This paper reports the two-photon absorbing and orange-red fluorescence emitting properties of a series of new 2,1,3-benzothiadiazole (BTD)-based D-pi-A-pi-D-type and star-burst-type fluorescent dyes. In the D-pi-A-pi-D-type dyes 1-6, a central BTD core was connected with two terminal N,N-disubstituted amino groups via various pi-conjugated spacers. The star-burst-type dyes 8 and 10 have a three-branched structure composed of a central core (benzene core in 8 and triphenylamine core in 10) and three triphenylamine-containing BTD branches. All the BTD-based dyes displayed intense orange-red color fluorescence in a region of 550-689 nm, which was obtained by single-photon excitation with good fluorescent quantum yield up to 0.98 as well as by two-photon excitation. Large two-photon absorption (TPA) cross-sections (110-800 GM) of these BTD dyes were evaluated by open aperture Z-scan technique with a femtosecond Ti/sapphire laser. The TPA cross-sections of D-pi-A-pi-D-type dyes 2-6 with a benzene, thiophene, ethene, ethyne, and styrene moiety, respectively, as an additional pi-conjugated spacer are about 1.5-2.5 times larger than that of 1c with only a benzene spacer. The TPA cross-sections significantly increased in three-branched star-burst-type BTDs 8 (780 GM) with a benzene core and 10 (800 GM) with a triphenylamine core, which are about 3-5 times larger than those of the corresponding one-dimensional sub-units 9 (170 GM) and 11 (230 GM), respectively. The ratios of sigma/e(pi) between three-branched and one-dimensional dyes were 6.5:3.8 (for 8 and 9) and 6.0:4.0 (for 10 and 11), which are larger than those predicted simply on the basis of the chromophore number density (1:1), according to a cooperative enhancement of the two-photon absorbing nature in the three-branched system.

NifS-like proteins catalyze the formation of elemental sulfur (S) and alanine from cysteine (Cys) or of elemental selenium (Se) and alanine from seleno-Cys. Cys desulfurase activity is required to produce the S of iron (Fe)-S clusters, whereas seleno-Cys lyase activity is needed for the incorporation of Se in selenoproteins. In plants, the chloroplast is the location of (seleno) Cys formation and a location of Fe-S cluster formation. The goal of these studies was to identify and characterize chloroplast NifS-like proteins. Using seleno-Cys as a substrate, it was found that 25% to 30% of the NifS activity in green tissue in Arabidopsis is present in chloroplasts. A cDNA encoding a putative chloroplast NifS-like protein, AtCpNifS, was cloned, and its chloroplast localization was confirmed using immunoblot analysis and in vitro import. AtCpNIFS is expressed in all major tissue types. The protein was expressed in Escherichia coli and purified. The enzyme contains a pyridoxal 5Ј phosphate cofactor and is a dimer. It is a type II NifS-like protein, more similar to bacterial seleno-Cys lyases than to Cys desulfurases. The enzyme is active on both seleno-Cys and Cys but has a much higher activity toward the Se substrate. The possible role of AtCpNifS in plastidic Fe-S cluster formation or in Se metabolism is discussed.NifS-like proteins are pyridoxal 5Ј phosphate (PLP)-dependent enzymes with sequence similarity to the Cys desulfurase encoded by nifS of Azotobacter vinelandii (Zheng et al., 1993). These proteins have been found in most organisms tested, where they play a role in S or Se metabolism (Mihara et al., 1997). NifS-like proteins catalyze the breakdown of Cys to form Ala and elemental S, or they may act on related substrates such as seleno-Cys to form Ala and elemental Se (Mihara et al., 1997). The nifS of A. vinelandii is required under nitrogen fixation conditions for the formation of Fe-S clusters in nitrogenase (Zheng et al., 1993). A. vinelandii NIFS is present in a gene cluster with several other genes (nifU, nifA, and cysE) all thought to be involved in Fe-S cluster formation. A second NifS-like protein of A. vinelandii, IscS, has a housekeeping function in the formation of other cellular Fe-S proteins (Zheng et al., 1993). Interestingly, iscS is present in a gene cluster that contains paralogs of the nif genes (iscU and iscA), thus, the nif and isc clusters share a similar organization (Zheng et al., 1998). Homologs of the nif/isc genes, all thought to play a role in cellular Fe-S cluster formation have been discovered in several other bacteria including in Escherichia coli (Zheng et al., 1998). In the eukaryotes, Fe-S clusters are essential cofactors for mitochondrial respiration, as well as for many cytosolic proteins. Recent work has suggested that in yeast and in mammals, all Fe-S clusters are made in the mitochondria (for review, see Lill and Kispal, 2000). Fe-S cluster formation in the mitochondria of eukaryotes involves homologs of the genes encoded by the nif/isc clusters of bacteria (Kispal et a...

