摘要:

AbstractProtein structure modelling offers a method of obtaining 3‐dimensional information that can be tested and used to plan mutagenesis experiments when a crystallographically determined structure is not available. At its simplest a model may consist of little more than a secondary structure prediction coupled with a determination of the likely regions of transmembrane/membrane surface/globular configuration. These methods can yield an interesting topology map of the protein, which places the residues in their likely positions with respect to, for example, the membrane interface. If it is a member of a large family of related proteins then aligned protein sequences can be used to predict the residues that have an important function as these. will be largely conserved in the alignments. Using all these methods a model can be constructed (using for example, the Nicholson Molecular Modelling Kit) to visualize the proposed structure in three dimensions following the premise of good design, that is, avoiding obvious steric clashes, packing of helices in a realistic manner, observing the correct H‐bond lengths,etc. In this latter exercise the review of Chothia (Annu. Rev. Biochem.53, 537–572, 1984) of the principles of protein structure is particularly helpful as it clearly sets out how proteins pack and their preferred configuration. There is a wealth of information about individual amino acid conformational preferences and observed frequencies of occurrence in known protein structures, which can help decide how the residues in the model can be oriented.In this article we have collated the various protein models of the bacterial light‐harvesting complexes and present our own model, which is a synthesis of the available biophysical data and theoretical predictions, and show its performance in explaining recent results of site‐directed mutants of the LHI and LH2 light‐harvesting complexes ofRhodobacter

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1994.tb05144.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractPorphyrin‐C60dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels‐Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn‐C60) and free base (P‐C60) dyads, determined by time‐resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet‐singlet energy transfer to C60The lifetime of Pzn‐1C60is ‐5 ps in toluene, whereas the singlet lifetime of an appropriate C60model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pznbull;+‐C60bull;‐. In toluene, P‐1C60is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of‐10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60triplet. In the more polar benzonitrile, P‐1C60underoes photoinduced electron transfer to give P•+‐C60bull;‐. The electron tran

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1994.tb05145.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractSome 2‐nitrothienyl derivatives characterized by the presence of an aryl or heteroaryl substituent in the C‐5 position were synthesized by using a photochemical coupling procedure. All synthesized compounds were demonstrated to be singlet oxygen sensitizers in the trans‐α,α'‐dimethylstilbene photo

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1994.tb05146.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractExperimental studies have recently demonstrated that fluorescence emission can be quenched by laser light pulses from modem high‐repetition rate lasers, a phenomenon we call “light quenching.” In this overview article, we describe the possible effects of light quenching on the steady‐state and time‐resolved intensity and anisotropy of fluorophores. One can imagine two classes of experiments. Light quenching can occur within the single excitation pulse, or light quenching can be accomplished with a second time‐delayed quenching pulse. The extent of light quenching depends on the amplitude of the emission spectrum at the quenching wavelength. Different effects are expected for light quenching by a single laser beam (within a single laser pulse) or for a time‐delayed quenching pulse. Depending upon the polarization of the light quenching beam, light quenching can decrease or increase the anisotropy. Remarkably, the light quenching can break the usual z‐axis symmetry of the excited state population, and the measured anisotropy (or polarization) depends upon whether the observation axis is parallel or perpendicular to the propagation direction of the light quenching beam. The polarization can increase to unity under selected conditions. Quenching with time‐delayed light pulses can result in step changes in the intensity or anisotropy, which is predicted to result in oscillations in the frequency‐domain intensity and anisotropy decays. These predicted effects of light quenching, including oscillations in the frequency‐domain data, were demonstrated to occur using selected fluorophores. The increasing availability and use of pulsed laser sources requires consideration of the possible effects of light quenching and offers the opportunity for a new class of two‐pulse or multiple‐pulse time‐resolved experiments where the sample is prepared by the excitation pulse and subsequent quenching pulses to modify the excited state population, followed by time‐ or frequency‐domain measurement of the opticall

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1994.tb05147.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractFluorescence, excitation and absorption spectra of octadecylrhodamine B (ORB) have been observed in solutions of water, methanol, ethanol and other simple alcohols. A remarkable increase in the ORB fluorescence intensity occurs when the probe is transferred from water to the alcohols. This effect is ascribed to the release of ORB fluorescent monomers from nonfluorescent multimers, which exist predominantly in water. The dependence of the fluorescence intensity on the composition of water‐alcohol binary solvent systems shows that hydrophobic interactions play an important role in the monomer = multimer equilibrium of ORB in these solutions. The fluorescence lifetimes of ORB in the alcohols are also reporte

