Comblike poly(alpha-alkyl gamma-glutamate)s: computer simulation studies of an intermediate thermal phase

Monte Carlo (MC) simulations have been used to study the structure of an intermediate thermal phase of poly(alpha-octadecyl gamma,D-glutamate). This is a comblike poly(gamma-peptide) able to adopt a biphasic structure that has been described as a layered arrangement of backbone helical rods immersed in a paraffinic pool of polymethylene side chains. Simulations were performed at two different temperatures (348 and 363 K), both of them above the melting point of the paraffinic phase, using the configurational bias MC algorithm. Results indicate that layers are constituted by a side-by-side packing of 17/5 helices. The organization of the interlayer paraffinic region is described in atomistic terms by examining the torsional angles and the end-to-end distances for the octadecyl side chains. Comparison with previously reported comblike poly(beta-peptide)s revealed significant differences in the organization of the alkyl side chains.     

Pharmacokinetic data of propranolol enantiomers in a comparative human study with (S)- and (R,S)-propranolol

The pharmacokinetics of (S)-propranolol were compared after the oral administration of a 40 mg dose of the pure enantiomer and an 80 mg dose of a racemic mixture of (R,S)-propranolol. The results of this study indicate that the bioavailability of (S)-propranolol, as expressed by the mean area under the concentration-time curve (AUC) and maximum serum concentration, is lower after 40 mg of the optically pure drug than after the racemic drug.     

Synthesis and HPLC enantioseparation of the cyclopropane analogue of valine (c3Val)

A new and efficient method is presented for the preparation of the N-Boc-protected cyclopropane analogue of valine, 1-(N-tert-butoxycarbonyl)amino-2,2-dimethylcyclopropanecarboxylic acid, both in racemic and enantiomerically pure forms. Cyclopropanation of the exocyclic double bond of 2-phenyl-4-isopropylidene-5(4H)-oxazolone with diazomethane followed by elaboration of the heterocyclic moiety provided multigram quantities of the racemic target compound. Subsequent HPLC resolution of a racemic precursor on a noncommercial chiral stationary phase has given access to enantiomerically pure products. Almost 1.5 g of the first-eluted enantiomer and 1.0 g of the second-eluted enantiomer have been isolated in optically pure form using a 150 x 20 mm ID column containing mixed 10-undecenoate/3,5-dimethylphenylcarbamate of cellulose covalently bonded to allylsilica gel with a mixture of hexanes/tert-butyl methyl ether/ethyl acetate as the mobile phase.     

A scalable synthesis of (+)-coronafacic acid

A facile, efficient, and scalable synthesis of optically pure coronafacic acid by resolution of racemic coronafacic acid obtained using an improved version of Watson's method has been developed. By optimizing the boron-mediated aldol reaction of Watson, we were able to prepare 2.1 g of racemic coronafacic acid. This was coupled with (S)-4-isopropyl-2-oxazolidinone to give a mixture of diastereomeric coronafacyl oxazolidinones, which were readily separable by silica-gel column chromatography to give 630 mg of optically pure (+)-coronafacic acid.     

First synthesis of the two enantiomers of alpha-methyldiphenylalanine [(alphaMe)Dip] by HPLC resolution.

A strategy for the preparation of enantiomerically pure (R)- and (S)-alpha-methyldiphenylalanine, constrained phenylalanine analogs, is described. A racemic precursor was prepared in high yield from easily available starting products and subjected to HPLC resolution on a noncommercial chiral stationary phase. More than 600 mg of each enantiomer was isolated in optically pure form by using a 150 x 20 mm ID column containing mixed 10-undecenoate/3,5-dimethylphenylcarbamate of cellulose covalently bonded to allylsilica gel and a mixture of n-hexane/2-propanol/acetone as the mobile phase.     

Synthesis and enantioselective rearrangement of (Z)-4-triphenylmethoxy-2,3-epoxybutan-1-ol enantiomers

Efficient enzyme catalyzed kinetic resolutions of a synthetically useful chiral building block, (Z)-4-triphenylmethoxy-2,3-epoxybutan-1-ol, are reported. The highest selectivities were achieved by Lipozyme TL IM and Amano Lipase PS enzymes in the presence of vinyl acetate. Enantiomeric enrichment of the optically active acetate isomer was accomplished by selective crystallization of the racemic part of the enantiomeric mixture. Enzyme catalyzed hydrolysis of the acetate also provided an optically pure epoxybutanol derivative. O-Benzylation of (+)-(Z)-1-hydroxy-4-triphenylmethoxy-2,3-epoxybutane followed by super base promoted diastereo- and enantio-selective rearrangement resulted in (+)-(2R,3R,1'R)-3-[1-hydroxy-2-(triphenylmethoxy)ethyl]-2-phenyloxetane in >98% ee and de. Configurations of the new optically active products were determined by chemical correlation.     

Thermal decomposition of fungal poly(beta,L-malic acid) and Poly(beta,L-malate)s

The thermal decomposition of poly(beta,l-malic acid), poly(alpha-methyl beta,l-malate), and ionic complexes of the polyacid with alkyltrimethylammonium salts was studied by TGA, GPC, and FTIR and NMR spectroscopy. It was found that poly(beta,l-malic acid) depolymerized above 200 degrees C by an unzipping mechanism with generation of fumaric acid which is then partially converted in a mixture of maleic acid and anhydride. On the contrary, random scission of the main chain was found to happen in the thermal decomposition of poly(alpha-methyl beta,l-malate). On the other hand, ionic poly(beta,l-malate)s degraded through a well defined three-stage process, the first one being depolymerization of the poly(malate) main chain along with decomposition of the ionic complex. Decomposition of the previously generated alkyltrimethylammonium salts followed by unspecific cracking of the resulting nitrogenated compounds happened at higher temperatures. Mechanisms partially explaining the decomposition processes of the three studied systems were proposed according to collected data.     

Molecular mobility and phase structure of biodegradable poly(butylene succinate) and poly(butylene succinate-co-butylene adipate)

Molecular mobility and phase structure of biodegradable poly(butylene succinate) (PBS) and poly(butylene succinate-co-20 mol % butylene adipate) [P(BS-co-20 mol % BA)] have been investigated by high-resolution solid-state (13)C NMR. For both samples, two components with different (13)C spin-lattice relaxation time (T(1C)) values have been observed in the crystalline region. The crystalline component with shorter T(1C) value is assignable to the interface near amorphous phase. The crystalline component with longer T(1C) value is ascribed to the inside of the crystalline region. On the basis of T(1C), it has been concluded that the BA units are not included in the crystalline region of P(BS-co-20 mol % BA). Molecular mobility and higher-ordered structure of amorphous phase have been also compared between the melt and solid state. Variable-temperature high-resolution (13)C NMR measurements for the amorphous phase have revealed the remarkable difference in dynamics and structure between the melt and solid state.     

Kinetically controlled optical resolution of racemic norbornene aldehyde derivatives

A new method for obtaining optically pure 5-norbornene 2-endo-aldehyde derivatives was developed. The reaction of a diastereomeric mixture of the ene acetals 2 and 2', derived from racemic norbornene aldehydes (+/-)-1 and chiral nonracemic (S,S)-hydrobenzoin 7, with NBS (0.5-0.6 eq.) in the presence of H(2)O proceeded in a kinetically controlled manner to give the optically pure hydroxy aldehydes 3 along with the intact ene acetals 2'. Both compounds 3 and 2' were converted into the optically pure norbornene aldehydes 1 and ent-1, respectively. This method opens the way to produce various types of 5-norbornene 2-endo-aldehydes with 3-exo- or 3-endo-substituents in optically pure forms.     

Crystalline/amorphous phase structure and molecular mobility of biodegradable poly(butylene adipate-co-butylene terephthalate) and related polyesters

Differential scanning calorimetry (DSC), atomic force microscopy (AFM), wide-angle X-ray scattering (WAXD), and solid-state (13)C NMR have been used to investigate the crystalline/amorphous structure and molecular mobility of biodegradable poly(butylene adipate-co-44 mol % butylene terephthalate) [P(BA-co-44 mol % BT)] copolyester sample crystallized from the melt. The DSC endothermic peak, which is ascribed to the melting of the crystalline region, was broad relative to those reported for conventional partially crystalline polyesters. In AFM observation, spherulitic morphology was not observed while small particles with a size of about 100 nm were detected. The WAXD pattern of the sample was very broad. These results have indicated that a melt-crystallized P(BA-co-44 mol % BT) sample contains small crystals with a wide distribution in size. A solid-state (13)C NMR technique was also used to perform molecular-level and selective analyses for both butylene terephthalate and butylene adipate units. For the butylene terephthalate units, the existence of two components with different microstructure and molecular mobility was detected: one component was assigned to the alpha-form crystal of poly(butylene terephthalate) homopolymer (PBT) and the other was in amorphous regions. In contrast, all of butylene adipate units were located in amorphous regions. Solid-state NMR data have suggested that sizes of crystalline regions are less than 3 nm.     

