolysis. The quantum yield of hydrated electron following flash photolysis of dopa (9.1%) was half the yield of dopasemiquinone (19.6%), implying that dopasemiquinone is formed via two primary photochemical mechanisms: photionisation (giving e) or photohomolysis (giving H^˙). Dopasemiquinone rapidly disproportionates to form dopaquinone and re-form dopa. Dopaquinone in turn decays via a base-catalysed unimolecular cyclisation eventually to form dopachrome. Assignment of the transient species was confirmed by previous pulse radiolysis studies of the one-electron oxidation of dopa. In contrast, flash photolysis of the cysteinyldopas, 5-SCD and 2,5-SCD results in lower photoionisation quantum yields and the production of initial transient species whose absorption spectra were markedly different from their semiquinone absorption spectra previously determined pulse radiolytically. These observations indicate that the primary cysteinyldopa photochemical species is not such a semiquinone, but rather results from S-CH₂ bond photohomolysis. Absorption spectra and rate constants for the formation and decay of various transient species are reported.     

Spectrophotometric determination of phosphate by extraction of reduced molybdoantimonylphosphoric acid with acetophenone-chloroform

A 90% ($\text{v}\text{v}$) mixture of acetophenone-chloroform is an effective extractant for reduced molybdoantimonylphosphoric acid. The extraction is quantitative over the acidity range of 0.04 to 4.8 N sulfuric acid. The mixed solvent extractant is virtually immiscible with water and can be used at aqueous/organic volume ratios from 1:1 to 40:1 with no loss of solvent or decrease in % extraction. Phosphate is easily determined from 5 to 1000 ppb. No interference from silicate was experienced.     

In vitro high throughput screening of compounds for favorable metabolic properties in drug discovery

Drug metabolism can have profound effects on the pharmacological and toxicological profile of therapeutic agents. In the pharmaceutical industry, many in vitro techniques are in place or under development to screen and optimize compounds for favorable metabolic properties in the drug discovery phase. These in vitro technologies are meant to address important issues such as: (1) is the compound a potent inhibitor of drug metabolising enzymes (DMEs)? (2) does the compound induce the expression of DMEs? (3) how labile is the compound to metabolic degradation? (4) which specific enzyme(s) is responsible for the compound's biotransformation? and (5) to which metabolites is the compound metabolized? Answers to these questions provide a basis for judging whether a compound is likely to have acceptable pharmacokinetic properties in vivo. To address these issues on the increasing number of compounds inundating the drug discovery programs, high throughput assays are essential. A combination of biochemical advances in the understanding of the function and regulation of DMEs (in particular, cytochromes P450, CYPs) and automated analytical technologies are revolutionizing drug metabolism research. Automated LC-MS based metabolic stability, fluorescence, radiometric and LC-MS based CYP inhibition assays are now in routine use. Automatible models for studying CYP induction based on enzyme activity, quantitative RT-PCR and reporter gene systems are being developed. We will review the utility and limitations of these HTS approaches and highlight on-going developments and emerging technologies to answer metabolism questions at the different stages of the drug discovery process.