Browsing by Author "Kinunda, Grace"
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Item The Activity of Invertase Immobilized on Cashew Nut Shell Liquid-Templated Large Pore Silica Hybrids(2011) Mubofu, Egid B.; Mdoe, James; Kinunda, GraceThe characterization and activity of invertase enzymes immobilized on large pore micelle templated silica (LP-MTS) hybrid materials is reported. The LP-MTS hybrid materials were prepared by a co-condensation of tetraethoxysilane (TEOS) and 3-aminopropyltrimethoxysilane (AMPTS) in a cashew nut shell liquid (CNSL) template. A commercially available dodecylamine template was also used to afford similar materials, hereinafter abbreviated as DDA-MTS, for comparison purposes. The prepared materials were characterized by different techniques to determine their physicochemical properties. The maximum loading for the amine groups in LP-MTS and DDA-MTS were 3.3 and 2.8 mmol g�1, respectively. Modification of the materials for immobilization of the invertase enzyme was done by reacting them with glutaraldehyde resulting in Glu-LP-MTS or Glu-DDA-MTS. The as-prepared hybrid materials have surface areas ranging from 100 to 214 m2 g�1 with pore diameters ranging from 3.1 to 25 nm. Scanning electron microscopy (SEM) images show that LP-MTS and DDA-MTS materials comprise of roughly spherical particles whereas enzyme and glutaraldehyde supported micelle templated silicas show a rupture of the spherical particles to a fine powder. The activities of free and immobilized invertases have been determined by measuring the amount of reducing sugar produced upon hydrolysis of sucrose at different temperatures, pH and substrate concentrations. Both free and immobilized invertase enzymes showed a maximum activity at a particular optimum temperature, pH and substrate concentration. The maximum activity for the free invertase was 15 229 U at pH 5.0 and at 40 1C whereas those of the LP-MTS immobilized invertases were 14 833 U and 14 625 U for covalent and cross-linked invertases, respectively, at pH 4.0 and 40 1C. The maximum activity for DDA-MTS covalently immobilized invertase was 4750 U, at pH 4.0 and 40 1C.Item The Activity of Invertase Immobilized on Cashew Nut Shell Liquid-Templated Large Pore Silica Hybrids,(Royal Society of Chemistry, 2011) Mubofu, Egid, B; Mdoe, James, E; Kinunda, GraceThe characterization and activity of invertase enzymes immobilized on large pore micelle templated silica (LP-MTS) hybrid materials is reported. The LP-MTS hybrid materials were prepared by a co-condensation of tetraethoxysilane (TEOS) and 3-aminopropyltrimethoxysilane (AMPTS) in a cashew nut shell liquid (CNSL) template. A commercially available dodecylamine template was also used to afford similar materials, hereinafter abbreviated as DDA-MTS, for comparison purposes. The prepared materials were characterized by different techniques to determine their physicochemical properties. The maximum loading for the amine groups in LP-MTS and DDA-MTS were 3.3 and 2.8 mmol g−1, respectively. Modification of the materials for immobilization of the invertase enzyme was done by reacting them with glutaraldehyde resulting in Glu-LP-MTS or Glu-DDA-MTS. The as-prepared hybrid materials have surface areas ranging from 100 to 214 m2 g−1 with pore diameters ranging from 3.1 to 25 nm. Scanning electron microscopy (SEM) images show that LP-MTS and DDA-MTS materials comprise of roughly spherical particles whereas enzyme and glutaraldehyde supported micelle templated silicas show a rupture of the spherical particles to a fine powder. The activities of free and immobilized invertases have been determined by measuring the amount of reducing sugar produced upon hydrolysis of sucrose at different temperatures, pH and substrate concentrations. Both free and immobilized invertase enzymes showed a maximum activity at a particular optimum temperature, pH and substrate concentration. The maximum activity for the free invertase was 15 229 U at pH 5.0 and at 40 °C whereas those of the LP-MTS immobilized invertases were 14 833 U and 14 625 U for covalent and cross-linked invertases, respectively, at pH 4.0 and 40 °C. The maximum activity for DDA-MTS covalently immobilized invertase was 4750 U, at pH 4.0 and 40 °CItem The Effect of Alkyl Chain Tethers on the Kinetics and Mechanistic Mehaviour of Bifunctional Dinuclear Platinum(II) Complexes Bearing N,N′-Dipyridylamine Ligands,(Royal Society of Chemistry, 2018) Wangoli, Panyako, A.; Kinunda, GraceIn the current paper, we report the kinetics of bifunctional