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Log K (log + k)
Selected AbstractsCompetition among zinc, manganese, and cadmium uptake in the freshwater alga Scenedesmus vacuolatusENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 3 2007Stefanie Töpperwien Abstract In the present study, Zn and Mn competition with Cd uptake was investigated in the freshwater alga Scenedesmus vacuolatus. Scenedesmus vacuolatus was exposed to experimental media with Cd and either Zn or Mn in short-term experiments; long-term experiments were undertaken to investigate the effect of growth on Cd accumulation. Cadmium accumulation in S. vacuolatus could be detected at very low exposure concentrations (free Cd2+, 2 × 10,14 to 1 × 10,11 M), and uptake was proportional to the free-Cd2+ concentration. Zinc was an effective competitive inhibitor of Cd uptake when the Zn2+ to Cd2+ ratio was greater than 14 in the exposure medium, whereas Mn competed with Cd for uptake above a Mn2+ to Cd2+ ratio of greater than 10,000. Binding constants for Cd and Zn affinity to the transport sites were determined (KZn and KCd). Values for KZn were slightly higher (log K = 9.4,9.8) than values for KCd (log K = 8.9,9.8). In contrast, Cd seemed not to compete with the Mn-binding sites for uptake over the Mn concentration range from 1 × 10,10 to 1 × 10,8 M. Determined values for the binding constants of Zn and Cd show that a simple model can be applied to predict Cd uptake at known Zn and Cd concentrations. The environmental implications of these results are discussed with respect to potential Cd toxicity for aquatic organisms. [source] Synthesis and Binding Properties of Dendritic Oxybathophenanthroline Ligands towards Copper(II)EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 22 2005Holger Stephan Abstract Dendritic oxybathophenanthroline ligands (generation 0 to 3) have been synthesized by treatment of 4,7-bis(4,-hydroxyphenyl)-1,10-phenanthroline with the corresponding Fréchet-type dendrons carrying a benzylic bromide function at the focal point. The complexation of copper(II) has been studied by liquid,liquid extraction using the radioisotope 64Cu and time-resolved laser-induced fluorescence spectroscopy (TRLFS) in organic media indicating the formation of 1:3 complexes (Cu:dendritic ligand). Electronic and EPR spectroscopy were used to characterize the copper(II) chromophore, which is shown to have the expected distorted square-planar geometry with two phenanthroline donors coordinated to the copper(II) center. The third dendritic ligand therefore is proposed to be bound by secondary interactions. The stability constants of the 1:3 complexes were found to be in the order of log K , 16 in CHCl3. On the other hand, increasing generation of the dendritic Fréchet-type branches leads to enhanced shielding of the copper ion from the environment. Additional information about this behaviour was obtained by the fluorescence lifetimes, which are much less influenced upon addition of copper(II) salt to solutions of the higher generation ligands. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Kinetic and Equilibrium Studies of Reactions of N-Heterocycles with Dimeric and Monomeric Oxorhenium(V) ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 10 2003James H. Espenson Abstract Equilibrium constants have been evaluated for the reaction {MeReO(edt)}2 + 2 L , 2 MeReO(edt)L, where edt is 1,2-ethanedithiolate and L is any of 13 N-donor heterocyclic ligands. The values of K range from 1.37(27)×10,2 for pyrimidine to 1.95(6)×106 for imidazole at 25 °C in chloroform. A successful correlation of logK with log (Ka) of HL+ was realized except in the case of the 2-substituted ligands 2-picoline and quinoline, where steric effects make K smaller than expected from the proton basicity of L. The kinetics of the same reactions were studied; the rate law for the reaction in the forward direction is given by ,d[{MeReO(edt)}2]/dt = {ka + kb[L]}[L] × [{MeReO(edt)}2]. Except for 2-picoline and quinoline, the major pathway is provided by the term that shows the quadratic dependence on [L]. Values of log (kb) also correlate with log K, and therefore necessarily with log (Ka). [source] Ester prodrugs of morphine improve transdermal drug delivery: a mechanistic studyJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 7 2007Jhi-Joung Wang Two alkyl esters of morphine, morphine propionate (MPR) and morphine enanthate (MEN), were synthesized as potential prodrugs for transdermal delivery. The ester prodrugs could enhance transdermal morphine delivery. The mechanisms of this enhancing effect were elucidated in this study. Both prodrugs were more lipophilic than their parent drug as evaluated by the skin/vehicle partition coefficient (log P) and capacity factor (log K,). The in-vitro skin permeation of morphine and its prodrugs from pH 6 buffer was in the order of MEN > MPR > morphine. MPR and MEN respectively enhanced the transdermal delivery of morphine by 2- and 5-fold. A contrary