摘要:

AbstractSolvolyses of tosylatesIIBandIICin a variety of solvents were found to give excellent linear log k ‐YBnOTsplots (R = 0.997 and SD = 0.022) and negative ΔS≠forIIB, but slight deviations (R = 0.987 and SD = 0.047) from lineation and positive ΔS≠for several cases forIIC. Limiting SN1 mechanisms with steric hindrance to resonance and solvent intervention at the cationic transition state could thus be confirmed. The inconsistency between theoretical and observed trend of ΔH≠is attributed to the neglect of solvent effect in MO calculations. Deductions linking theoretical calculation and solvolytic experiments should be drawn wi

ISSN:0009-4536    

DOI:10.1002/jccs.199400048

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe application of enzymes to perform selective transformations is the central aim of industrial biocatalysis. Since a simple handled C‐N ligase is not available, the only alternative of general practical interest for enzymatic peptide bond formation are proteases due to the principle of microscopic reversibility. The stereo‐ and regiospecifitity of proteases, which guarantee racemization‐free segment condensation and require only minimal side‐chain protection, are significant for preparative purposes. Unfortunately, proteases are not perfect acyltransferases. Both unwanted proteolytic side reactions and the hydrolysis of the acyl‐enzyme in the kinetic approach are the major problems in enzymatic peptide synthesis. The object of this paper was to demonstrate several new synthesis strategies including medium engineering that enables one to bypass unfavorable reaction routes. Planning and optimization of enzymatic peptide synthesis require the S′ subsite mapping of proteases and the knowledge of additional basic parameters that determine the reac

ISSN:0009-4536    

DOI:10.1002/jccs.199400049

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe Chinese medicinal plant‐derived antineoplastic alkaloids and their analogs are reviewed with emphasis on those discovered from the authors' laboratory. The active compounds include camptothecin, colchicine, ellipticine, harringtonine,Vincaalkaloids, indirubin, (‐)‐sophocarpine, lycobetaine, monocrotaline, d‐tetrandrine, indicineN‐oxide, maytansine, aporphine type alkaloids, palmatine, murrapanine, emarginatine alkaloids, and the alkaloids fromSecurinega virosa. The compounds are discussed briefly with the recent advances concerning their antitumor activity, structure‐activity relationships and mechanism

ISSN:0009-4536    

DOI:10.1002/jccs.199400050

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractWe report a classical trajectory study of the molecular desorption of a vibrationaly excited HF adsorbed on a corrugated LiF(001) surface. The desorption is induced by either a vibrational deexcitation of the adsorbed molecule (vibrational desorption) or an active motion of the surface ion (thermal desorption). A brief description of the calculation of the gas molecule/surface interaction potential is first presented. By applying this potential, Hamilton's equations of motion can be numerically integrated with the initial conditions selected, thus the dynamics is simulated. The result gives a clear indication about the features of the evolution of the vibrational, rotational and translational energies with increase of time. The angular momentum and translational energy distributions of the desorbed molecule are presented. Desorption probabilities for different initial states of the adsorbed molecule are also predicted, which are influenced by the polar‐angle‐dependent potential and the excitation of the initial vibrational level. The surface phonon mode is found to be strongly coupled to the vibrational, rotational, translational modes of the molecule and surface b

ISSN:0009-4536    

DOI:10.1002/jccs.199400051

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractWe apply a quantum‐mechanical theory of rate processes to treat the recombination of ligands to haeme proteins such as myoglobin. Emphasis is placed on the effect of temperature in the low‐temperature regime.We present a theory of time‐resolved spectra pertaining to protein dynamics. We emphasize the analysis of the entire spectrum rather than spectral shifts as a function of

ISSN:0009-4536    

DOI:10.1002/jccs.199400052

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractConformational analysis of biphenyl‐2,2′‐diacetate by dynamic NMR and UV spectra and by plots of enzyme activity vs temperature plots is described. From dynamic NMR spectra of the biphenyl‐2,2′‐diacetate with a chiral shift reagent, the coalescence temperature (Tc), the Gibbs energy of activation (ΔG≠), and the rate coefficient (k) of bipbenyl‐2,2′‐diacetate were −5 °C, 59.5 kJ/mol, and 13.3 s−1, respectively. From analysis of the conformational break in the UV spectra and the discontinuity in the plots of enzyme activity vs temperature, the racemerization temperature of bipbenyl‐

ISSN:0009-4536    

DOI:10.1002/jccs.199400053

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractSolubilities are reported for three Co(III) complexes of 2‐aminophenol, 2‐amino‐4‐nitrophenol and 6‐amino‐2,4‐dimethylphenol. Transfer chemical potentials have been derived from appropriate solubility measurements in aqueous methanol. The trends in transfer chemical potentials are discussed in terms of the nature of the coordinated ligands and are compared with those for selected of other inorganic Co(III) complex ions. The kinetics of base hydrolysis of Co(III) complexes in aqueous methanol solutions are described and the change in the activation barrier δmΔG≠is determined. Solvent effects on reactivity trends are analysed into initial state and transition state components. The small decrease in the rate constant, with increasing proportion of methanol, is ascribed to a greater effect of solvent on

ISSN:0009-4536    

DOI:10.1002/jccs.199400054

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractA scheme for separation and detection of eleven priority phenols using capillary zone electrophoresis (CZE) coupled with amperometric detection is described. With a capillary of I.D. 50 μm and length 62.5 cm at 9 kV and an electrophoretic buffer of 20 mM CHES (pH 10.1), complete separation of the eleven compounds was achieved in less than 17 min. Amperometric detection was carried out using a carbon fiber microelectrode of diameter 9 μm inserted into the end of the detection capillary. Linearity over two orders of magnitude was generally obtained for the eleven priority phenols. With an electrode potential+1.10 V (vs. Ag/AgCl reference), the concentration limits of detection were in the sub‐ppm (10−6M) level. This method was successfully applied to analysis of priority phenols in industrial waste

ISSN:0009-4536    

DOI:10.1002/jccs.199400055

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractMicroscopic differential scanning calorimetry and Fourier transform infrared spectrometry (DSC‐FTIR) were combined to investigate the thermal response and IR spectra of lipid and protein in the process of a phase transition in porcine stratum corneum (SC) by KBr disc method. The alterations of bands associated with the CH2stretching vibrations near 2850 and 2920 cnv−1were used to determine the phase transformation of lipid with temperature. The peaks of amide I and II of protein were used to investigate the thermal conversion of protein. A reheating process was performed. The results indicate that the bands of lipid near 2900 cm−1shifted to greater wavenumber with increased temperature, but reversibly. The band due to deformation mode of the lipid altered from shoulder to smooth with increased temperature. During heating, α‐keratin of the protein transformed gradually to p‐keratin, but irreversibly. Thermal transitions that occurred near 78 °C and 115 °C for the sample on first heating were associated with phase transition of the lipid‐protein complex and the protein in porcine SC, respectively. After reheating, this phase transitional temperature of the lipid‐protein complex in porcine SC decreased from 78 to 68 °C, and the transition of protein near 115 °C almost disappeared. This behaviour indicates that porcine SC after heating might alter its structure. The thermally altered proportion of lipid was 43.98% and the thermally induced proportion of protein was 41.48% during the first heating process, but the restoration of lipid during the cycle of heating, cooling and reheating was 37.64%. The variation is attributed to the denaturation of protein to alter the structure of lipid‐protein complex after first heating. This technique was simple, precise and reproducible for simple determination of stratum corneum or biological sampl

ISSN:0009-4536    

DOI:10.1002/jccs.199400056

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe pyrimidine bridged binuclear complex (CN)5FepymRu(NH3)5‐ (I) was prepared in aqueous solution by mixing cquimolar of Fe(CN)5OH23−and Ru(NH3)5pym2+. Its mixed valence state molecule (CN)5FepymRu(NH3)5(II) was obtained upon oxidation of I by one equivalent of peroxydisulfate ion. Both binuclear complexes and corresponding Fe(II) and Ru(II) mononuclear complexes displayed a metal‐to‐ligand charge transfer absorption in 400–450 nm region. Rate constants of formation and dissociation of I and II were measured, and the values of kf(−103M−1s−1) and kd(−10−3‐10−4s−1) were consistent with kinetic results expected for the substitution of Fe(CN)5OH23−with di‐ and trivalent ligands. Cyclic voltammetry of I exhibited two one‐electron steps of oxidation corresponding to [III, L, II] + e → [II, L, II]and [III, L, III] + e → [III, L, II], respectively. The mixed valence binuclear complex II showed an intervalence band at 955 nm with a molar extinction coefficient 5.80 × 102M−1cm−1and a half‐width 5100 cm−l. The properties of the IT band conform to Hush's theory. Spectroscopic, electrochemical and kinetic results of II suggest that the mixed valence complex features a trapped ‐ valence formulation with lo

ISSN:0009-4536    

DOI:10.1002/jccs.199400057

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY