摘要:

Monomethyl esters of acyl phosphates (mixed anhydrides of carboxylic and phosphoric acids) are being developed as site-directed acylating agents for amino groups in proteins. In an illustrative application, these anionic materials are shown to bind to a positively charged region of hemoglobin where they convert amino groups to amides. Bifunctional acyl methyl phosphates cross-link hemoglobin to give a variety of products, some of which have the oxygen-binding properties anticipated for materials that can serve as an alternative to red cells in transfusions. Higher yields of desired products result from the use of a trifunctional analogue. Kinetic patterns of the reactions of a series of alkyl amines and methyl aroyl phosphates indicate that the transition state for formation of the amide involves almost complete development of positive charge on nitrogen.

ISSN:0008-4042    

DOI:10.1139/v94-279

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

HCl-catalyzed reactions of the hemiperacetals 3-methoxy-3-methyl-7-hydroxy-1,2-dioxepane (1a) and 3-methoxy-3-methyl-6-hydroxy-1,2-dioxane (1b) and of the related ozonides 1-methyl-6,7,8-trioxabicyclo[3.2.1]octane (2a) and 1-methyl-5,6,7-trioxabicyclo[2.2.1]heptane (2b) gave peroxidic monocyclic (11—13), bicyclic (14), and (or) tricyclic (15) products. By contrast, reactions of the peracetals corresponding to1aand1b, viz., 3-methyl-3,7-dimethoxy-1,2-dioxepane (13a) and 3-methyl-3,6-dimethoxy-1,2-dioxane (13b) gave only non-peroxidic products. Reactions of the persubstituted peracetals 3,6-dimethoxy-3,6-dimethyl-1,2-dioxane (25) and 3,6-dimethoxy-3-isopropyl-6-methyl-1,2-dioxane (27) also gave non-peroxidic products, which resulted from fragmentation of the carbon skeletons.

ISSN:0008-4042    

DOI:10.1139/v94-280

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The unimolecular metastable ion and low-energy collision-induced fragmentation reactions of alkyl ammonium ions RnH4−nN+(n = 2 to 4, R = n-C3H7andn-C4H9) have been studied and breakdown graphs expressing the % fragment ion abundances as a function of the centre-of-mass collision energy have been established. The dialkyl and trialkyl ammonium ions fragment primarily by olefin ([R − H]) elimination or by formation of the alkyl ion R+(with rearrangement to the more stable secondary structure). By contrast the tetraalkyl ammonium ions show both elimination of [R − H] and elimination of an alkane, which is C5H12for the propyl compound C7H16for the butyl compound. The results are interpreted in terms of competition between heterolytic bond cleavage to form a [R+---NH4−nRn−1] complex and homolytic bond cleavage to form a [Rn−1H4−nN+•---•R] complex. The former complex fragments either to form R+or undergoes internal proton transfer to form Rn−1H4−nNH+ + [R − H], while the latter complex fragments by alkane elimination resulting from attack of R•on the α-CC bond of the alkyl group. For the propyl ammonium ions plausible potential energy profiles are established which show that the former complex is favoured forn = 2, 3 while the two complexes have similar energies forn = 4.

ISSN:0008-4042    

DOI:10.1139/v94-281

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The liquidus curves at 75 °C, 1 atm, of the system under consideration have been studied systematically by means of solubilities determined conductimetrically, and lines of constant water content have been deduced. The study used isoplethic sections chosen to complete data on previous measurements of limiting ternary systems and lines of double saturation. Four isoplethic sections: K2SO4–KH2PO4, K2SO4–K2HPO4, KHSO4–K2HPO4, and KHSO4–KH2PO4were determined, the first two of which are quasi-ternary systems. A total of nine portions of the curves were measured to extend knowledge of the liquidus. The limiting curves in the field of KHSO4and K8(H2PO4)1+X(HSO4)7−Xshow curvature, which reveals retrograde solubility as a function of water content.

ISSN:0008-4042    

DOI:10.1139/v94-282

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The NMR spectra of the proton located in a N+---H---−O bond formed in the reaction of a conjugated Schiff base (SB) with a carboxylic acid (HA) in a non-polar solvent indicate that this proton is rapidly exchanging with other protons present in the system. For this study, two SB were used: all-trans-retinylidenetert-butylamine (RBA) andtrans,tran-2,4-heptadienylidenetert-butylamine (HBA), and five carboxylic acids having pKa's between 0.66 and 4.87.1H NMR spectra were taken at 293 and 233 K in CDCl3of systems of SB:HA using different ratios of acid to SB. Results indicate that the rapid proton exchanges are temperature and concentration dependent. Finally, the results are related to the natural pigments rhodopsin and bacteriorhodopsin.