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1994.tb05148.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract‐The photochemistry of 4′‐aminomethyl‐4,5′,8‐trimethylpsoralen (AMT) with poly(dA‐dT) and calf thymus DNA was studied. The extent of photoadduct formation and the distribution of photoadducts (3,4– and 4′,5′‐monoadducts and crosslinks) were determined by liquid scintillation analysis and HPLC, respectively. The adducts were characterized on the basis of their UV absorption spectra and mass spectral analysis. The high DNA binding constant for AMT (1.5 x 105M−1) led to a high fraction of intercalated molecules, which contributed to the high level of AMT photoadduct formation, as many as 102 adducts per kilobase pair. In addition, there is a distinct difference in the adduct distribution compared to the previously studied 8‐methoxypsoralen (8‐MOP). Under the conditions employed for the photochemical studies, virtually all of the AMT molecules in solution are intercalated, occupying 25% of the base pair sites. Under similar conditions, 8‐MOP molecules occupied 10 times fewer sites. Thus, for AMT, DNA base pair sites other than 5′TA, the well‐characterized strong binding for psoralens in general, are an additional target for photomodification, which results in the formation of a higher percentage of monoadducts. The proportion of photoadducts formed was virtually independent of AMT concentration and U

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1994.tb05149.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractSunlight has been implicated in the high incidence of skin cancer found in patients receiving 6‐mercaptopurine (PSH) in the form of its pro‐drug azathioprine. In this study we have used EPR spectroscopy in conjunction with the spin‐trapping technique to determine whether PSH and its metabolic or photochemical oxidation products generate highly reactive free radicals upon UV irradiation. When an aqueous anaerobic solution (pH 5 or 9) of PSH (pK2= 7.7) and either 2‐methyl‐2‐nitrosopropane (MNP) or nitromethane (NM) were irradiated (λ>300 nm) with a Xe arc lamp, the corresponding purin‐6‐thiyl (PS.) radical adduct and the reduced form of the spin trap (MNPIH’or CH3N02) were observed. However, no radical adducts were detected when PSH and 5,5‐dimethyl‐1‐pyrroline‐N‐oxide (DMPO) were irradiated (λ= 320 nrn) in oxygen‐free buffer. These findings suggest that PSH does not photoionize but that instead MNP and NM are reduced by direct electron transfer from excited state PSH,1,3(PSH)*. In aerobic solution, oxygen can act as an electron acceptor and the O2*‐ and PS radicals are formed and trapped by DMPO. 6‐Mercaptopurine did photoionize when irradiated with a Nd:YAG laser at 355 nm as evidenced by the appearance of the DMPO/H (eeq+ H+) adduct, which decreased in intensity in the presence of N2O.1,3(6‐Mercap‐topurine)* oxidized ascorbate, formate and reduced glutathione to the corresponding ascorbyl, CO2.‐ or glutathiyl radicals. The photochemical behavior of 6‐thioxanthine and 6‐thiouric acid was similar to PSH. However, the excited states of these metabolic oxidation products exhibited stronger reducing properties than1,3(PSH)*. Photolysis of PSH photoproducts purine‐6‐sulfonate or purine‐6‐sulfinate resulted in homolysis of the C‐S bond and the appearance of the SO3′‐ and SO2−‐‐ radicals, respectively, which were detected by direct EPR. These studies demonstrate that UV irradiation of PSH, its photoproducts and metabolites generates a variety of free radicals that may b

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1994.tb05150.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract‐Time‐resolved reflectance was used to measure the absorption spectrum of hematoporphyrin derivative (HpD)in vivoin a murine tumor model. Reflectance measurements were performed in the 600–640 nm range on mice bearing the L1210 leukemia. Then the animals were administered 25 mg/kg body weight of HpD intraperito‐neally. One hour later the reflectance measurements were repeated. Fitting of the data using the diffusion theory allowed assessment of the absorption coefficient before and after the administration. As a difference between the latter and the former data, thein vivoabsorption spectrum of HpD was evaluated. Maximum absorption was measured at 620–625 nm. Similar spectral behavior was obtained for HpD in solution in the presence of low‐density l

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1994.tb05151.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractTiaprofenic acid is a photosensitizing nonsteroidal anti‐inflammatory drug, whose major photoproduct (decarboxytiaprofenic acid) is also a potent photosensitizer. Because of the lack of the carboxylate moiety, this photoproduct is more lipophilic and might bind more efficiently to cell membranes, thereby causing phototoxic damage. To verify the feasibility of this hypothesis, we have prepared the3H‐labeled analogs of tiaprofenic acid and its photoproduct and examined the binding, persistence and phototoxicity of the photoproduct using poorly metabolizing (fibroblasts) and actively metabolizing cells (hepatocytes). The photoproduct of tiaprofenic acid accumulates in both cell types as it is formed. Upon removal of the photoproduct from the culture medium, it rapidly disappears from hepatocytes but not from fibroblasts. Consequently, irradiation of fibroblasts previously incubated with the photoproduct and kept in culture in the dark for 20 h results in generalized cell damage while this effect is not observed in hepatocytes. Because of its long persistence in poorly metabolizing skin cells and its reluctance to photobleaching, the formation of this photoproduct in skin may be of relevance to explain thein vivophototoxicity of tiaprofenic a

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1994.tb05152.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract‐A number of carboxylic acid derivatives of the photoactive terthiophene, α‐terthienyl, were found to possess impressive UVA‐dependent activity against the human immunodeficiency virus, HIV‐1; but only when assayed in the absence of serum, indicating that the latter contained interfering components. Good antiviral activity required a high rate of singlet oxygen production, in accordance with previous observations on th

ISSN:0031-8655    

DOI:10.1111/j.1751-1097.1994.tb05153.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY