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91.
Raffaello Romeo Giuseppe Arena Luigi Mons Scolaro Maria Rosario Plutino 《Inorganica chimica acta》1995,240(1-2):81-92
The rates of displacement of dimethyl sulfoxide from the cation [Pt(phen) (CH3) (Me2SO)]+ by a series of uncharged and negatively charged nucleophiles have been measured in a methanol/water (19:1 vol./vol.) mixture. The starting complex and the reaction products were characterized either as solids or in solution by their IR and 1H NMR spectra. The substitution reactions take place by way of a direct bimolecular attack of the ligand on the substrate. The sequence of reactivity observed is as expected on the basis of a nucleophilicity scale relevant for + 1 charged substrates ([Pt(en) (NH3)Cl]+ used as standard). The difference of reactivity between the first (t-BuNH2) and the last (SeCN−) members of the series spans five orders of magnitude. The value measured for the nucleophilic discrimination (1.55) is the highest found so far for cationic substrates. This is a result of the easy transfer of some of the electron density brought in by the incoming ligand into the ancillary ligands. When the reaction is carried out in a series of protic and dipolar aprotic solvents, using chloride ion as nucleophile, the rate of formation of [Pt (phen) (CH3)Cl] is dominated by the extent of solvation of Cl−, as measured by its values of the Gibbs molar energy of transfer ΔtG0. Conductivity measurements at 25°C in dichloromethane were fitted to the Fuoss equation and the values of the dissociation constants Kd for the ion pairs were calculated as follows: 2.27 × 10−5 M for Bu4NCl, 2.75 × 10−5 M for Bu4NSCN and 17.05 × 10−5 M for [Pt(phen) (CH3) (Me2SO)]PF6. The pseudo-first-order rate constants kobs for the reactions with Bu4NCl, Bu4NBr, Bu4NSCN and Bu4NI showed a curvilinear dependence on the concentration of the salt which levels off very soon (at concentrations higher than 0.005 M the kinetics are zero order in [Bu4NX]). On addition of the inert electrolyte Bu4NPF6 the rates slow down and the kinetics follow the rate law kobs = kKip[Bu4NX]/[Bu4NPF6] + Kip[Bu4NX]). These findings fit well with a reaction scheme which involves a pre-equilibrium Kip between ion pairs, followed by unimolecular substitution within the contact ion pair [Pt(phen) (CH3) (Me2SO)X]ip. Values of the equilibrium constants Kip for ion-pair exchange and of the internal substitution rates k were derived. The latter showed that the discrimination in reactivity between Cl−, Br−, SCN− and I− is greatly reduced with respect to aqueous solutions. The reason behind this may be desolvation of the ions coupled to the fact that a contact ion pair is already at a certain distance along the reaction coordinate in the direction of the transition state. Applications of the special salt effect and of ion pairing to synthesis are discussed. 相似文献
92.
Juventino J. Garcia María L. Hernndez Hugo Torrens Atilano Gutierrez Federico del Río 《Inorganica chimica acta》1995,230(1-2):173-176
By reacting [(C5Me5)M(SRF)2] (forM = Ir, Rf = C6F5 (1a) or C6F4H-p (1b); for M = Rh, Rf = C6F5 (2a) or C6F4H-p (2a)) in toluene with Na[AuCl4], ionic binuclear compounds with the general formula [(C5Me5)M(μ-SRF)2AuCl2]Cl for M = Ir, R = C6F5 (3a) or C6F4H-p (3a); for M = Rh, RF = C6F5 (4a) or C6F4H-p (4b) can be obtained, together with small amounts of [(C5Me5)2Rh2(μ-SRF)(μ-Cl)2]Cl (RF = C6F5 (5a) or C6F4H-p (5b)) as by-products when 2a and 2b were used. 相似文献
93.
In vitro recognition of the replication origin of pLS1 and of plasmids of the pLS1 family by the RepB initiator protein. 总被引:4,自引:1,他引:3
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Rolling-circle replication of plasmid pLS1 is initiated by the plasmid-encoded RepB protein, which has nicking-closing (site-specific DNA strand transferase) enzymatic activity. The leading-strand origin of pLS1 contains two regions, (i) the RepB-binding site, constituted by three directly repeated sequences (iterons or the bind region), and (ii) the sequence where RepB introduces the nick to initiate replication (the nic region). A series of plasmids, belonging to the pLS1 family, show features similar to those of pLS1 and have DNA sequences homologous to the pLS1 nic region. In addition, they all share homologies at the level of their Rep proteins. However, the bind regions of these plasmids are, in general, not conserved. We tested the substrate specificity of purified RepB of pLS1. The RepB protein has a temperature-dependent nicking-closing action on supercoiled pLS1, as well as on recombinant plasmid DNAs harboring the pLS1 nic region. The DNA strand transferase activity of pLS1-encoded RepB was also assayed on two plasmids of the pLS1 family, namely, pE194 and pFX2. DNAs from both plasmids were relaxed by RepB, provided they had a proper degree of supercoiling; i.e., it was necessary to modulate the supercoiling of pE194 DNA to achieve RepB-mediated DNA relaxation. Single-stranded oligonucleotides containing the nic regions of various plasmids belonging to the pLS1 family, including those of pE194 and pFX2, were substrates for RepB. In vitro, the RepB protein does not need to bind to the iterons for its nicking-closing activity. 相似文献
94.
Manganese and lignin peroxidase (MnP, LiP) activities were measured in straw extracts from cultures of Phanerochaete chrysosporium. Out of six MnP substrates, the MBTH/DMAB (3-methyl-2-benzothiazolinone hydrazone/3-(dimethylamino)benzoic acid), gave the highest MnP activity. Detection of LiP activity as veratryl alcohol oxidation was inhibited by phenols in the straw culture extracts. Appropriate levels of veratryl alcohol and peroxide (4 mM and 0.4 mM, respectively), and a restricted sample volume (not larger than 10%) were necessary to detect activity. 相似文献
95.
96.
Identification and Characterization of a New Serotonergic Recognition Site with High Affinity for 5-Carboxamidotryptamine in Mammalian Brain 总被引:1,自引:0,他引:1
Ma. Elena Castro Tamara Romón Ma. Josefa Castillo Elena del Olmo Angel Pazos Carmen del Arco 《Journal of neurochemistry》1997,69(5):2123-2131
Abstract: We analyzed the existence of an additional serotonin (5-HT) receptor subtype, sensitive to 5-carboxamidotryptamine, in the mammalian brain. Radioligand binding studies with [3 H]5-HT were carried out in rat, guinea pig, and human brain membranes, in the presence of unlabeled drugs to mask the binding to all known 5-HT receptors, with the exception of 5-HT1E sites. Under these conditions, unlabeled 5-carboxamidotryptamine still showed a biphasic competition curve with a nanomolar affinity component. Saturation studies with 5-[3 H]carboxamidotryptamine were carried out in the presence of (±)-8-hydroxy-2-(di- n -propylamino)tetralin, mesulergine, and ergotamine, to mask the binding to all receptors known to be labeled by 5-carboxamidotryptamine. These studies showed the existence in cortex and hippocampus from guinea pig and human brain of a remaining binding site with high affinity ( pK D = 7.8–8.1) and a unique pharmacological profile. 5-HT and 5-carboxamidotryptamine showed nanomolar affinity, whereas 5-methoxytryptamine recognized this binding site with intermediate affinity. Other drugs exhibited low or very low potency in inhibiting this binding. The addition of 5'-guanylylimidodiphosphate significantly reduced the number of binding sites labeled by 5-[3 H]carboxamidotryptamine, in the presence of the masking drugs described above, indicating the interaction with a GTP-binding protein. Preliminary autoradiographic studies in human brain appear to indicate that this 5-HT binding site is present in areas such as the globus pallidus, neocortex, and hippocampus, among others. 相似文献
97.
BCL-2-Related Protein Expression in Apoptosis: Oxidative Stress Versus Serum Deprivation in PC12 Cells 总被引:5,自引:0,他引:5
Abstract: Expression of the BCL-2 protein family members, BAX, BAK, BAD, BCL-xL, BCL-xS, and BCL-2, was measured (by western blotting using specific antibodies) in PC12 cells before and during apoptosis induced by either H2 O2 treatment or by serum deprivation and during rescue from apoptosis by nerve growth factor (NGF). H2 O2 -induced apoptosis, as measured by DNA fragmentation, caused: (a) a dose-dependent increase in BAX, (b) a dose-independent increase in BAK, and (c) a dose-dependent inhibition of BAD expression. By comparison, apoptosis induced by serum deprivation resulted in a time-dependent decrease in both BAX and BAK, along with a dramatic and sudden decrease in BAD expression. However, when PC12 cells were incubated in an apoptosis-sparing medium (i.e., NGF-supplemented serum-free medium), both BAX and BAK were increased significantly, whereas BAD expression remained inhibited. BCL-xL expression was increased by H2 O2 but unaffected by serum deprivation or long-term NGF treatment. Neither BCL-2 nor BCL-xS expression could be detected in PC12 cells under the experimental conditions tested. Our results show that the expression of BAX, BAK, BAD, and BCL-xL is altered in a stimulus-dependent manner but cannot be used to define whether a cell will undergo or survive apoptosis. The similarity between changes in expression of BCL-2-related proteins induced by H2 O2 exposure and NGF rescue could reflect activation in part of a common antioxidant pathway. 相似文献
98.