dinuclear platinum(II) complexes viz., 1,2-N,N′-di-(2,2-dipyridylamine)ethanetetraaquaplatinum(II), PtL2, 1,3-N,N′-di-(2,2-dipyridylamine)propanetetraaquaplatinum(II), PtL3, 1,4-N,N′-di-(2,2-dipyridylamine)butanetetraaquaplatinum(II), PtL4, 1,5-N,N′-di-(2,2-dipyridylamine)pentanetetraaquaplatinum(II), PtL5 and 1,6-N,N′-di-(2,2-dipyridylamine)hexanetetraaquaplatinum(II), PtL6. The substitution reactions were carried out on tetraaqua complexes with thiourea nucleophiles under pseudo-first-order conditions as a function of nucleophile concentration and temperature by stopped-flow and UV-vis spectrophotometric techniques. An experimental study was conducted with the aim of determining the influence of alkyl chains on the steric and electronic structure of dinuclear platinum(II) complexes. The reactivity of these complexes was dependent on the length of the alkyl spacer. The results obtained herein demonstrate the intriguing odd–even effects induced by the alkyl chain on the complexes. Artificial constraints imposed by the alkyl chain significantly affect their conformational structure to be either synperiplanar (syn-) or antiperiplanar (anti-) characterized by the odd and even effect. The kinetic, mechanistic and conformational behaviour was influenced by the size of the alkyl chain in accordance with odd–even alterations of the spacer. Computational modeling using density functional theory (DFT) calculations supplemented experimental findings that structural features and the reactivity pattern of these organometallic complexes are governed by both steric and electronic effects arising from the flexibility and inductive nature of the alkyl spacer. The strong σ-donicity of longer alkyl chains favours sufficient accumulation of electron density at the metal centre and stabilizes a 14-electron intermediate. The study shows the HOMO–LUMO energy (ΔE) is affected by the length of the spacer. Kinetic and DFT data indicate electron donation by the alkyl spacer. The low positive values of enthalpy of activation and significantly large negative values of entropy of activation indicate an associative mechanism of substitution.Item Kinetic and Mechanistic Studies of Cisplatin Analogues Bearing 2, 2′-Dipyridylalkylamine Ligands(Springer, 2016-03-01) Kinunda, Grace; Jaganyi, DeogratiusA series of mononuclear Pt(II) complexes of the type diaqua(2,2′-dipyridylalkylamine)platinum(II) (where the alkyl group = methyl, ethyl, propyl, butyl or hexyl) were synthesized to investigate their nucleophilic substitution behaviour and the influence of the alkyl chain bonded to the tertiary nitrogen atom joining the two pyridine rings on the reactivity of the chosen complexes. The trend in rate constant shows that introduction of the σ-donating alkyl chain on the tertiary nitrogen joining the two pyridine moieties reduces the π-acceptor ability of the cis coordinated pyridine rings resulting in a less reactive Pt(II) centre which causes a decrease in the reaction rate. This is well supported by data from DFT calculations. It is also evident that the alkyl chain also introduces a steric effect which blocks the approach of the nucleophile to the Pt(II) centre. The boat-like structure of the six-membered chelate ring also contributes to the steric effect. The study has also shown that two substitution processes going through an associative mode of activation are observed. The first is the simultaneous substitution of the two aqua ligands, and the second is due to the dechelation of the ligand, an indication of possible disintegration of the complex if used as a drug.Item Kinetic and mechanistic studies of cisplatin analogues bearing 2,2'-dipyridylalkylamine ligands(Springer, 2016) Kinunda, Grace; Jaganyi, DeogratiasA series of mononuclear Pt(II) complexes of the type diaqua(2,2′-dipyridylalkylamine)platinum(II) (where the alkyl group = methyl, ethyl, propyl, butyl or hexyl) were synthesized to investigate their nucleophilic substitution behaviour and the influence of the alkyl chain bonded to the tertiary nitrogen atom joining the two pyridine rings on the reactivity of the chosen complexes. The trend in rate constant shows that introduction of the σ-donating alkyl chain on the tertiary nitrogen joining the two pyridine moieties reduces the π-acceptor ability of the cis coordinated pyridine rings resulting in a less reactive Pt(II) centre which causes a decrease in the reaction rate. This is well supported by data from DFT calculations. It is also evident that the alkyl chain also introduces a steric effect