ISSN:0947-6539    

DOI:10.1002/chem.19970030703

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe use of hexamethylphosphoramide (HMPA) or other sterically demanding groups as stabilizing ligands allows the successful isolation of a series of structurally characterizable complexes of ketyl radicals with alkali, alkaline earth, and lanthanide metal. It has been demonstrated that the stability and reactivity of the ketyl radicals strongly depend on both the nature of the metals to which they are bound and the steric and electronic properties of the ancillary ligands, as well as the structure of their parent ketones.

ISSN:0947-6539    

DOI:10.1002/chem.19970030704

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractWe have been investigating the synthesis of endohedral metallofullerenes since we successfully opened an orifice on the framework of C60in the form of a cobalt complex having the metal sitting on top of the opening. A second approach aims at the synthesis of spherically‐shaped acetylenic macrocycles, which are expected to rearrange to endohedral metal complexes of fullerenes in a controlled process analogous to the gas‐phase coalescence of mono‐ and polycyclic polyynes during fullerene formation by the graphite evaporation method. Since the potential benefits from obtaining endohedral metal complexes of C60are enormous, both approaches are being actively pursued in our

ISSN:0947-6539    

DOI:10.1002/chem.19970030705

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA 32‐residue α‐helical peptide with a sequence similiar to that of the GCN4 leucine zipper region is shown to catalyze its own formation by accelerating the amide bond formation of a 17‐residue peptide, preactivated as a thiobenzyl ester, and a 15‐residue peptide with aN‐terminal cysteine. The self‐replication process displays parabolic growth characteristics as revealed by a detailed kinetic analysis. Control reactions with single‐mutant peptides strongly support a mechanism in which a ternary and/or quaternary complex of the product with both peptide fragments act(s) as the catalytically active intermediate(s). Furthermore, these experiments reveal a remarkable sequence selectivity, as evidenced by the loss of autocatalytic activity as a result of a single replacement of leucine or valine residues with an alanine at the recogni

ISSN:0947-6539    

DOI:10.1002/chem.19970030706

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractPhenylboronic acid groups as sugar recognition sites were successfully introduced into native myoglobin by a cofactor‐reconstitution method. Spectrophotometric pH titration demonstrated the sugar‐induced pKashift of the H2O coordinated to the heme center of the semisynthetic myoglobin bearing phenyl‐boronic acids (met‐Mb(PhBOH)2). By means of circular dichroism (CD) and paramagnetic1H NMR spectroscopies, it was proven that sugars that were bound to phenylboronic acid sites induced the rearrangement of the heme crevice to reinforce the heme cofactor‐apoprotein interactions. The structural changes that were induced by the binding of sugars subsequently enhanced the dioxygen storage activity of Mb(PhBOH)2. Such sugar‐induced structural and functional changes did not occur for other modified Mbs that had no sugar‐recognition units. Interestingly, a randomly modified Mb with phenylboronic acid units did not show any sugar response. In Mb(PhBOH)2, the information from the sugar‐binding event was efficiently transmitted to the active center, so that the activity was efficiently altered upon sugar binding. In conclusion, the active site specific incorporation of molecular recognition units as nonnatural functional molecules can provide a novel strategy for the design of stimuli‐responsive semis

ISSN:0947-6539    

DOI:10.1002/chem.19970030707

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA skin sample from the Iceman (Ötzi, Similaun Man, Man from Hauslabjoch) was studied by means of IR spectroscopy and histology, and the results were compared to those obtained from nine other more recent human tissue samples with known case reports. Attenuated total reflection IR spectroscopy was used for studying the fate of proteins and lipids in these rare skin tissues. This technique provided a simple yet powerful means for semiquantitative determination of the main compound classes found in skin samples, namely, triacylglycerols, their main breakdown products (fatty acids), and proteins. When combined with histology, IR spectroscopy provided reliable information about the main conditions under which mummification of the samples had taken place. In the case of the 5300‐year‐old Iceman, preserved collagen contributed to the conservation of morphological structures of the skin, although no cellular structures such as nuclei survived. In addition, post‐mortal alterations such as hydrolysis of triacylglycerols and phospholipids into fatty acids were unequivocally demonstrated. The solidified fatty acids provided a certain degree of preservation to the tissue characteristic of adipocere. Based on this observation, taken together with the concomitant loss of epidermis, we suggest that Iceman's body was submerged in water for a period of at least several months prior to desiccation. Results from other research disciplines support our concl