Preparation of an enantiomerically pure, highly porous metalloorganic crystal

We report the synthesis and the structure determination of tris-(4,5-diazo-spiro-bifluorene)ruthenium(II) chloride, a chiral building block whose racemic mixture solution spontaneously resolves and forms two crystalline, enantiomerically pure, porous networks composed exclusively of the Λ or Δ atropisomers. The extended chiral channels are occupied by water molecules (approximately 20% by weight) and the chloride counter-ions.     

Cocrystallization model for synthetic biodegradable poly(butylene adipate-co-butylene terephthalate)

Synthetic biodegradable poly(butylene adipate-co-butylene terephthalate), P(BA-co-BT), with 56 mol % butylene adipate, BA, was characterized by solid-state NMR spectroscopy, thermal analysis, X-ray diffraction, computer modeling, and polarization microscopy. The NMR study showed the presence of BA and butylene terephthalate, BT. T(1C) NMR measurements proved that some BA and BT units were in crystalline regions. Thermal analysis showed one glass-transition temperature and a single diffuse melting endotherm corresponding to a large melting-point depression of about 100 degrees C compared with poly(butylene terephthalate), PBT. These results suggest that there is only one crystalline phase. An X-ray fiber diagram of a stretched film could be indexed with the same unit cell as that for PBT. Computer modeling showed that the adipate unit fits into the crystal structure of PBT by adopting a TTGTG dihedral angle sequence in the crystalline conformation proposed for the cocrystallization model. The predicted fiber diagram from the proposed model qualitatively agrees with the experimental one. Polarization microscopy revealed that the spherulite growth rate of P(BA-co-BT) was similar to that for poly(butylene adipate), PBA.     

On the structure of poly d(A-S4T).

The helical twist of poly d(A-s4T) was determined from the periodicity of the cleavage patterns of the double stranded polydeoxynucleotide adsorbed on calcium phosphate and found to be 14 bp per turn. Both cleavage patterns and 31P NMR spectra indicate a mononucleotide structure rather than an alternating B DNA like poly d(A-T). The failure of nucleosome formation excludes a B type structure. The discrepancy of the mononucleotide structure found in 31P NMR spectra and the dinucleotide structure given by X ray fiber diffraction is explained by an alternating tilt of the planes of the base pairs (base roll) as a consequence of a strong propeller twist. The importance of interstrand stacking interactions of adjacent 4-thiothymidines for the helical stability is discussed.     

An alternating conformation characterizes the phosphodiester backbone of poly(dA-dT) in solution.

A highly homogeneous 145-base-pair fragment of double helical poly(dA-dT) . poly(dA-dT) was obtained by micrococcal nuclease digestion of a semisynthetic chromatin prepared from the nucleosome core histones (H2A, H2B, H3, H4) and the synthetic polydeoxyribonucleotide. In contrast to higher molecular weight alternating copolymers, this fragment displayed two resolved 31P NMR signals, separated by 24 Hz at 10.93 MHz. The two signals were of equal intensity at all temperatures less than the Tm for the fragment. Analyses of the possible origins for the two reasonances leads to the conclusion that the phosphodiester backbone of this DNA contains two distinct phosphorus environments, probably in an alternating array. We suggest that this may indicate the presence of sequence-dependent local variation in the helical structure of DNA in general.     

Methylated pyrimidines stabilize an alternating conformation of poly(dA-dU).poly(dA-dU)

We have investigated the effect of increasing percentages of methylated pyrimidines on the structure of poly(dA-dU).poly(dA-dU). This was done by synthesizing analogous polynucleotides that contained deoxythymidine residues as well as deoxyuridine residues and observing their 31P NMR spectra in increasing amounts of CsF. The results show that methylated pyrimidines play a large role in the stabilization of the "alternating B" conformation of DNA.     

Enzymatic hydrolysis of bacterial poly(3-hydroxybutyrate-co-3-hydroxypropionate)s by poly(3-hydroxyalkanoate) depolymerase from Acidovorax Sp. TP4