ISSN:0008-4042    

DOI:10.1139/v94-283

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Contrary to expectations, many primary hydroxy groups are completely unreactive in glycosylation reactions, or give the desired glycosides in very low yields accompanied by products of many side reactions. Hydrogens of such primary hydroxyls are shown to be intramolecularly hydrogen bonded. Intermediates formed by nucleophilic attack by these hydroxyls on activated glycosylating agents may resist hydrogen abstraction. This resistance to proton loss is postulated to be the origin of the observed unreactivity. It is shown that successful glycosylations take place under acidic conditions under which such hydrogen bonds cease to exist. Accordingly, direct galactosylations of the normally unreactive 5′-hydroxyls of nucleosides were accomplished for the first time with a galactose trichloroacetimidate donor in chloroform under silver triflate promotion. It is noted that such galactosylated anticancer nucleosides may have improved biological specificity.

ISSN:0008-4042    

DOI:10.1139/v94-284

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The spectroscopic and photophysical parameters of neutral and cationic species of the following molecules have been discussed: 2-phenyl-3,3-dimethyl-3H-indole (1), 2-[(p-amino)phenyl]-3,3-dimethyl-3H-indole (2), 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-3H-indole (3), 2-[(p-amino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (4), 2-[(p-methylamino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (5), 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (6). Solvatochromic shifts have been interpreted in terms of the nature of the substituent groups and the state of solute–solvent interactions and complexation. The theoretical radiative decay rate constantalong with the bandwidth of the absorption profile of the different species involved have been used to discuss the geometrical changes from one species to the other in the ground state. The mirror-image relationship between absorption and fluorescence spectra has proven to be a good tool to discuss any geometrical changes occurring in the excited state. A radiationless torsional mechanism takes place in the excited state relaxation of the various species. The protonation of the ring nitrogen atom generates a highly planar cationic species which retains its conformation in the relaxed excited state. The very effective quenching of the monocation fluorescence is interpreted by the formation of a non-emissive TICT state.

ISSN:0008-4042    

DOI:10.1139/v94-285

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Products obtained from the oxidation of thianthrene 5-oxide,SSO, have been used to compare oxygen transfer mechanisms for three high-valent transition metals. Oxidation ofSSOby benzyltriethylammonium permanganate in methylene chloride gives the corresponding sulfone, thianthrene 5,5-dioxide (SSO2), as the exclusive product. Oxidation ofSSOby ruthenium tetroxide also givesSSO2as the predominant product along with minor amounts of the disulfoxide, thianthrene 5,10-dioxide (SOSO). However, the converse is observed when chromyl chloride is used as the oxidant;SOSOis the major product. It is suggested that oxygen transfers from permanganate and ruthenium tetroxide are initiated by complexation between the central metal atom and the oxygen end of the S=O dipole, while oxidation by chromyl chloride is likely initiated by an alternative mechanism, possibly a single electron transfer.

ISSN:0008-4042    

DOI:10.1139/v94-286

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

A triblock copolymer thermoplastic elastomer (SEBS) consisting of an ethylene–butylene random-copolymer rubbery domain flanked by rigid polystyrene short blocks has been analyzed using novel solid-state NMR techniques.13C NMR relaxation measurements suggest that at theT1(H) level (minimum observable phase size of about 18 nm) the material appears homogeneous, but at theT1ρ(H) level (minimum observable phase size of about 3 nm) the expected two distinct phases are noted. Two-dimensional WISE spectra (high-resolution13C – broadline1H) show the different mobility in the two phases. A broad component associated with the highly mobile rubbery phase indicates the presence of an interface. The mobile phase is selected using a dipolar filter pulse sequence and the magnetization is allowed to transfer to the rigid polystyrene phase during a period called "mixing time." From the growth of the polystyrene magnetization during the mixing period, the size of the dispersed polystyrene phase can be calculated to be 16 ± 2 nm.

ISSN:0008-4042    

DOI:10.1139/v94-287

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The tube-inhabiting marine worm,Cephalodiscusgilchristihas been found to produce a series of cytotoxic and antineoplastic disteroidal alkaloids. A substantial study concerned with discovery of new cell growth inhibitory constituents of this marine animal has led to isolation of cephalostatins 14 (5a) and 15 (5b). The structures were determined by interpretation of spectral data, principally high-field (400 and 500 MHz) NMR and HRFABMS combined with NMR spectral relationships to cephalostatin 1 (4) where the structure was previously assigned by X-ray crystal structure analysis. The stereochemical assignments were established employing the ROESY NMR technique. Both new cephalostatins significantly inhibited growth of the P388 lymphocytic leukemia and a selection of human cancer cell lines. The two new cephalostatins further enhance knowledge of structure–activity parameters for these potentially important steroidal alkaloids.

ISSN:0008-4042    

DOI:10.1139/v94-288

出版商:NRC Research Press    

年代:1994

数据来源: NRC