which blocks the approach of the nucleophile to the Pt(II) centre. The boat-like structure of the six-membered chelate ring also contributes to the steric effect. The study has also shown that two substitution processes going through an associative mode of activation are observed. The first is the simultaneous substitution of the two aqua ligands, and the second is due to the dechelation of the ligand, an indication of possible disintegration of the complex if used as a drug.Item Kinetic and Mechanistic Studies of Cisplatin Analogues Bearing 2,2’-dipyridylalkylamine Ligands(Springer, 2016) Kinunda, Grace; Jaganyi, DeogratiasA series of mononuclear Pt(II) complexes of the type diaqua(2,2′-dipyridylalkylamine)platinum(II) (where the alkyl group = methyl, ethyl, propyl, butyl or hexyl) were synthesized to investigate their nucleophilic substitution behaviour and the influence of the alkyl chain bonded to the tertiary nitrogen atom joining the two pyridine rings on the reactivity of the chosen complexes. The trend in rate constant shows that introduction of the σ-donating alkyl chain on the tertiary nitrogen joining the two pyridine moieties reduces the π-acceptor ability of the cis coordinated pyridine rings resulting in a less reactive Pt(II) centre which causes a decrease in the reaction rate. This is well supported by data from DFT calculations. It is also evident that the alkyl chain also introduces a steric effect which blocks the approach of the nucleophile to the Pt(II) centre. The boat-like structure of the six-membered chelate ring also contributes to the steric effect. The study has also shown that two substitution processes going through an associative mode of activation are observed. The first is the simultaneous substitution of the two aqua ligands, and the second is due to the dechelation of the ligand, an indication of possible disintegration of the complex if used as a drug.Item A Kinetic Study of Aqua Ligand Substitution in Dinuclear Pt (II) Complexes Containing Four Non-Coplanar Pyridine Ligands(Springer, 2014-10-01) Kinunda, Grace; Jaganyi, DeogratiusSubstitution reactions of the aqua ligands from azine-bridged dinuclear platinum(II) complexes of the type [{cis-Pt(py)2(OH2)}2(μ-pzn)](ClO4)4 [pzn = pyrazine (Pt-PZN), 2,3-dimethylpyrazine (Pt-2,3PZN), 2,5-dimethylpyrazine (Pt-2,5PZN) or 2,6-dimethylpyrazine (Pt-2,6PZN)] by thiourea nucleophiles were investigated under pseudo first-order conditions as a function of concentration and temperature using the stopped-flow technique. The experimental results are discussed in reference to structures obtained by DFT calculations. The results are in good agreement with the pKa values of the complexes as well as DFT calculations. Compared to [{cis/trans-Pt(NH3)2(OH2)}2(μ-pzn)](ClO4)4, the complexes in this series react faster by a factor of 10 or 23 respectively due to the presence of pyridine rings, which forces the geometry to allow π-back bonding to take place such that the electrons from the metal centres are accepted to the empty π*-orbitals of the pyridine subunits. The reactivity of the nucleophile is sterically dependent, with N,N,N′,N′-tetramethylthiourea reacting three times slower than thiourea. In all complexes and for both substitution steps, the mode of activation remains associative in nature.Item A Kinetic Study of Aqua Ligand Substitution in Dinuclear Pt(II) Complexes Containing Four Non-Coplanar Pyridine Ligands(Springer, 2014) Kinunda, Grace; Jaganyi, DeogratiasSubstitution reactions of the aqua ligands from azine-bridged dinuclear platinum(II) complexes of the type [{cis-Pt(py)2(OH2)} 2(μ-pzn)](ClO4)4 [pzn = pyrazine (Pt-PZN), 2,3-dimethylpyrazine (Pt-2,3PZN), 2,5-dimethylpyrazine (Pt-2,5PZN) or 2,6-dimethylpyrazine (Pt-2,6PZN)] by thiourea nucleophiles were investigated under pseudo first-order conditions as a function of concentration and temperature using the stopped-flow technique. The experimental results are discussed in reference to structures obtained by DFT calculations. The results are in good agreement with the pKa values of the complexes as well as DFT calculations. Compared to [{cis/trans-Pt(NH3)2(OH2)} 2(μ-pzn)](ClO4)4, the complexes in this series react faster by a factor of 10 or 23 respectively due to the presence of pyridine rings, which forces the geometry to allow π-back bonding to take place such that the electrons from the metal centres are accepted to the empty π*-orbitals of the pyridine subunits. The reactivity of the nucleophile is sterically dependent, with