result was observed when using sesame oil as the vehicle. The prodrugs were stable against chemical hydrolysis in an aqueous solution, but were readily hydrolysed to the parent drug when exposed to skin homogenate and esterase. Approximately 98% MPR and ,75% MEN were converted to morphine in an in-vitro permeation experiment. The viable epidermis/dermis contributed to a significant resistance to the permeation of ester prodrugs. According to the data of skin permeation across ethanol-, ,-terpineol-, and oleic acid-pretreated skin, MEN was predominantly transported via lipid bilayer lamellae in the stratum corneum. The intercellular pathway was not important for either morphine or MPR permeation. [source] Permeability of the continental crust: dynamic variations inferred from seismicity and metamorphismGEOFLUIDS (ELECTRONIC), Issue 1-2 2010S. E. INGEBRITSEN Geofluids (2010) 10, 193,205 Abstract The variation of permeability with depth can be probed indirectly by various means, including hydrologic models that use geothermal data as constraints and the progress of metamorphic reactions driven by fluid flow. Geothermal and metamorphic data combine to indicate that mean permeability (k) of tectonically active continental crust decreases with depth (z) according to log k , ,14,3.2 log z, where k is in m2 and z in km. Other independently derived, crustal-scale k,z relations are generally similar to this power-law curve. Yet there is also substantial evidence for local-to-regional-scale, transient, permeability-generation events that entail permeabilities much higher than these mean k,z relations would suggest. Compilation of such data yields a fit to these elevated, transient values of log k , ,11.5,3.2 log z, suggesting a functional form similar to that of tectonically active crust, but shifted to higher permeability at a given depth. In addition, it seems possible that, in the absence of active prograde metamorphism, permeability in the deeper crust will decay toward values below the mean k,z curves. Several lines of evidence suggest geologically rapid (years to 103 years) decay of high-permeability transients toward background values. Crustal-scale k,z curves may reflect a dynamic competition between permeability creation by processes such as fluid sourcing and rock failure, and permeability destruction by processes such as compaction, hydrothermal alteration, and retrograde metamorphism. [source] Forcing highly connected subgraphsJOURNAL OF GRAPH THEORY, Issue 4 2007Maya Jakobine Stein Abstract A theorem of Mader states that highly connected subgraphs can be forced in finite graphs by assuming a high minimum degree. We extend this result to infinite graphs. Here, it is necessary to require not only high degree for the vertices but also high vertex-degree (or multiplicity) for the ends of the graph, that is, a large number of disjoint rays in each end. We give a lower bound on the degree of vertices and the vertex-degree of the ends which is quadratic in k, the connectedness of the desired subgraph. In fact, this is not far from best possible: we exhibit a family of graphs with a degree of order 2k at the vertices and a vertex-degree of order k log k at the ends which have no k -connected subgraphs. Furthermore, if in addition to the high degrees at the vertices, we only require high edge-degree for the ends (which is defined as the maximum number of edge-disjoint rays in an end), Mader's theorem does not extend to infinite graphs, not even to locally finite ones. We give a counterexample in this respect. But, assuming a lower bound of at least 2k for the edge-degree at the ends and the degree at the vertices does suffice to ensure the existence (k + 1)- edge -connected subgraphs in arbitrary graphs. © 2006 Wiley Periodicals, Inc. J Graph Theory 54: 331,349, 2007 [source] The nitro anomaly and Brønsted ,nuc values in SN2 reactions on chlorine,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2002Linoam Eliad Abstract The kinetics of chlorine transfer reactions between N -chlorosuccinimide (NCS) and four carbon nucleophiles (the conjugated bases of phenyldinitromethane, Meldrum's acid, phenylmalononitrile and phenylnitro-methane) in water were determined. A plot of log k for the SN2 reactions vs the pKa of the first three conjugated acids of the nucleophiles gave a straight line with a slope (,nuc) of 1.8. The data point for the mononitro derivative, phenylnitromethane, deviates negatively from the line by 6.7 log units. This deviation is typical of proton transfer reactions and was recently shown to occur also in SN2 reactions on bromine. Copyright © 2002 John Wiley & Sons, Ltd. [source] The lipophilicity indices of flavonoids estimated by reversed-phase liquid chromatography using different computation methodsJOURNAL OF SEPARATION SCIENCE, JSS, Issue 12 2009Rodica Domnica Briciu Abstract The chromatographic behavior of some flavonoids was established by RP-HPLC on RP-18 (LiChroCART, LiChrosphere RP-18e), RP-8 (Zorbax, Eclipse