ISSN:0947-6539    

DOI:10.1002/chem.19970030708

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe pentahydridosulfonium trication SH3+5was found by ab initio MP2/ 6‐31G**and QCISD(T)/6‐311G**levels of calculation to be a stable minimum withCssymmetric structure1. It is isostructural with the parent pentacoordinate carbonium ion CH+5. Structure1resembles a complex between SH3+3and molecular hydrogen forming a 2e–3c bond. This structure involving a pentaco‐ordinate sulfur atom is unprecedented. Rotation of the H2unit around the pseudo‐C3axis of SH3is facile. The structure of SF3+5was also calculated and compared with that of1. In addition, our studies at the ab initio MP2/6‐31G**level showed that the parent pentacoordinate oxonium trication OH3+5is not a minimum on its potential energy surface, and dissociation into OH2+4and H+occurs upon o

ISSN:0947-6539    

DOI:10.1002/chem.19970030709

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractOcta‐tert‐butylphthalocyanina‐tonickel(II) (4) was synthesized from 3,5‐di‐tert‐butylphthalonitrile (1) and nickel acetate. A nonstatistical mixture of the four structural isomers4a–dwas obtained. The isolation of the structural isomers4a–dby preparative HPLC (MPLC) allowed their unequivocal identification and characterization by spectroscopic techniques. Moreover, a new family oftert‐butyl‐substituted phthalocya‐nines6–10containing dienophilic functionalities were prepared by condensation of 3,5‐di‐tert‐butylphthalonitrile (1) with 6,7–dicyano‐1,4‐epoxy‐1,4‐dihydronaphthalene (5). The separation of the phthalocyanines6–10and of some of their structural isomers was accomplished by normal column chromatography. The structural isomers were characterized by spectro

ISSN:0947-6539    

DOI:10.1002/chem.19970030710

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe complexation of three guests containing 4,4′‐bipyridinium redoxactive residues by β‐cyclodextrin (β‐CD) and its heptakis‐(2,6‐O‐dimethyl) analogue (DM‐β‐CD) was investigated by means of voltammetric techniques. The three 4,4′‐bipyridinium (viologen) derivatives used as guests were designed to be water‐soluble in all three accessible oxidation states. TheN‐substituents chosen to enhance aqueous solubility were: 2‐(2‐(2‐ethoxy)ethoxy)ethanol (guest12+), 6‐hexanoate (guest2), and 3‐propanesulfonate (guest3). Detailed analysis of the voltammetric results by digital simulation techniques revealed that the oxidized forms of the guests did not interact appreciably with either CD host; the two‐electron reduced guests formed extremely stable inclusion complexes, with association constants in the range 103–104M−1, while the cation radical forms exhibited intermediate binding affinities (≈︁102M−1). In all cases, DM‐β‐CD was f

ISSN:0947-6539    

DOI:10.1002/chem.19970030711

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

Abstractcloso−1,2‐(SiMe3)2−1,2‐C2B4H4undergoes thermal conversion to 1,6‐(SiMe3)2−1,6‐C2B4H4. The reaction pathway was monitored by11B NMR spectroscopy. The structures of the 1,2‐ and 1,6‐isomers were optimized at the HF/6‐31 G*ab initio level. Gas‐phase electron diffraction studies for both isomers are reported, as well as low‐temperature X‐ray crystal structure determinations. Comparison of calculated structural data with the data obtained experimentally shows good agreement betw

ISSN:0947-6539    

DOI:10.1002/chem.19970030712

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY