Influence of the formation of an oxidized layer on the dynamic-mechanical properties of natural rubber

Natural rubber cylinders vulcanized with two different systems, TMTD and S/CBS, were aged for more than 2000 h in an air oven at 100 °C. During the ageing an oxidized layer formed on the surfaces of the specimens. To study the differences between the interior of the materials and their surfaces, and to find out how the interior of the materials was affected by the ageing, measurements were performed both on whole specimens and on specimens from which the surface layer had been removed. Dynamic mechanical (complex) modulus and loss-angle were recorded intermittently. The surface layer was hard and brittle because of thermo-oxidative crosslinking and probably had a low permeability to oxygen. The modulus of the interior material decreased slightly during the ageing. After ageing, the specimens cracked easily during deformation. The difference between surface and interior was greater, and the formation of the surface layer started earlier, when the material was vulcanized with S/CBS.     

INTERACTION BETWEEN DNA AND 8–METHOXYPSORALEN STUDIED BY LINEAR DICHROISM

Abstract— High sensitivity linear dichroism on DNA oriented by flow is shown to provide conclusive information about the interaction with 8–methoxypsoralen (8–MOP) both in the dark and by irradiation. 8–MOP forms directly in the dark a weak complex with DNA with a structure consistent with the intercalation model of Lerman. When aqueous DNA-psoralen solutions are UV irradiated (30800 nm). a photoadduct is formed. By comparison with experiments in ice matrix, it is suggested that it is the primarily bound 8–MOP that is responsible for the final product. It is inferred that this product is the 4 ¹ ,5 ^I -photoadduct of 8–MOP. The orientation of the photo adduct on DNA is not consistent with intercalation.     

Reference points for comparisons of two-dimensional maps of proteins from different human cell types defined in a pH scale where isoelectric points correlate with polypeptide compositions

A highly reproducible, commercial and nonlinear, wide-range immobilized pH gradient (IPG) was used to generate two-dimensional (2-D) gel maps of [³⁵S]methionine-labeled proteins from noncultured, unfractionated normal human epidermal keratinocytes. Forty one proteins, common to most human cell types and recorded in the human keratinocyte 2-D gel protein database were identified in the 2-D gel maps and their isoelectric points (pI) were determined using narrow-range IPGs. The latter established a pH scale that allowed comparisons between 2-D gel maps generated either with other IPGs in the first dimension or with different human protein samples. Of the 41 proteins identified, a subset of 18 was defined as suitable to evaluate the correlation between calculated and experimental pI values for polypeptides with known composition. The variance calculated for the discrepancies between calculated and experimental pI values for these proteins was 0.001 pH units. Comparison of the values by the t-test for dependent samples (paired test) gave a p-level of 0.49, indicating that there is no significant difference between the calculated and experimental pI values. The precision of the calculated values depended on the buffer capacity of the proteins, and on average, it improved with increased buffer capacity. As shown here, the widely available information on protein sequences cannot, a priori, be assumed to be sufficient for calculating pI values because post-translational modifications, in particular N-terminal blockage, pose a major problem. Of the 36 proteins analyzed in this study, 18–20 were found to be N-terminally blocked and of these only 6 were indicated as such in databases. The probability of N-terminal blockage depended on the nature of the N-terminal group. Twenty six of the preteins had either M, S or A as N-terminal amino acids and of these 17–19 were blocked. Only 1 in 10 proteins containing other N-terminal groups were blocked.     

Photoinitiated free radical carbonylation enhanced by photosensitizers

[reaction: see text] The enhancing effect of several photosensitizers in photoinitiated radical carbonylation is demonstrated and applied to accelerate the synthesis of compounds labeled with short-lived 11C. With the sensitizers, the synthesis of [carbonyl-11C]esters and acids from alkyl iodides, [11C]carbon monoxide, alcohols, and water provided up to 75-85% decay-corrected radiochemical yields in 6-min reactions under mild conditions. Acetone was used as a sensitizer in preparing 13C-substituted 1,10-decanedicarboxylic acid from (13C)carbon monoxide.     