XDB-C8), CN (Säulentechnik, LiChrosphere CN100) columns. The mobile phases were mixtures of methanol-water in different volume proportions from 70 to 80% v/v for RP-18 and RP-8, while for the CN column the proportions were between 66 and 70% v/v. The lipophilicity was expressed through different lipophilicity descriptors such as mean of k (mk), mean of log k (mlog k), log kW, S, f0, and scores of k and log k corresponding to the first principal component. The experimental lipophilicity indices are directly correlated with the computed values, via computer software and internet module, at a high analytical level. Furthermore, the results obtained applying principal component analysis to k and/or log k values allow the prediction and explanation of the interaction involved in the retention mechanism which takes place between the compounds and the employed stationary phases during the development. [source] Comparison of ethylammonium formate to methanol as a mobile-phase modifier for reversed-phase liquid chromatographyJOURNAL OF SEPARATION SCIENCE, JSS, Issue 5 2006Martin M. Waichigo Abstract Ethylammonium formate (EAF), (C2H5NH3+HCO2,), is a room-temperature ionic liquid that has a polarity similar to that of methanol (MeOH) or acetonitrile. The separation at 1 mL/min of a test mixture of vitamins or phenols on a polystyrene-divinylbenzene column using either an EAF- or MeOH-water mobile phase is similar in terms of both resolution and analysis time. Because the viscosity of EAF is higher than that of MeOH, the plate count for phenol at room temperature is lower by about a factor of 1.1,1.4 depending on the flow rate. However, van Deemter plots show that this loss in plate count at 1 mL/min can be recovered and improved from 1500 to 2400 plates by working at a slightly elevated temperature of 55°C. A slower flow rate such as 0.8 mL/min can also substantially improve the plate count as compared to 1,1.5 mL/min. Log P (octanol partition coefficient) versus log k, data for a variety of neutral test solutes are again similar whether EAF or MeOH is used as the organic modifier. Resolution of certain peak pairs such as 2,4-dinitrophenol/2,4,6-trinitrophenol and p -aminobenzoate/benzoate is enhanced using EAF as compared to MeOH. One advantage of EAF is that control of retention of solutes such as water-soluble vitamins under totally aqueous mobile phase conditions is environmentally preferable for quality control applications. In addition, EAF seems to be a milder mobile-phase modifier than MeOH for certain proteins such as lysozyme. [source] Cost minimization in wireless networks with a bounded and unbounded number of interfacesNETWORKS: AN INTERNATIONAL JOURNAL, Issue 3 2009Ralf Klasing Given a graph G = (V,E) with |V| = n and |E| = m, which models a set of wireless devices (nodes V) connected by multiple radio interfaces (edges E), the aim is to switch on the minimum cost set of interfaces at the nodes to satisfy all the connections. A connection is satisfied when the endpoints of the corresponding edge share at least one active interface. Every node holds a subset of all the possible k interfaces. Depending on whether k is a priori bounded or not, the problem is called Cost Minimization in Multi-Interface Networks or Cost Minimization in Unbounded Multi-Interface Networks, respectively. We distinguish two main variations for both problems by treating the cost of maintaining an active interface as uniform (i.e., the same for all interfaces), or nonuniform. For bounded k, we show that the problem is APX-hard while we obtain an approximation factor of min for the uniform caseand a (k , 1)-approximation for the nonuniform case. For unbounded k, i.e., k is not set a priori but depends on the given instance, we prove that the problem is not approximable within O(log k) while the same approximation factor of the k -bounded case holds in the uniform case, and a min -approximation factor holds for the nonuniform case. Next, we also provide hardness and approximation results for several classes of networks: with bounded degree, trees, planar, and complete graphs. © 2008 Wiley Periodicals, Inc. NETWORKS, 2009 [source] The fractional congestion bound for efficient edge disjoint routingNETWORKS: AN INTERNATIONAL JOURNAL, Issue 3 2008Alok Baveja Abstract This article investigates the following problem: Given the fractional relaxation of the edge disjoint routing problem, how small a fractional congestion is sufficient to guarantee efficient edge disjoint routing? That is, what is the largest possible value v such that a fractional flow with congestion at most v, can be efficiently converted into an edge disjoint routing? Leighton, Lu, Rao, and Srinivasan (SIAM J Comput 2001) have established that fractional congestion of at most the order of O(1/(d log k)) is sufficient, where d is the maximum path length in the fractional relaxation, and k is the number of pairs to be routed. It is also known that ,(1/d) is the correct bound, if we are only