New developments in the synthesis of pyrrolizidinone-based dipeptide isosteres

1,3-Dipolar cycloaddition of acrylamide with the cyclic nitrone derived from proline tert-butyl ester has been employed in the synthesis of bicyclic Gly-(s-cis)Pro isosteres suitably protected for the Fmoc-based solid phase peptide synthesis. (R)-1-Phenylethylamine was introduced as chiral auxiliary to resolve racemic intermediates and obtain enantiopure compounds. Using methacrylamide as dipolarophile, the analogous Ala-Pro mimetics have been prepared in racemic form, whereas the same strategy applied to methyl itaconate failed to give the corresponding Asp-Pro mimetic.     

Ion chromatography characterization of polysaccharides in ancient wall paintings

Specific programming of automated HPLC systems allows total on-line qualification, validation and stability monitoring using the concept of deferred standards. Setting up such a process for routine analyses in an automated HPLC system requires specific autosampler programming as well as specific monitoring software. With an autosampler, a double injection procedure is programmed, the first introducing the sample, and the second, a few minutes deferred, the deferred control standard. Two additional compounds are therefore added to the sample before and during the chromatographic process: the intemal standard for sample quantification and the deferred standard for system control. Specific methodologies are described of how to obtain classical quantitative analysis information as well as system qualification validation stability information. Experiments were performed to develop specified methodologies to monitor the quality of quantitative analysis during the life of the column by using the deferred standard concept to probe the effects of column ageing on separation characteristics.     

Poly(4‐vinylpyridine) as the host ligand of metal‐containing chromophores for second‐order nonlinear optical active materials

Two formulas of grafted polymers with metal‐containing chromophores, potentially suitable for second‐order nonlinear optics applications, are described. Two chromophores were obtained from a tridentate ligand coordinated to Cu(II) or Pd(II) ions. The organometallic chromophore fragments were grafted to poly(4‐vinylpyridine) by the pyridinic nitrogen of the host polymer. Some qualities displayed by the new metallated polymers are remarkable: (1) a high value of the first hyperpolarizability coefficient of the chromophores, (2) a high content of the grafted chromophore in the polymers (up to 60 wt %), (3) a considerable increase in the glass‐transition temperatures (up to 240 °C), (4) good thermal stability in air (ca. 280 °C), and (5) good optical transparency of the films. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2987–2993, 2002     

Electrophoretic properties of complexes between DNA and the cationic surfactant cetyltrimethylammonium bromide

We use agarose gel electrophoresis to characterize how the monovalent catioinic surfactant cetyltrimethylammonium bromide (CTAB) compacts double-stranded DNA, which is detected as a reduction in electrophoretic DNA velocity. The velocity reaches a plateau at a ratio R = 1.8 of CTAB to DNA-phosphate charges, i.e., above the neutralization point, and the complexes retain a net negative charge at least up to R = 200. Condensation experiments on a mixture of two DNA sizes show that the complexes formed contain only one condensed DNA molecule each. These CTAB-DNA globules were further characterized by time-resolved measurements of their velocity inside the gel, which showed that CTAB does not dissociate during the migration but possibly upon entry into the gel. Using the Ogston-model for electrophoresis of spherical particles, the measured in-gel velocity of the globule is quantitatively consistent with CTAB having two opposite effects, reduction of both the electrophoretic charge and DNA coil size. In the case of CTAB the two effects nearly cancel, which can explain why opposite velocity shifts (globule faster than uncomplexed DNA) have been observed with some catioinic condensation agents. Dissociation of the complexes by addition of anionic surfactants was also studied. The DNA release from the globule was complete at a mixing ratio between anionic and cationic surfactants equal to 1, in agreement with equilibrium studies. Circular DNA retained its supercoiling, and this demonstrates a lack of DNA nicking in the compaction-release cycle which is important in DNA transfection and purification applications.     

The effects of cationic and zwitterionic micelles on the keto-enol interconversion of 2-phenylacetylfuran and 2-phenylacetylthiophene

The presence of micelles from cationic and zwitterionic surfactants increases the apparent acidity of either the keto and the enol forms of 2-phenylacetylfuran (2PAF) and 2-phenylacetylthiophene (2PAT). This effect can be attributed to the affinity of the surfactant micelles for the enolate of the two substrates. Although the equilibrium constants for keto-enol tautomerism of 2PAF and 2PAT, K_T=[enol]/[ketone]=pK_a^KH−pK_a^EH, do not change much, the presence of micelles provides an efficient method for producing appreciable quantities of the enolates under mild experimental conditions and in aqueous solutions. The obtained rate-profiles for the ketonisation reactions and the consistency of the kinetic rate constants over a wide range of ‘pH’ in several overlapping buffers indicate that the pH of the aqueous pseudophase (but not that at the micellar surface) can be controlled by buffers. Moreover, the increase of the acidity and the decrease of the ‘water’ rate of ketonisation of the enols of 2PAF and 2PAT upon addition of surfactants allow the uncovery of a metal ion catalysed pathway that cannot be observed in absence of surfactants.     