Enzymatic degradability has been investigated for a series of bacterial poly(3-hydroxybutyrate-co-3-hydroxypropionate)s (P(3HB-co-3HP)s) with 3-hydroxypropionate (3HP) unit contents from 11 to 86 mol % as well as poly(3-hydroxybutyrate) (P(3HB)) and chemosynthesized poly(3-hydroxypropionate) (P(3HP)). The behavior of degradation by two types of extracellular poly(3-hydroxyalkanoate) (PHA) depolymerases purified from Ralstonia pikettii T1 and Acidovorax Sp. TP4, defined respectively as PHA depolymerase types I and II according to the position of the lipase box in the catalytic domain, were compared in relation to the thermal properties and crystalline structures of the PHA samples elucidated by differential scanning calorimetry and wide-angle X-ray diffraction. The degradation products were characterized by high-performance liquid chromatography and one- (1D) and two-dimension (2D) (1)H NMR spectroscopy. It was found that the PHA depolymerase of Acidovorax Sp. TP4 showed degradation behavior different from that shown by depolymerase of R. pikettii T1. PHA depolymerase from Acidovorax Sp. TP4 degraded the P(3HB-co-3HP) films with lower crystallinity in higher rates than those with higher crystallinity, no matter what kinds of crystalline structures they formed. In contrast, PHA depolymerase from R. pikettii T1 degraded P(3HB-co-3HP) films forming P(3HB) crystalline structure in higher rates than those forming P(3HP)s. The increase in amorphous nature of the P(3HB-co-3HP) films with P(3HB)-homopolymer-like crystalline structure increases and then decreases the rate of degradation by depolymerase from R. pikettii T1. The 3-hydroxybutyrate (3HB) monomer was produced as a major product by the hydrolysis of P(3HB) film by PHA depolymerase from Acidovorax Sp. TP4. The P(3HB-co-3HP) films could be degraded into 3HB and 3-hydroxypropionate (3HP) monomer at last, indicating that the catalytic domain of the enzyme recognized at least two monomeric units as substrates. While the PHA depolymerase from R. pikettii T1 hydrolyzed P(3HB) film into 3HB dimer as a major product, and the catalytic domain recognized at least three monomeric units. The degradation behavior of P(3HB-co-3HP) films by the PHA depolymerase of Acidovorax Sp. TP4 could be distinguished from that by the depolymerase of R. pikettii T1.     

Mechanisms for the formation of isoprostane endoperoxides from arachidonic acid. "Dioxetane" intermediate versus beta-fragmentation of peroxyl radicals

The isoprostanes are a class of autoxidation products generated from arachidonic acid (or its esters) by a free radical initiated process. The potent biological activity of these compounds has been attracting intense research interest since they were detected in humans as well as animal models in the early 1990s. The measurement of these compounds has been regarded as one of the most useful non-invasive biomarkers for oxidative stress status. Two mechanisms for the formation of these compounds have been proposed. In the first mechanism, a peroxyl radical undergoes successive 5-exo cyclizations analogous to the enzymatic mechanism proposed for prostaglandin biosynthesis. The second mechanism starts with a 4-exo cyclization of a peroxyl radical leading to an intermediate dioxetane, a mechanism that has also been proposed for prostaglandin biosynthesis as well as for the formation of 4-hydroxy nonenal (HNE). Autoxidation of cholesteryl-15-HpETE under free radical conditions provides Type IV isoprostanes. The "dioxetane" mechanism for isoprostane generation from 15-HpETE requires that optically pure products are formed from an optically pure reactant, whereas an alternate mechanism for the process involving beta-fragmentation of the 15-peroxyl would give racemic isoprostane products. We have carried out a test of the mechanism based upon these stereochemical requirements. The results of analysis of the product mixture derived from autoxidation of optically pure Ch-15-HpETE by atmospheric pressure chemical ionization-mass spectrometry coupled with chiral high performance liquid chromatography indicate that the major isoprostane diastereomers are formed as a racemic mixture. These experimental results are consistent with a mechanism for isoprostane formation involving beta-fragmentation of the 15-peroxyl radical followed by re-addition of oxygen to form the 11-HPETE peroxyl, and they exclude a mechanism proceeding through the formation of a dioxetane intermediate.     

Poly(ester urethane)s consisting of poly[(R)-3-hydroxybutyrate] and poly(ethylene glycol) as candidate biomaterials: characterization and mechanical property study

Poly(ester urethane)s with poly[(R)-3-hydroxybutyrate] (PHB) as the hard and hydrophobic segment and poly(ethylene glycol) (PEG) as the soft and hydrophilic segment were synthesized from telechelic hydroxylated PHB (PHB-diol) and PEG using 1,6-hexamethylene diisocyanate as a nontoxic coupling reagent. Their chemical structures and molecular characteristics were studied by gel permeation chromatography, 1H NMR, and Fourier transform infrared spectroscopy. Results of differential scanning calorimetry and X-ray diffraction indicated that the PHB segment and PEG segment in the poly(ester urethane)s formed separate crystalline phases with lower crystallinity and a lower melting point than those of their corresponding precursors, except no PHB crystalline phase was observed in those with a relatively low PHB fraction. Thermogravimetric analysis showed that the poly(ester urethane)s had better thermal stability than their precursors. The segment compositions were calculated from the two-step thermal decomposition profiles, which were in good agreement with those obtained from 1H NMR. Water contact angle measurement and water swelling analysis revealed that both surface hydrophilicity and bulk hydrophilicity of the poly(ester urethane)s were enhanced by incorporating the PEG segment into PHB polymer chains. The mechanical properties of the poly(ester urethane)s were also assessed by tensile strength measurement. It was found that the poly(ester urethane)s were ductile, while natural source PHB is brittle. Young's modulus and the stress at break increased with increasing PHB segment length or PEG segment length, whereas the strain at break increased with increasing PEG segment length or decreasing PHB segment length.     

