ISSN:0947-6539    

DOI:10.1002/chem.19970030303

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractWith the tight focus of a gaussian laser beam, single molecules in solution at room temperature can be trapped and detected by observation of fluorescent photons. The focus defines an ultrasmall probe volume on the order of 1 femtoliter, and the electric field gradient associated with this focus enables an individual molecule to be trapped and manipulated.

ISSN:0947-6539    

DOI:10.1002/chem.19970030304

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractIn this paper we describe the design, synthesis, and spectroscopic characterization of a covalent helix–heme–helix sandwich named FeIIImimochrome I. It contains deuterohemin bound through both propionyl groups to two identicalN‐andC‐terminal protected nonapeptides as α‐helical scaffolds. Each peptide moiety bears a His residue in the central position, which acts as axial ligand to the metal ion. The newly developed synthetic strategy is based on a combination of solution and solid‐phase methodologies. It represents a powerful method for obtaining a large variety of analogues containing two symmetric or unsymmetric peptide chains covalently bound to the deuteroporphyrin ring. UV/Visible spectroscopic characterization in buffered 2,2,2‐trifluoroethanol/water solution proves low‐spin bis(his‐tidine) iron(III) coordination; circular dichroism (CD) measurements show an α‐helical conformation for the peptide moieties. Thus, all the data are in agreement with the designed hypothetical model regarding both the iron(III) coordination and the peptide chain st

ISSN:0947-6539    

DOI:10.1002/chem.19970030305

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractFeIIImimochrome I is the prototype of a new class of hemoprotein models characterized by a covalent helix–heme–helix sandwich. It contains deuterohemin bound through two propionyl groups to two identicalN‐ andC‐terminal protected α‐helical nonapeptides, each of which bears a His residue (a potential axial lig‐and of the iron ion) in the central position. In order to understand better the three‐dimensional structure of FeIIImimochrome I and its correlation with spectral properties, we have characterized the fully diamagnetic parent compound CoIIImimochrome I by UV/visible, CD, and NMR spectroscopy, coupled with conformational energy calculations. CoIIImimochrome I is a highly water‐soluble compound present in solution as two isomers, which slowly interconvert only at very low pH values. These isomers were isolated and separately characterized. Their UV/visible spectral properties are very similar, while their CD spectral properties differ markedly in both the far UV and Soret regions. The isomers were identified by1H NMR spectroscopy as diastereomers of the δ and δ type. This is the first example of an accurate three‐dimensional structure determination in solution of a hemoprotein mimetic that allows a straightforward correlation between structure and

ISSN:0947-6539    

DOI:10.1002/chem.19970030306

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

Abstract5‐Deuteriophycocyanobilin (4), 5,15,182,182‐tetradeuteriobiliverdin IXα dimethyl ester (6), and 5,15‐dideuteriomesobiliverdin IXα dimethyl ester (8) have been prepared and analyzed by resonance Raman spectroscopy in the C–H out‐of‐plane region, based on a previous normal‐mode analysis of biliverdin XI α dimethyl ester (5). In this way most of the modes containing out‐of‐plane vibrations of the methine bridges could be assigned unambiguously. These results are of special relevance with regard to the interpretation of the resonance Raman spectra of the chrom

ISSN:0947-6539    

DOI:10.1002/chem.19970030307

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractOne‐electron reductions of the tetraaryldiborane(4) compound Mes2‐BBMes(Ph) (1) (Mes = 2,4,6‐Me3C6H2) with KC8to afford the singly reduced radical anions [K(DME)3][Mes2BBMes(Ph)] (2) and [K([18]crown‐6)(THF)2][Mes2‐BBMes(Ph)] (3) are described. Both2and3were characterized by IR and EPR spectroscopy, and X‐ray diffraction studies of3showed it to be a solvent‐separated ion pair. Similarly, reduction of Mes(MeO)‐BB(OMe)Mes with lithium in diethyl ether under controlled conditions furnished the radical anion [Li(OEt2)2]‐[Mes(MeO)BB(OMe)Mes](4), which has a contact‐ion‐pair structure in which lithium is solvated by oxygen atoms from ether molecules and methoxy groups. The X‐ray crystallographic studies of [K([18]‐crown‐6)(THF)2][Mes2BBMes(Ph)] (3) and [Li(OEt2)2][Mes(MeO)BB(OMe)‐Mes](4) revealed shortened B–B bond lengths in both compounds, consistent with the presence of partial (bond order 0.5) B–B π bonds. Interestingly, the B–B distances in the singly reduced species are very similar to those in the doubly reduced dianions [R2BBR2]2‐, which have formal π‐bond orders of unity. The synthesis and characterization of 2,6‐Mes2‐C6H3(MeO)BB(OMe)2(5) and 2,6‐Mes2‐C6H3(MeO)BB(OMe)Mes (6) are also reported. Reduction of6or5with lithium in diethyl ether solution did not lead to multiply bonded B–B species, but to the isolation of the rearranged products

ISSN:0947-6539    

DOI:10.1002/chem.19970030308

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractAlk‐5‐enoyl radicals were made to cyclize inexoandendomodes to give the corresponding cycloketone radicals, which are related through one‐carbon ring expansion. Relative kinetic data were determined for the ring closure of the 2‐methylhept‐5‐enoyl radical generated by the reaction of the corresponding phenylseleno ester with Bu3SnH over the temperature range 233–323 K. The conversion to absolute rates provided Arrhenius expressions for the 5‐exo‐trigand 6‐endotrigcyclizations.Ab initioand semiempirical (AM1) calculations were performed on the hex‐5‐enoyl and hept‐5‐enoyl radicals, respectively, and the outcomes aided in the rationalization of the preexponential factors and activation energies. Both 1,5‐and 1,6‐ring closure occur via a lower energy “chairlike” transition state. The observed high regioselectivity is due to favorable entropic and enthalpic factors associated with the formation of the smaller ring. The stereoselectivity was higher in the 1,6‐ring closure (70:30) than in the 1,5‐ring closure (55:45), thetransisomer being predominant in both. For the onecarbon ring expansion studies, the radicals of interest were obtained by deoxygenation of suitable alcohols via theO‐phenyl thiocarbonates with (TMS)3‐SiH. The one‐carbon ring expansion in the cyclopentanone series for the secondary alkyl radicals was studied over the temperature range 343–413 K by means of free‐radical clock methodology and yielded the Arrhenius expression. The rate constant was 4.2 x 103s−1at room temperature and the reverse reaction (ring contraction) was found to be at least 10 times slower. Since the intermediacy of acyl radicals can be excluded, the reaction must occur via 3‐membere

ISSN:0947-6539    

DOI:10.1002/chem.19970030309

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractSteric blockage of the N1 and N7 sites of 9‐methyladenine for metal complexation is achieved by twofold methylation of the exocyclic amino group. With 6′,6′,9‐trimethyladenine (TrimeA), binding of MII(dien) (M = Pt, Pd) as well as oftrans‐[Pt(NH3)2Cl]+therefore takes place through N 3. X‐ray crystal structure determinations and NMR spectroscopic studies of three compounds are reported, and the effects of PtIIand PdIIon the geometry and the acid‐base properties of the TrimeA ligand have been measured and compared with those of the free base. TrimeA has a very pronounced self‐stacking tendency in water (K85M−1according to the isodesmic model of indefinite noncooperative self‐association). Acidity constants derived from potentiometric pH titrations, spectrophotometry, and NMR shift experiments display excellent agreement with each other. Twofold protonated TrimeA, that is H2(TrimeA)2+, carries protons at the N7 and N1 sites; the acidity constants are pKHH2(TrimeA)= – 0.75 and pKHH(TrimeA)= 4.15. These values compare well with those of other adenine residues. Protonation of [Pt(dien)‐(TrimeA‐N3)]2+occurs at the N7 position, as shown by spectrophotometry and NMR spectroscopy. The acidity constant of the H+(N7) site in this complex is low, that is pKHH[Pt(dien)(TrimeA)]= 0.3 (as determined by spectrophotometry), but it is not as low as that for the same site when a proton resides at N1 of unmetalated TrimeA. The pKaof the doubly protonated complex, pKHH2[Pt(dien)(TrimeA)], in which the second acidic proton is situated at N1, is about–1.2 ± 0.3. These findings indicate that upon metalation of N3, the sequence of adenine protonation is reversed. While the N7 site still displays basic properties, the N1 site has undergo

ISSN:0947-6539    

DOI:10.1002/chem.19970030310

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractWe designed and targeted for synthesis the C‐ring aryl taxoids2a‐cin order to develop methods for the construction of the taxoid skeleton and to test their cytotoxicity against tumor cells. Compound2awas synthesized by a convergent route from hydrazone5and aldehyde4. Key steps included a Shapiro reaction to join5and4, a McMurry coupling to construct the 8‐membered ring, a carbonate opening to introduce the 2‐benzoate group, and an allylic oxidation followed by side‐chain attachment. A similar sequence led to compound2c, whereas attempts to attain2bwere thwarted by the lability of the benzyl group during the carbonate opening. The biological activity of2aand2cagainst tumor cells was considerably less than that

ISSN:0947-6539    

DOI:10.1002/chem.19970030311

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractUsing a convergent synthetic strategy starting from nicotinic acid and imidazole, we have prepared the (E) and (Z) isomers of 1‐benzyl‐3‐carbamoyl‐5‐[2‐(ethoxycarbonylmethylene)‐2‐(1‐(p‐to‐lylsulfamoyl)imidazol‐4‐yl)ethyl]pyridinium bromide (21) as models of the urocanase reaction. Domino reactions of both (E)‐21and (Z)‐21 led to the same spirocyclic compound, (3aRS)‐11‐[9‐([D7]benzyl)‐5‐ethoxy‐1‐(p‐tolylsulfamoyl)‐1H,9H‐furo[2,3‐g]imidazo[5,4‐f]isoquinolyl]carboxamide (33), which was isolated and spectroscopically characterised. A possible sequence of reactions leading to33shows a number of analogies to the conversions catalysed by the enzyme urocanase. Removal of thep‐tolylsulfamoyl protecting group of (E)‐21and (Z)‐21under mild conditions led to the highly reactive model compounds (E)‐4and (Z)‐4, which were identified by1H NMR spectroscopy, but could not be isolated, owing to their instability. To facilitate the monitoring of the reaction cascade by NMR spectroscopy (Z)‐21was prepared in which the benzyl group was fully deuterated. Its deprotection to (Z)‐4started a reaction cascade, which led, after purification, to a main product. According to investigations by UV and1H NMR spectroscopy it seems very likely that 1‐([D7]benzyl)‐3‐carbamoyl‐7‐(ethoxycarbonylmethyl)‐imidazo[4,5‐f]isoquinolinium bromide (27) was formed. The presumed mechanism of its format

ISSN:0947-6539    

DOI:10.1002/chem.19970030312

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY