New organotin(IV) complexes with l-Arginine, Nα-t-Boc-l-Arginine and l-Alanyl-l-Arginine: Synthesis, structural investigations and cytotoxic activity

Novel diorganotin(IV) derivatives of l-Arginine (HArg), N_α-(tert-Butoxycarbonyl)-l-Arginine (Boc-Arg-OH) and l-Ala-l-Arg (H₂Ala-Arg), H₂NC(NH)NH(CH₂)₃CH(NHR′)CO₂H, where R′ = H in HArg, R′ = C(O)OC(CH₃)₃ in Boc-Arg-OH, R′ = H₂NCH(CH₃)CO in H₂Ala-Arg and triorganotin(IV) derivatives of Boc-Arg-OH have been synthesized and structurally characterized. The complexes were investigated by FT-IR and ¹¹⁹Sn Mössbauer in the solid state and by ¹H, ¹³C, ¹¹⁹Sn and ¹H-¹H COSY NMR spectroscopy, in solution. The spectroscopic characterization leading to the proposed molecular structures was accomplished on the basis of these experiments. l-Arginine appears to behave as a chelating ligand through carboxylate and -NH₂ groups in Me₂Sn(Arg)₂, while in N_α-t-Boc-l-Arginine complex, the N_α-protected amino group being exempted from coordination, only the carboxylate groups are effectors of bonding to the organometallic moieties. FT-IR spectra give a clear indication that guanidino groups in all the complexes are not involved in coordination, since ν(CN-H) frequency of the terminal guanidino group is fairly constant and unshifted relative to the free ligand. The biological activity of organotin(IV)-complexes was also investigated by use of human HT29 colorectal carcinoma cells. The cytotoxic activity of the compounds was determined by the MTT quantitative colorimetric assay, capable of detecting viable cells in comparison with that exerted by cisplatin. A marked cytotoxic activity for nearly all complexes, is evident being higher than that exerted by cisplatin, while no significant improvement of activity was observed for Me₂Sn(Arg)₂ and Me₂Sn(Ala-Arg), which was confirmed by IC₅₀ values. Then, we assessed whether the cytotoxicity induced by organotin(IV) complexes was associated with the induction of apoptosis. Light microscopy analysis, performed to study the morphological changes induced in HT29 cells, confirmed the results obtained with MTT test. No significant morphological alterations were observed in HT29 cells after treatment with Me₂Sn(Ala-Arg) and Me₂Sn(l-Arg)₂. Cells treated with ⁿBu₂Sn(Boc-Arg)₂, ⁿBu₂Sn(Ala-Arg), ⁿBu₃Sn(Boc-Arg) and Me₃Sn(Boc-Arg), appeared rounded, isolated and detached from culture substrate, indicating the commitment to apoptotic cell death.     

Structure-based study of antiamyloidogenic cyclic d,l-α-peptides

Protein misfolding and aggregation are the hallmarks of many devastating diseases. We have previously shown that cyclic d,l-α-peptide CP-2 reacts and stabilizes less toxic forms of amyloid β (Aβ), and protects the cells from Aβ-induced toxicity. Here, we performed extensive structure-based studies on CP-2 to elucidate the contribution of each residue to the total antiamyloidogenic activity and determine the interactions that are involved between CP-2 and Aβ. We showed that the hydrophobicity of CP-2 analogs correlates with their antiamyloidogenic potency, however, aromatic interactions are even more important for this activity. The antiamyloidogenic activity of CP-2 analogs also correlates with their ability to self-assemble, as shown by the critical micelle concentration measurements. The cell survival studies performed on rat pheochromocytoma PC-12 cells suggest that incorporation of an additional aromatic residue to the CP-2's sequence increases its protective effect against Aβ42-induced toxicity.     

Direct access to new β-d-galactofuranoconjugates: application to the synthesis of galactofuranosyl-l-cysteine and l-serine

Galactofuranose post-translational modifications, although quite rare, were detected in some biomolecules produced by parasites. While hexopyranosides were already linked to various peptides and proteins, few hexofuranosides have been artificially conjugated to amino acids. We thus report herein a robust glycosylation methodology to obtain S-alkyl, O-serine and S-cysteine-β-d-galactofuranosides starting from readily available galactofuranose donors. O-Acetyl, thioimidoyl and acetimidoyl donors were compared in terms of yields and selectivity when reacted with mercaptans, l-cysteine and l-serine. Acetimidates turned out to be the best notably for amino acids glycosylation.     