Conformational variability of poly(dA-dT).poly(dA-dT) and some other deoxyribonucleic acids includes a novel type of double helix

The article reviews data indicating that poly(dA-dT).poly(dA-dT) is able of adopting three distinct double helical structures in solution, of which only the A form conforms to classical notions. The other two structures have dinucleotides as double helical repeats. At low salt concentrations poly(dA-dT).poly(dA-dT) adopts a B-type alternating conformation which is exceptionally variable. Its architecture can gradually move in the limits demarcated by the CD spectra with inverted long wavelength CD bands and the 31P NMR spectra with a very low and a 0.6 ppm separation of two resonances. Contrary to Z-DNA, the 31P NMR spectrum of the limiting alternating B conformation of poly(dA-dT).poly(dA-dT) is characterized by an upfield shift of one resonance. We attribute the exceptional conformational flexibility of the alternating B conformation to the unequal tendency of bases in the dA-dT and dT-dA steps to stack. However, by assuming the limiting alternating B conformation, the variability of the synthetic DNA is not exhausted. Specific agents make it isomerize into another conformation by a fast, two-state mechanism, which is reflected by a further deepening of the negative long wavelength CD band and a downfield shift of the 31P NMR resonance of poly(dA-dT).poly(dA-dT) that was constant in the course of the gradual alterations of the alternating B conformation. These changes are, however, qualitatively different from the way poly(dG-dC).poly(dG-dC) behaves in the course of the B-Z isomerization. Poly(dG-dC).poly(dG-dC) displays purine-pyrimidine (dGpdC) resonance in the characteristic downfield position, while the downfield resonance of poly(dA-dT).poly(dA-dT) belongs to the pyrimidine-purine (dTpdA) phosphodiester linkages. Consequently, phosphodiester linkages in the purine-pyrimidine steps play a similar role in the appearance of the Z form to the pyrimidine-purine phosphodiesters in the course of the isomerization of poly(dA-dT).poly(dA-dT). This excludes that the high-salt structures of poly(dA-dT).poly(dA-dT) and poly(dG-dC).poly(dG-dC) are members of the same conformational family. We call the high-salt conformation of poly(dA-dT).poly(dA-dT) X-DNA. It furthermore follows from the review that synthetic molecules of DNA with alternating purine-pyrimidine sequences of bases can adopt either the Z form or the X form, or even both, depending on the environmental conditions. This introduces a new dimension into the DNA double helix conformational variability. The possible biological relevance of the X form is suggested by experiments with linear molecules of natural DNA.(ABSTRACT TRUNCATED AT 400 WORDS)     

Stereoselective features of (R)- and (S)-atenolol: Clinical pharmacological,pharmacokinetic, and radioligand binding studies

In a randomized, double-blind, cross-over study in 12 healthy volunteers, the effects of single oral doses of 100 mg rac-atenolol were compared during exercise to those of equal amounts of the optically pure enantiomers, i.e., 50 mg (R)- and 50 mg (S)-atenolol. The mean rate pressure product decreased with rac-atenolol (?37%; P < 0.01) and half-dosed (S)-atenolol (?35%; P < 0.01) to the same extent, whereas (R)-atenolol caused no effect. Radioligand binding studies in beta-adrenergic receptors of the guinea pig heart yielded a eudismic ratio of 46 for (S)- to (R)-atenolol. The mean AUCs, maximal plasma concentrations, and plasma half-lives of the enantiomers were similar regardless of whether they were administered as optically pure enantiomers or as racemic mixture. On the other hand, the AUC of (R)-atenolol was 1.08-fold greater (P < 0.01) than that of the (S)-enantiomer. The reason for this finding remains unclear. We conclude that only (S)-atenolol, but not (R)-atenolol, contributes to the beta-blocking effect of currently used rac-atenolol since the same effect can be elicited with the (S)-enantiomer alone. © 1993 Wiley-Liss, Inc.