Preparation and properties of some water-soluble,comb-shaped,amphiphilic polymers

Water-soluble comb-shaped polymers were prepared through grafting of poly(ethylene glycol) monomethyl ethers (MPEG) onto acrylic and methacrylic ester copolymers by transesterification reactions. The grafting was alkali-catalyzed, and performed in refluxing toluene solution or in melt at 155°C. The grafting efficiency was found to be on the order of 1 graft/10 monomer units. Epoxy groups in glycidyl methacrylate copolymers were also utilized for grafting. The crude graft copolymers were purified through chromatography and characterized by NMR and IR spectroscopy. Polymers prepared from MPEG 2000 were crystalline with melting points 10–15°C lower than the MPEG used. All polymers were shown to be surface active with CMC on the order of 1.5 g/L, and surface tensions of 38–45 dyn/cm. When used as emulsifiers the graft copolymers containing bulky lipophilic ester groups (2-ethylhexyl t-butyl) gave oil-in-water (o/w) and water-in-oil (w/o) emulsions from xylene/water with higher stability than those containing straight chain ester groups (methyl nbutyl n-docecyl). The most stable emulsions were obtained by dissolving the polymers in the organic phase.     

Sample preparation effects in matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry of partially depolymerised methyl cellulose

Methyl cellulose (MC) was partially depolymerised and the oligomers thus obtained were studied by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOFMS). The depolymerisation was either enzymatic or acidic. Fractions of enzymatically depolymerised MC were collected from size-exclusion chromatography and subjected to a sample preparation investigation. Several MALDI matrices and solvents were evaluated. The results showed that the solvent choice had a significant effect on the measured degree of substitution (DS). Aprotic solvents produced higher DS values, which was most likely due to poor solubility of species with low DS. The obtained signal intensity, however, did not correlate with the solubility but seemed to be more dependent on certain matrix/solvent combinations. All the matrices attempted produced mass spectra with sufficient signal intensity for accurate peak area calculation. The choice of matrix did not have any significant effect on the measured DS. Sample spots obtained from organic solvents had a more homogeneous distribution of the analyte and smaller crystals than those obtained from water. This increased both the reproducibility and peak resolution and in addition the analysis time was shorter. DS measurements were performed on two acidically depolymerised MCs with different nominal DS values. It was easy to distinguish between the two MCs, and the measured DS values agreed well with the values supplied by the manufacturers.     

Novel chiral pyromellitdiimide (1,2,4,5-benzenetetracarboxydiimide) dimers and trimers: exploring their structure,electronic transitions,and exciton coupling

The chiral but highly symmetrical acyclic and cyclic pyromellitic diimide dimers and trimers 2-5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecules are linked by a rigid diequatorially 1,2-disubstituted cyclohexane skeleton. The structures of the compounds have been determined in detail by molecular modeling and, in the case of cyclic dimer 4 and trimer 5, by means of X-ray diffraction analysis. The electronically excited states of the pyromellitdiimide chromophore (1a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. CD spectra of the rigid cyclic trimer 5 have provided the most detailed information on the excited states of the pyromellitdiimide chromophore. The low-energy tail (340-360 nm) of the absorption envelope can be assigned to out-of-plane polarized n-pi* transitions (I, II). The higher energy bands are due to contributions from up to six pi-pi* transitions, these being polarized either along the long (IV-VI, VIII) or short axis (III, VII). The results of ab initio CIS/cc-pVDZ and semiempirical INDO/S-CI calculations have been compared with the experimental data. CD Cotton effects in the region 200-260 nm, which result from exciton interactions between electric dipole allowed transitions of two pyromellitdiimide chromophores in compounds 2-5, provide reliable and useful information concerning the conformation and absolute configuration of these molecules, which may be extrapolated to other oligoimide systems.     

Chromatography of amphiphilic graft copolymers

Amphiphilic comb polymers were prepared through grafting poly(ethylene glycol) methyl ether (MPEG 2000) onto acrylic and methacrylic copolymers. The graft copolymers were purified from unreacted MPEG by partition chromatography on methanol pretreated fibrous cellulose using ethyl acetate and methanol as eluents. The separation was found to depend on the water contents of the cellulose and the eluents. It is proposed that one parameter of importance for the chromatographic separation is the formation of hydrated inverted micelles. The amphiphilic comb polymers were shown by gel chromatography on Sepharose to form high molecular weight aggregates in water. On addition of sodium lauryl sulphate or inorganic salts to the eluent at low ionic strengths these aggregates dissociated and were fractionated by the gel. It was also shown that on GPC in THF solution on Styragel columns the polymers exhibited apparent molecular weights equal or smaller than those of the corresponding backbones. This effect may be a consequence of the graft copolymers having relatively small hydrodynamic volumes, and this idea is supported by the fact that their intrinsic viscosities generally were lower than those of the backbones. However, adsorption on the Styragel columns also may be of importance.     