Blends as a strategy towards tailored hydrolytic degradation of poly(?-caprolactone-co-d,l-lactide)-poly(ethylene glycol)-poly(?-caprolactone-co-d,l-lactide) co-polymers

Binary blends were prepared from poly(?-caprolactone) (PCL), and P(CL-co-d,l-lactic acid)-P(ethylene glycol)-P(CL-co-d,l-lactic acid) co-polymers, where the d,l-LA content in the side chains varied from 0 to 70 mol%. Blend discs were fabricated by melt-molding, and the effect of blend composition on hydrolytic degradation was studied. Variations in medium pH were monitored, and morphological changes were observed using scanning electron microscopy. Blending of these co-polymers was found to constitute a simple means by which intermediate rates of water absorption and mass loss were obtained, compared to those observed in pure co-polymer preparations. In one of the blends, prepared from the two components containing 70 or 0 mol% d,l-LA in the side chains and thereby exhibiting large differences in degradation rate, hydrolysis resulted in the formation of a porous material over time. Furthermore, all blend samples maintained their initial shape throughout the study. Such materials may be interesting for further investigations for applications in cellular therapy and controlled release.     

l-Galactose as a natural product: isolation from a marine octocoral of the first α-l-galactosyl saponin

Saponine 1 (6′-O-acetyl-3β-pregna-5,20-dienyl-α-l-galactopyranoside), that contains a l-galactose moiety linked to the aglycone through an infrequent α-glycosidic bond, has been isolated from the marine octocoral Muricea c.f. purpurea. This constitutes the first report on the occurrence of l-Gal as a nonpolymeric natural product. A CD procedure for the absolute stereochemical assignment of saponins, based on the CD analysis of its perbenzoylated derivative, is proposed.     

Increasing the inhibitory potency of l-arabino-imidazolo-[1,2]-piperidinose towards β-d-glucosidase and β-d-galactosidase

The synthesis of some potent inhibitors of two retaining β-glycosidases was achieved by introducing aglycon-mimics into the imidazole moiety of l-arabino azasugar 1. The strongest inhibition was observed with the phenyl-ethyl substituent at C(2) of 1 against β-d-galactosidase and β-d-glucosidase, whereas the hydroxymethyl group at C(2) increased only slightly the inhibitory properties.     

A practical synthesis of fully protected l-γ-carboxyglutamic acid

We have developed a new synthetic route for the preparation of Fmoc protected l-γ-carboxyglutamic acid in 60% overall yield (>99% ee) via a six-step synthesis from d-Garner’s aldehyde. An aldol condensation and the selective cleavage of the acetonide protective group are key steps.     

Synthesis of chiral thiourea–phosphine organocatalysts derived from l-proline

A novel class of thiourea–phosphine was prepared from l-proline as a chiral renewable resource. The original structure of the chiral framework offers an interesting potential to the construction of bifunctional organocatalysts for asymmetric transformations.     

Decomposing or subliming? An investigation of thermal behavior of l-leucine

The thermal behavior of l-leucine under inert conditions was investigated by TGA, FTIR and TG-FTIR. The TG results showed that only one mass loss stage of more than 99% happened when l-leucine was under program heating with temperature ranging from 30 to 600 °C. The apparent activation energy, pre-exponential factor and the most probable model function were obtained by using of master plots method. The results of kinetic study showed that the decrease in mass of l-leucine was due to subliming rather than decomposing. And this was proved by the FTIR spectrum analysis and the directly observed subliming phenomenon. The results of TG-FTIR experiments showed that there was only one stage of decomposition process that happened after the subliming of leucine. The gas products were CO₂, NH₃, CO and some organic compounds such as 3-methyl-1-butanamine, and the main primary decomposition was decarboxylation.     

Very short OH?hydrogen bond in l-histidinium difluoride

The crystal structure of C₆H₁₁N₃O₂²⁺·2F²⁻ is reported. The structure contains a fluoride ion strongly H-bonded to a carboxylic O atom, a rare, very strong, hydrogen bond. The donor-acceptor distance is 2.3818(10) Å, the shortest value reported to date, considerably less than the sum of the van der Waals radii of the atoms implicated, as expected from a very strong hydrogen bond. The di-cation has an open conformation. There is an extensive H-bonding network between anions and cations assembling rings on the ac plane and chains in several directions. Two extra intermolecular interactions of the type CH?π are found, exhausting the aromatic π electron system ability to act as a proton acceptor.     

Synthesis of d-fructopyranosides with 2-O-acyl-β-d-fructopyranosides

Glycosylation of 2-O-acyl fructopyranosides was investigated, which were shown to be effective glycosyl donors for d-fructopyranoside synthesis with good β-selectivity and yields. For bulky acceptor 4e, α-anomer 5e was obtained with α/β = 65:23. Unexpected ring-opening was observed during acetylation of 9, indicating the sensitivity of the fructopyranosyl ring.     

Entry to nitrogen-containing heterocycles by based-promoted heterocyclization on allenylamides of l-α-aminoacids

Allenylamides of Boc-protected α-aminoacids easily gave in basic medium heterocyclic products arising from attack of the NH group on the inside C-C double bond of the 1,2-diene moiety, namely imidazolidinones, pyrazinones, and a pyrrole compound. The microwave-assisted heterocyclization occurred cleanly at C-β of the allenyl group with formation of pyrazin-2-ones having an endo- or exocyclic double bond.     

Synthesis and RNA-selective hybridization of α-l-ribo- and β-d-lyxo-configured oligonucleotides

Three α-l-ribofuranosyl analogues of RNA nucleotides (α-l-RNA analogues) have been synthesized and incorporated into oligonucleotides using the phosphoramide approach on an automated DNA synthesizer. The 4′-C-hydroxymethyl-α-l-ribofuranosyl thymine monomer was furthermore synthesized. Relative to the unmodified duplexes, incorporation of a single α-l-RNA monomer into a DNA strand leads to reduced thermal stability of duplexes with DNA complements but unchanged thermal stability of duplexes with RNA complements, whereas incorporation of more than one α-l-RNA monomer lead to moderately decreased thermal stability also of duplexes with RNA complements. Efficient hybridization with an RNA complement and no melting transition with a DNA complement were observed with stereoregular chimeric oligonucleotides composed of a mixture of α-l-RNA and affinity enhancing α-l-LNA monomers (α-l-ribo-configured locked nucleic acid). Furthermore, duplexes formed between oligodeoxynucleotides containing an α-l-RNA monomer and complementary RNA were good substrates for Escherichia coli RNase H. RNA-selective hybridization was also achieved by the incorporation of 1-(4-C-hydroxymethyl-β-d-lyxofuranosyl)thymine monomers into a DNA strand, whereas stable duplexes were formed with both complementary DNA and RNA when these monomers were incorporated into an RNA strand.     

Supramolecular structures from self-assembled poly(γ-benzyl-l-glutamate)-polydimethylsiloxane-poly(γ-benzyl-l-glutamate) triblock copolypeptides in thin films

We describe the self-assembly of A-B-A triblock copolymers in thin films composed of a soft polydimethylsiloxane (PDMS) central block (B) and two polypeptidic (A) blocks, poly(γ-benzyl)-l-glutamate (PBLG). The PBLG segment exhibits depending on the chain length two distinct secondary conformations either a β-sheet or a α-helical conformation. The direct relationship between the surface morphology and the secondary conformation of the polypeptide segment has been evidenced by atomic force microscopy. For chain lengths below 20 U the polypeptide segments adopt preferentially a β-sheet secondary structure and the triblock copolymer self-assembled in fibers. Moreover, the fiber diameters increased with the chain length of the triblock copolymer. For chain lengths above 20, the α-helical structure is stabilized and a lamellar morphology is formed driven by rod-rod interactions in spite of the very asymmetric composition of the triblock copolymer. However, decreasing the film thickness from 25 to 8 nm, i.e., below the L/2 and due to the preferential attraction of the polypeptide block for the hydrophilic substrate employed, instead of a lamellar morphology a rod-like morphology could be found. Thus, the use of hybrid block copolymer containing polypeptides with particular secondary structures offers novel alternatives to control the self-assembly in thin films compared to traditional amorphous block copolymers.     

An enantioselective nucleophilic addition of α,β-unsaturated trifluoromethylketones catalyzed by l-proline derivatives

An unexpected enantioselective 1,2-aldol reaction of acetone with α,β-unsaturated trifluoromethylketone catalyzed by l-proline derivative was described. The absolute configuration of the resulting chiral product was assigned based on a single crystal X-ray diffraction analysis. Structure-reactivity study of this organocatalytic system was briefly discussed. A reaction mechanism was tentatively postulated.     

Enantioselective epoxidation of α,β-unsaturated ketones catalyzed by stapled helical l-Leu-based peptides

Stapled helical l-leucine-based heptapeptides were synthesized and used as catalysts for the enantioselective epoxidation of α,β-unsaturated ketones. All N-terminal free stapled peptides were successfully used as chiral catalysts. Among them, the use of H-hS₃,₇hS-10 gave epoxide products with high enantioselectivities of up to 99% ee. Furthermore, the dominant conformations of the N-terminal protected stapled peptides R₃,₇R-10 and hS₃,₇hS-10 were investigated by ¹H NMR, IR, CD spectra, and X-ray crystallographic analysis. The peptide R₃,₇R-10 formed a right-handed (P) α-helix in solution and in the crystalline state, while hS₃,₇hS-10 formed a right-handed (P) 3₁₀-helix in solution.     

Synthesis and anti-HIV activity of l-β-3′-C-cyano-2′,3′-unsaturated nucleosides and l-3′-C-cyano-3′-deoxyribonucleosides

An efficient synthetic method was developed for l-β-3′-C-cyano-2′,3′-unsaturated nucleosides and l-3′-C-cyano-3′-deoxyribonucleosides. The key intermediate 11 was obtained from l-xylose, from which a series of pyrimidine and purine nucleosides were prepared in high yield by the coupling of 11 and various silyl-protected bases in the presence of TMSOTf. These nucleosides were eliminated, followed by deprotecting to give l-β-3′-C-cyano-2′,3′-unsaturated nucleosides. When selectively deprotected by hydrazine hydrate in buffered acetic acid-pyridine followed by treatment with potassium carbonate in methanol, l-3′-C-cyano-3′-deoxyribonucleosides were obtained. The synthesized nucleosides were tested for anti-HIV activity.     

Enzymatic synthesis of ω-carboxy-β-hydroxy-(l)-α-amino acids

Commercially available ω-carboxy-aldehydes and glycine have been subjected to the catalytic action of an l-threonine aldolase from Escherichia coli to give the corresponding β-hydroxy-α-(l)-amino acids as a mixture of erythro/threo epimers.Specifically, the reaction with glyoxylic acid (2) gave the epimeric β-hydroxy-(l)-aspartates (t,e)-9 that could be isolated by ion-exchange chromatography in 67% yield. Following esterification and N-Boc protection, the two epimers could be isolated as pure compounds.Similarly, the aldolase-catalyzed addition of glycine to succinic semialdehyde (4) gave the expected mixture of β-hydroxy-l-α-aminoadipic acids (t)-12 and (e)-12 in 34% yield.     

Effect of partial crosslinking on morphology and properties of the poly(β-hydroxybutyrate)/poly(d,l-lactic acid) blends

Poly(β-hydroxybutyrate) (PHB) is a bio-based and biodegradable aliphatic polyester, however its application is limited by some disadvantages such as high price, brittleness, poor processability and low melt-strength due to serious thermal degradation. Partial crosslinking initiated by dicumyl peroxide (DCP) was applied in this work to improve the performance of poly(β-hydroxybutyrate)/poly(d,l-lactic acid) (PHB/PDLLA) blends. The partial crosslinking of the blends and its effect on the properties, morphology, rheology and thermal behavior of the blends were investigated. The tensile strength and impact toughness of the PHB were increased by incorporation of the PDLLA, which were improved further after the partial crosslinking because of an increased compatibility between the PHB and the PDLLA phases. The rheological study revealed that the storage modulus (G′) and complex viscosity (η*) of the blends were increased after addition of the DCP. On the other hand, the crystallization of PHB in the blends was restricted to a certain extent by the formation of partially crosslinked network while its crystal form was not modified.     

Solvent-induced morphological diversification in poly(l-lactide-b-?-caprolactone) block copolymer thin films

Biodegradable block copolymer of poly(l-lactide-b-?-caprolactone) (P(LA-b-CL)) was dissolved in various solvents with different solubility as well as volatility, and spin-cast on a highly oriented pyrolytic graphite (HOPG) to prepare thin films. The surface morphologies were observed by using atomic force microscopy (AFM) in a dynamic force (tapping) mode. Particle like morphology was found in the thin films prepared form the dichloromethane and acetone. Higher volatility of dichloromethane and acetone resulted in the reflection of the particle like objects in the solution to HOPG substrate. In contrast, the P(LA-b-CL)s in toluene and 1,4-dioxane exhibited different morphologies compared to those in dichloromethane and acetone. Lower volatility of toluene and 1,4-dioxane assisted the epitaxial crystallization of PCL component along the HOPG lattice, that was revealed by enzymatic degradation of PLLA component by proteinase K. Thus, adjusting the solubility and solvent volatility for the film formation provided morphological divergence of the P(LA-b-CL) block copolymer, and this technique would be applicable for the surface patterning of biodegradable polymers.