IscS and IscU from Escherichia coli cooperate with each other in the biosynthesis of iron-sulfur clusters. IscS catalyzes the desulfurization of L-cysteine to produce L-alanine and sulfur. Cys-328 of IscS attacks the sulfur atom of L-cysteine, and the sulfane sulfur derived from L-cysteine binds to the S␥ atom of Cys-328. In the course of the cluster assembly, IscS and IscU form a covalent complex, and a sulfur atom derived from L-cysteine is transferred from IscS to IscU. The covalent complex is thought to be essential for the cluster biogenesis, but neither the nature of the bond connecting IscS and IscU nor the residues involved in the complex formation have been determined, which have thus far precluded the mechanistic analyses of the cluster assembly. We here report that a covalent bond is formed between Cys-328 of IscS and Cys-63 of IscU. The bond is a disulfide bond, not a polysulfide bond containing sulfane sulfur between the two cysteine residues. We also found that Cys-63 of IscU is essential for the IscU-mediated activation of IscS: IscU induced a six-fold increase in the cysteine desulfurase activity of IscS, whereas the IscU mutant with a serine substitution for Cys-63 had no effect on the activity. Based on these findings, we propose a mechanism for an early stage of iron-sulfur cluster assembly: the sulfur transfer from IscS to IscU is initiated by the attack of Cys-63 of IscU on the S␥ atom of Cys-328 of IscS that is bound to sulfane sulfur derived from L-cysteine.

The development of a unique class of non-planar push-pull chromophores by means of [2 + 2] cycloaddition, followed by cycloreversion, of electron-deficient olefins, such as tetracyanoethene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)-TCNQ), as well as dicyanovinyl (DCV) and tricyanovinyl (TCV) derivatives, to donor-substituted alkynes is explored in this feature article. This high-yielding, "click-chemistry"-type transformation with acetylenic dendrimers affords dendritic electron sinks capable of multiple electron uptake within a narrow potential range. An [AB]-type oligomer with a dendralene backbone was synthesised by a one-pot, multi-component cascade reaction of polyyne oligomers with TCNE and tetrathiafulvalene (TTF). In most cases, the resulting chromophores feature intense intramolecular charge-transfer bands extending far into the near infrared region and some of them display high third-order optical nonlinearities. Despite substitution with strong donors, the electron-withdrawing moieties in the new chromophores remain potent acceptors and a number of them display positive first reduction potentials (vs. the ferrocenium/ferrocene (Fc(+)/Fc) couple in CH(2)Cl(2)), which rival those of parent TCNE, TCNQ and F(4)-TCNQ. The non-planarity of the chromophores strongly enhances their physical properties when compared to planar push-pull analogues. They feature high solubility, thermal stability and sublimability, which enables formation of amorphous, high-optical-quality thin films by vapour phase deposition and makes them interesting as advanced functional materials for novel opto-electronic devices.

Novel donor-pi-bridge-acceptor-pi-bridge-donor (D-pi-A-pi-D) type 2,1,3-benzothiadiazole fluorescent dyes connected to the N,N-diarylamino terminus via various type pi-conjugate spacers exhibits large two-photon absorption cross-sections and high fluorescent quantum yields in orange-red color.

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The assembly of iron-sulfur (Fe-S) clusters is mediated by complex machinery which, in Escherichia coli, is encoded by the iscRSUA-hscBA-fdx-ORF3 gene cluster. Here, we demonstrate the network of protein-protein interactions among the components involved in the machinery. We have constructed (His)(6)-tagged versions of the components and identified their interacting partners that were co-purified from E. coli extracts with a Ni-affinity column. Direct associations of the defined pair of proteins were further examined in yeast cells using the two-hybrid system. In accord with the previous in vitro binding and kinetic experiments, interactions were observed for the combinations of IscS and IscU, IscU and HscB, IscU and HscA, and HscB and HscA. In addition, we have identified previously unreported interactions between IscS and Fdx, IscS and ORF3, IscA and HscA, and HscA and Fdx. We also found, by site-directed mutational analysis combined with the two-hybrid system, that two cysteine residues in IscU are essential for binding with HscB but not with IscS. Despite the complex network of interactions in various combinations of components, heteromultimeric complexes were not observed in our experiments except for the putative oligomeric form of IscU-IscS-ORF3. Thus, the sequential association and dissociation among the IscS, IscU, IscA, HscB, HscA, Fdx, and ORF3 proteins may be a critical process in the assembly of Fe-S clusters.

A large series of conjugated carbazole dimers, namely bicarbazoles 1-12, were synthesized by Suzuki-Miyaura, Sonogashira, Hay, and McMurry coupling reactions. In 1-12, the two carbazole moieties are linked at the 1-, 2-, or 3-position directly or via an acetylenic or olefinic spacer. The structure-property relationships, particularly the effects of the conjugation connectivity and the π-conjugated spacers on the electronic, photophysical, and electrochemical properties of 1-12, were studied by extensive UV-vis and fluorescence spectroscopic measurements, cyclic voltammetry (CV), and theoretical calculations as well as X-ray crystallographic analyses. The connection at the 1-position of carbazole ensures high extent of π-conjugation, while that at the 3-position enhances the electron-donating ability. Both acetylenic and olefinic spacers allow the extension of π-conjugation, and the latter also causes the increase of the donor ability. Moreover, the structural variations were found to affect the fluorescence quantum yields significantly, which are up to 0.84.