interested in an existence result (Leighton, Rao, and Srinivasan, Hawaii International Conference on System Sciences, 1998). Motivated by the fact that d is small for many types of routing problems, specifically, polylogarithmic for expander graphs, this article improves upon the former result by showing O(1/(d log d)) fractional congestion to suffice. © 2007 Wiley Periodicals, Inc. NETWORKS, 2008 [source] Maximum independent set for intervals by divide and conquer with pruningNETWORKS: AN INTERNATIONAL JOURNAL, Issue 2 2007Jack Snoeyink Abstract Suppose a given set of n intervals contains a maximum independent set of k disjoint intervals. This brief note demonstrates that "divide and conquer with pruning" produces an easy, output-sensitive O(n log k)-time algorithm to compute such a maximum independent set. © 2006 Wiley Periodicals, Inc. NETWORKS, Vol. 49(2), 158,159 2007 [source] Coloring H-free hypergraphsRANDOM STRUCTURES AND ALGORITHMS, Issue 1 2010Tom Bohman Abstract Fix r , 2 and a collection of r -uniform hypergraphs . What is the minimum number of edges in an -free r -uniform hypergraph with chromatic number greater than k? We investigate this question for various . Our results include the following: An (r,l)-system is an r -uniform hypergraph with every two edges sharing at most l vertices. For k sufficiently large, there is an (r,l)-system with chromatic number greater than k and number of edges at most c(kr,1 log k)l/(l,1), where This improves on the previous best bounds of Kostochka et al. (Random Structures Algorithms 19 (2001), 87,98). The upper bound is sharp apart from the constant c as shown in (Random Structures Algorithms 19 (2001) 87,98). The minimum number of edges in an r -uniform hypergraph with independent neighborhoods and chromatic number greater than k is of order kr+1/(r,1) log O(1)k as k , ,. This generalizes (aside from logarithmic factors) a result of Gimbel and Thomassen (Discrete Mathematics 219 (2000), 275,277) for triangle-free graphs. Let T be an r -uniform hypertree of t edges. Then every T -free r -uniform hypergraph has chromatic number at most 2(r , 1)(t , 1) + 1. This generalizes the well-known fact that every T -free graph has chromatic number at most t. Several open problems and conjectures are also posed. © 2009 Wiley Periodicals, Inc. Random Struct. Alg., 2010 [source] Determination of lipophilicity of , -(4-phenylpiperazine) derivatives of N -benzylamides using chromatographic and computational methodsBIOMEDICAL CHROMATOGRAPHY, Issue 4 2008Marek Bajda Abstract This paper describes the evaluation of lipophilicity of ,- (4-phenylpiperazine) derivatives of N -benzylamides. We employed reversed-phase thin-layer chromatography (RP-TLC) and reversed-phase high performance liquid chromatography (RP-HPLC) as experimental methods, using mixtures of acetonitrile and water as the mobile phases with addition of 0.1%TFA in the HPLC experiments. Retention parameters (RM) and capacity factors (log k) determined by applying these methods were linearly dependent on the acetonitrile concentration and enabled us to estimate the relative lipophilicity factors: RM0 and log k0. These factors were compared with the calculated partition coefficients C log P obtained using several software packages. The results indicate that both experimental methods (RP-TLC and RP-HPLC) yielded similar results, and these methods enable determining the lipophilicity of ,- (4-phenylpiperazine) derivatives of N -benzylamides. Significant correlations were found between log P values calculated by Pallas, ALOGPS and C log P Chem3D programs and the experimental data. Copyright © 2007 John Wiley & Sons, Ltd. [source] Determination of lipophilic descriptors of antihelmintic 6,7-diaryl-pteridine derivatives useful for bioactivity predictionsBIOMEDICAL CHROMATOGRAPHY, Issue 6 2003Mario Reta Abstract The liquid chromatographic retention factors extrapolated to pure water, k,w, for several 6,7-diaryl-pteridine derivatives in both an octadecylsilane (ODS) and an immobilized arti,cial membrane column (IAM.PC.DD2), using acetonitrile,aqueous buffer pH = 7.45 as mobile phase, were obtained. The logarithms of the k,w values in the IAM.PC.DD2 column, log k,wIAM, show good correlation with the calculated values of the octanol,water partition coef,cients, log Po/w, showing that the chromatographic parameter can be used as lipophilicity descriptor for the studied pteridines. However, interactions other than the lipophilic ones seem to be involved in the ODS column. Previous studies have shown that pteridines have antihelmintic properties. In spite of the complexity of the studied biological system as compared with the chromatographic one, good correlation between the descriptors obtained in the IAM column and biological activity (expressed as the log of the inhibitory concentration required to obtain up to 50% in the reduction of population growth of nematodes, log IC50) was observed. Copyright © 2003 John Wiley & Sons, Ltd. [source] | |||||||||||||