Preparation and properties of centrally bridgehead-substituted hexacyclo[4.4.0.02,1.03,5.04,8.07,9]decanes ("diademanes") and related (CH)10 hydrocarbons

6-Trimethylsilyl- (1 b), 6-hydroxymethyl- (1 e), and 6-methyldiademane (1 f) have been prepared by irradiation of the corresponding snoutene derivatives, in 23, 2.8, and 17 % yields, respectively, together with the isomeric 1-trimethylsilyl- (10 b) and 1-methyldiademane (10 f) (8 and 2 % yields, respectively). The starting 4-trimethylsilyl- (9 b) and 4-(trimethylsilyloxymethyl)snoutene (9 d) were prepared from the correspondingly substituted cyclooctatetraenes 4 b and 4 c in several steps in 20 and 8 % overall yields, respectively. Upon heating, as well as under the conditions of gas-chromatographic separation, diademanes 1 b, 10 b, 1 f, and 10 f rearranged into the corresponding C10- and C1-substituted triquinacenes 3 b, 3 f, 11 b, and 11 f, respectively. Rough kinetic measurements of these rearrangements indicate some acceleration of the reaction caused by the presence of a methyl substituent and retardation by that of a trimethylsilyl substituent, relative to the parent diademane 1 a. At this insufficient precision, however, the activation energies (E(a)) of 29.0 and 28.1 kcal mol(-1), respectively, are essentially the same as that reported for 1 a (28.3 kcal mol(-1)). An X-ray crystal structure analysis of trimethylsilylsnoutene 9 b revealed a significant lengthening of the distal (with respect to the substituent) bond (1.534 versus 1.505 A) in the unsubstituted cyclopropane ring. In the substituted cyclopropane ring, the two proximal bonds are lengthened (1.530 A) and the distal bond is slightly shortened 1.492 A). This indicates a small, but significant electron-withdrawing effect of the trimethylsilyl group in 9 b. An X-ray crystal structure analysis of 6-hydroxymethyldiademane 1 e showed pronounced alternation of the bond lengths in the six-membered ring, with 1.494(4) between and 1.539(4) A within the three cyclopropane moieties, in close agreement with computations at different theoretical levels. This structural feature corroborates a predisposition of the tris-sigma-homobenzene skeleton of this molecule in the ground state to undergo the facile [sigma(2)(s) + sigma(2)(s) + sigma(2)(s)] cycloreversion to the triquinacene skeleton observed for the parent diademane 1 a, its derivative 1 b and 1 f, as well as for other tris-sigma-homobenzene derivatives.     

Synthesis and nmr spectra of the six isomeric thieno[c]-fused 1,7- and 1,8-naphthyridines

Through the use of Pd(0)-catalyzed coupling between 2- and 4-formyl-3-thiopheneboronic acid and 4-iodo-3-aminopyridine ( 1 ) and 3-bromo-2-aminopyridine, convenient one-pot procedures for the preparation of thieno[2,3-c]-1,7-naphthyridine ( 2 ), thieno[3,4-c]-1,7-naphthyridine ( 3 ), thieno[2,3-c]-1,8-naphthyridine ( 6 ) and thieno[3,4-c]-1,8-naphthyridine ( 7 ) have been developed. Thieno[3,2-c]-1,7-naphthyridine ( 4 ) and thieno[3,2-c]-1,8-naphthyridine ( 8 ) were obtained through the coupling of 2-tri-n-butylstannyl-3-thiophenaldehyde with 2,2-dimethyl-N-(4-iodo-3-pyridinyl)propanamide and 3-bromo-2-acetamidopyridine ( 1 ). The yield of 8 was further increased when copper(II) oxide was used as the co-reagent. The ¹³C nmr spectra of the six isomeric thieno[c]-fused 1,7- and 1,6-naphthyridines are discussed.