Binary and ternary chromium(III) complexes with cellular reductants and DNA components isolated from redox type systems. Part 1. Chromium(VI)–cysteine–adenine (adenosine,ATP)

The reduction of CrO₃ with an excess of L-cysteine and its interaction with DNA fragments (adenine, adenosine) and ATP nucleotide was studied by analysis of the isolated solid products. The precipitates were characterised by elemental analyses, FAB mass spectral data, spectroscopic methods (u.v.–vis., f.i.r., i.r.) and magnetic measurements. The Cr^III complexes obtained appear to be various Cr^III cysteinate and adenine or ATP (but not adenosine) ternary species of the addition type bound through the hydrogen bonding network.     

Second-generation mimics of ganglioside GM1 oligosaccharide: a three-dimensional view of their interactions with bacterial enterotoxins by NMR and computational methods

As a step to delineate a strategy of ligand design for cholera toxin (CT), NMR studies were performed on several mimics of the GM1 ganglioside oligosaccharide. The conformation of these analogues was investigated first in solution and then upon binding to cholera toxin by transferred nuclear Overhauser effect (TR-NOE) measurements. It was demonstrated that CT selects a conformation similar to the global minima of the free saccharides from the ensemble of presented conformations. No evidence of major conformational distortions was obtained, but one or two of the available conformers of the hydroxyacid side chain appear to be selected in the bound state. The NMR data were interpreted with the aid of computer models, generated and analyzed by using a combination of different approaches (MacroModels' MC/EM and MC/SD, Autodock, and GRID). Analysis of the NMR data supported by computational studies allowed us to interpret the experimental observations and to derive workable models of the ligand:toxin complexes. These models suggest that the higher affinity of the (R)-lactic acid derivative 3 may stem from lipophilic interactions with a hydrophobic area in the toxin binding site located in the vicinity of the sialic acid side chain binding region of the CT:GM1 complex, and formed by the side chain of Ile-58 and Lys-34. Thus, the models obtained have allowed us to make useful design suggestions for the improvement of ligand affinity.     

Synthesis, structure and conformation of partially-modified retro- and retro-inverso psi[NHCH(CF3)]Gly peptides

Partially modified retro- (PMR) and retro-inverso (PMRI) psi[NHCH(CF(3))]Gly peptides, a conceptually new class of peptidomimetics, have been synthesized in wide structural diversity and variable length by aza-Michael reaction of enantiomerically pure alpha-amino esters and peptides with enantiomerically and geometrically pure N-4,4,4-trifluorocrotonoyl-oxazolidin-2-ones. The factors underlying the observed moderate to good diastereocontrol have been investigated. The conformations of model PMR-psi[NHCH(CF(3))]Gly tripeptides have been studied in solution by (1)H NMR spectroscopy supported by MD calculations, as well as in the solid-state by X-ray diffraction. Remarkable stability of turn-like conformations, comparable to that of parent malonyl-based retropeptides, was evidenced, as a likely consequence of two main factors: 1) severe torsional restrictions about sp(3) bonds in the [CO-CH(2)-CH(CF(3))-NH-CH(R)-CO] module, which is biased by the stereoelectronically demanding CF(3) group and the R side chain; 2) formation of nine-membered intramolecularly hydrogen-bonded rings, which have been clearly detected both in CHCl(3) solution and in some crystal structures. The former factor seems to be more important, as turn-like conformations were found in the solid-state even in the absence of intramolecular hydrogen bonding. The relative configuration of the -C*H(CF(3))NHC*H(R)- stereogenic centers has a major effect on the stability of the turn-like conformation, which seems to require a syn stereochemistry. X-ray diffraction and ab initio computational studies showed that the [-CH(CF(3))NH-] group can be seen as a sort of hybrid between a peptide bond mimic and a proteolytic transition state analogue, as it combines some of the properties of a peptidyl -CONH- group (low NH basicity, CH(CF(3))-NH-CH backbone angle close to 120 degrees, C-CF(3) bond substantially isopolar with the C=O) with some others of the tetrahedral intermediate [-C(OX)(O(-))NH-] involved in the protease-mediated hydrolysis reaction of a peptide bond (high electron density on the CF(3) group, tetrahedral backbone carbon).     

Site-selective labeling strategies for screening by NMR

NMR based screening has become an important tool in the pharmaceutical industry. Methods that provide information on the location of small molecule binding sites on the surface of a drug target (e. g. SAR-by-NMR and related techniques) are of particular interest. In order to extend the applicability of such techniques to drug targets of higher molecular weight, selective labeling strategies may be employed. Dual-amino acid selective labeling and site directed non-native amino acid replacement (SNAAR) allow for the selective detection of NMR resonances of a specific amino acid residue. This results in significantly reduced spectral complexity, which not only enables application to higher molecular weight systems, but also eliminates the need for sequential resonance assignment in order to identify the binding site. Regio-selective (or segmental) labeling of an entire protein domain of a multi domain protein may also be achieved. Labeling only a selected part of a multi domain protein (e. g. a catalytic or ligand binding domain) is an attractive way to simplify the spectral interpretation without disturbing the system under study.     

Solvent-free Mukaiyama aldol reaction of O-silyl dienolates catalyzed by benzoic acid

The vinylogous aldol reaction of O-silyl dienolates deriving from 2,2-dimethyl-[1,3]-dioxin-4-ones proceeds in moderate to excellent yields in the presence of catalytic amounts of PhCOOH under solvent-free conditions. Modest to good yields can be obtained by using silica gel or 3 Å molecular sieves as heterogeneous catalysts.