The Uba2 and Ufd1 proteins of Saccharomyces cerevisiae interact with poly(A) polymerase and affect the polyadenylation activity of cell extracts 总被引:2,自引:0,他引:2
Poly(A) polymerase is responsible for the addition of the adenylate tail to the 3′ ends of mRNA. Using the two-hybrid system,
we have identified two proteins which interact specifically with the Saccharomyces cerevisiae poly(A) polymerase, Pap1. Uba2 is a homolog of ubiquitin-activating (E1) enzymes and Ufd1 is a protein whose function is probably
also linked to the ubiquitin-mediated protein degradation pathway. These two proteins interact with Pap1 and with each other,
but not with eight other target proteins which were tested in the two-hybrid system. The last 115 amino acids of Uba2, which
contains an 82-amino acid region not present in previously characterized E1 enzymes, is sufficient for the interaction with
Pap1. Both Uba2 and Ufd1 can be co-immunoprecipitated from extracts with Pap1, confirming in vitro the interaction identified
by two-hybrid analysis. Depletion of Uba2 from cells produces extracts which polyadenylate precursor RNA with increased efficiency
compared to extracts from nondepleted cells, while depletion of Ufd1 yields extracts which are defective in processing. These
two proteins are not components of polyadenylation factors, and instead may have a role in regulating poly(A) polymerase activity.
Received: 6 January 1997 / Accepted: 27 February 1997 相似文献
99.
Plasmid rolling circle replication and its control 总被引:10,自引:0,他引:10
Manuel Espinosa Gloria del Solar Fernando Rojo Juan C. Alonso 《FEMS microbiology letters》1995,130(2-3):111-120
Abstract This review summarises current information on rolling circle replicating plasmids originally isolated from Gram-positive bacteria with a low guanine and cytosine content in their DNA. It focuses on the peculiar biological features of these small, high copy number plasmids that replicate via an asymmetric RC mechanism. The regulation of plasmid copy number is also discussed. 相似文献
100.
M. Teresa García-López Ibon Alkorta M. José Domínguez Rosario González-Mu?iz Rosario Herranz Nils L. Johansen Kjeld Madsen Henning Th?gersen Peter Suzdak 《Letters in Peptide Science》1995,1(6):269-276
Summary In order to enforce different spatial orientations in the C-terminal hexapeptide of neurotensin (NT8–13) and to gain information about the importance of the 10–11 peptide bond for binding to NT receptors, the Pro10-Tyr11 fragment has been replaced with (2R,8S,8aR)-, (2S,8S,8aR)-, (2S,8S,8aS)-, (2S,8R,8aS)- and (2R,8R,8aS)-8-amino-2-benzyl-3-oxoindolizidine-2-carboxylic acid. Molecular dynamics calculations and energy minimization studies have shown that, contrarily to the Pro-Tyr moiety, none of these indolizidines display a tendency to adopt type I and III -turns, but those having (8S,8aR) or (8R,8aS) stereochemistry essentially adopt extended conformations and the (8S,8aS) stereoisomer prefers a nonstandard folding. The four diastereomeric NT8–13 analogues incorporating (8S,8aR) or (8R,8aS) indolizidines displayed binding affinities for the brain NT receptor similar to that of [Ala11]-NT8–13 and only five- to ninefold lower than that of the corresponding analogue, [Phe11]NT8–13. Although this slight decrease could be attributed to differences in conformational behavior between these constrained NT8–13 analogues and [Phe11]NT8–13 or NT8–13, it is not clear whether the -turn around Pro10-AA11 (AA=Phe, Tyr) is conserved upon receptor binding. An excessive restriction in the motions of the aromatic side chain, imposed by the highly steric constraint of the indolizidine moiety, emerges as an alternative explanation. The findings reported here demonstrate the possibility of replacing the Pro10-Tyr11 dipeptide in NT8–13 with a non-peptide residue without affecting considerably the affinity for brain NT receptors. 相似文献