N,N,N′,N′-tetramethylthiourea reacting three times slower than thiourea. In all complexes and for both substitution steps, the mode of activation remains associative in nature.Item Mechanistic study of the substitution reactions of [Pt(II)(bis(2-pyridylmethyl)amine)H2O](ClO4)2 and [Pt(II)(bis(2-pyridylmethyl)sulfide)H2O](ClO4)2 with azole Nucleophiles. Crystal structure of [Pt(II)(bis(2-pyridylmethyl)sulfide)Cl]ClO4(Elsevier, 2017) Nkabinde, Slindokuhle, V.; Kinunda, Grace; Jaganyi, DeogratiusThe substitution kinetics of the complexes [Pt(II)(bis(2-pyridylmethyl)amine)H2O](ClO4)2, Ptdpa and [Pt(II)(bis(2-pyridylmethyl)sulfide)H2O](ClO4)2, Ptdps, with a series of azole nucleophiles: Imidazole (Im), 1-methylimidazole (MIm), 1,2-Dimethylimidazole (DIm), 1,2,4-triazole (Trz) and pyrazole (Pyz), were studied in an aqueous medium at constant ionic strength (0.1 M NaClO4). The substitution of the coordinated water ligand on the Pt(II) complexes by the azoles was studied under pseudo-first order conditions as a function of the incoming nucleophiles concentration and temperature using either stopped-flow techniques or UV-Vis spectroscopy. Ptdps was found to be more reactive (three magnitude higher) than Ptdpa. The second-order rate constant, k2, for all the nucleophiles ranged between 0.087 ± 0.005 and 0.926 ± 0.05 M−1 s−1 for Ptdpa and between 146 ± 4 and 1458 ± 10 M−1 s−1 for Ptdps. The rate of substitution of the aqua ligand is dependent on the strength of the σ-donor character and the π-acceptability of the trans atom to the leaving group. The observed reactivity trend for the azoles followed the trend, MIm > Im > DIm > Trz > Pyz. This reactivity trend correlates with the basicity, steric and electrophilic effects of the nucleophiles. The X-ray crystal structure of Ptdps–Cl is reported.Item Steam treatment of a hollow lithium phosphate catalyst: enhancing carbon deposition resistance and improving the catalytic performance of propylene oxide rearrangement(Royal Society of Chemistry, 2016) Wang, Yanan; Li, Zhishan; Ma, Weihua; Kinunda, Grace; Qua, Hongxia; Zhonga, QinCarbon deposition is a great problem for most solid acid and base catalysts applied in organic catalytic reactions. Basic lithium phosphate catalyst, when used for propylene oxide rearrangement, is also easily deactivated due to carbon deposition. In this paper, a green surface modification technique of steam treatment was employed to suppress carbon deposition on the basic lithium phosphate catalyst and to improve its catalytic performance. The results showed that the catalyst which was pre-treated with steam at 300 C for 30 minutes exhibited excellent catalytic activity. Furthermore, the amount of carbon deposition was 15.1%, much lower than that of the untreated catalyst (21.5%). The steam treatment could increase the amount of hydroxyl and adjust the distribution of the acid and base sites. The decrease of the amount of Brønsted acid sites resulted in the reduction of carbon deposition. The enhancement of activity could be attributed to the increase of synergistic sites, and this could be due to an increase in the amount of Lewis acid sites and in the strength of the base sites.Item Steam Treatment of a Hollow Lithium Phosphate Catalyst: Enhancing Carbon Deposition Resistance and Improving the Catalytic Performance of Propylene Oxide Rearrangement(RSC, 2016-04-27) Wang, Qin Z. Y.; Li, Zhishan; Ma, Weihua; Kinunda, Grace; Qu, HongxiaCarbon deposition is a great problem for most solid acid and base catalysts applied in organic catalytic reactions. Basic lithium phosphate catalyst, when used for propylene oxide rearrangement, is also easily deactivated due to carbon deposition. In this paper, a green surface modification technique of steam treatment was employed to suppress carbon deposition on the basic lithium phosphate catalyst and to improve its catalytic performance. The results showed that the catalyst which was pre-treated with steam at 300 °C for 30 minutes exhibited excellent catalytic activity. Furthermore, the amount of carbon deposition was 15.1%, much lower than that of the untreated catalyst (21.5%). The steam treatment could increase the amount of hydroxyl and adjust the distribution of the acid and base sites. The decrease of the amount of Brønsted acid sites resulted in the reduction of carbon deposition. The enhancement of activity could be attributed to the increase of synergistic sites, and this could be due to an increase in the amount of Lewis acid sites and in the strength of the base sites.Item A trans influence and π-conjugation effects on ligand substitution reactions of Pt(II) complexes with tridentate pendant N/S-donor ligands(College of Natural and Applied Sciences, 2018) Kinunda, GraceThe rate of displacement of the chloride ligands by three neutral nucleophiles (Nu) of different steric demands, namely thiourea (TU), N,N’-dimethylthiourea (DMTU) and N,N,N,’N-tetramethylthiourea (TMTU) in the complexes viz; [Pt(II)(bis(2-pyridylmethyl)amine)Cl]ClO4, (Pt1), [Pt(II){N-(2-pyridinylmethyl)-8-quinolinamine}Cl]Cl, (Pt2), [Pt(II)(bis(2-pyridylmethyl)sulfide)Cl]Cl, (Pt3) and [Pt(II){8-((2-pyridylmethyl)thiol)quinoline}Cl]Cl, (Pt4) was studied under pseudo first-order conditions as a function of concentration and temperature using a stopped-flow technique and UV-Visible spectrophotometry. The observed pseudo first-order rate constants for substitution reactions obeyed the simple rate law. The results have shown that the chloro ligand in Pt(N^S^N) complexes is more labile by two orders of magnitude than Pt(N^N^N) complexes due to the high trans labilizing effect brought by the S-donor atom. The quinoline based Pt(II) complexes (Pt2 and Pt4) have been found to be slow than their pyridine counterparts Pt1 and Pt3 due to poor π-acceptor ability of quinoline. Second-order kinetics and large negative activation entropies support an associative mode of activation.Item A trans influence and π-conjugation effects on ligand substitution reactions of Pt(II) complexes with tridentate pendant N/S-donor ligands(College of Natural and Applied Sciences (CoNAS) of the University of Dar es Salaam (UDSM), 2018) Kinunda, GraceThe rate of displacement of the chloride ligands by three neutral nucleophiles (Nu) of different steric demands, namely thiourea (TU), N,N’-dimethylthiourea (DMTU) and N,N,N,’Ntetramethylthiourea (TMTU) in the complexes viz; [Pt(II)(bis(2-pyridylmethyl)amine)Cl]ClO4, (Pt1), [Pt(II){N-(2-pyridinylmethyl)-8-quinolinamine}Cl]Cl, (Pt2), [Pt(II)(bis(2-pyridylmethyl)sulfide)Cl]Cl, (Pt3) and [Pt(II){8-((2-pyridylmethyl)thiol)quinoline}Cl]Cl, (Pt4) was studied under pseudo first-order conditions as a function of concentration and temperature using a stopped-flow technique and UV-Visible spectrophotometry. The observed pseudo first order rate constants for substitution reactions obeyed the simple rate law kobs =[Nu] k2. The results have shown that the chloro ligand in Pt(N^S^N) complexes is more labile by two orders of magnitude than Pt(N^N^N) complexes due to the high trans labilizing effect brought by the S donor atom. The quinoline based Pt(II) complexes (Pt2 and Pt4) have been found to be slow than their pyridine counterparts Pt1 and Pt3 due to poor π-acceptor ability of quinoline. Second-order kinetics and large negative activation entropies support an associative mode of activation.Item Understanding the Electronic and Π-Conjugation Roles of Quinoline on Ligand Substitution Reactions of Platinum (II) Complexes(Springer, 2014-05-01) Kinunda, Grace; Jaganyi, DeogratiusA kinetic and mechanistic study of chloride substitution by thiourea nucleophiles, namely thiourea, N-methylthiourea, N,N-dimethylthiourea and N,N,N′,N′-tetramethylthiourea in the complexes chlorobis-(2-pyridylmethyl)amineplatinum(II) (Pt1), chloro N-(2-pyridinylmethyl)-8-quinolinamineplatinum(II) (Pt2), chloro N-(2-pyridinylmethylene)-8-quinolinamineplatinum(II) (Pt3) and chlorobis(8-quinolinyl)amineplatinum(II) (Pt4) was undertaken under pseudo-first-order conditions using UV–visible spectrophotometry. The study showed that lability of the chloro leaving group is dependent on the strength of π-interactions between the filled dπ-orbitals of the metal and the empty π*-orbitals of the chelating ligand in the following manner: Pt1 > Pt3 > Pt2 > Pt4. Introduction of the quinoline moiety within the non-labile chelated framework of the Pt(II) complexes results in a more electron-rich metal centre which retards the approach of the nucleophile through repulsion. Moreover, the net σ-effect of the ligand moiety plays a significant role in controlling the reactivity of the complexes. The experimental results are interpreted with the aid of computational data obtained by density functional theory (B3LYP(CPCM)/LANL2DZp//B3LYP/-LANL2DZp) calculations. The mode of substitution remains associative as supported by negative entropies and the dependence of the second-order rate constants on the concentration of entering nucleophiles.Item Understanding the Electronic and π-conjugation Roles of Quinoline on Ligand Substitution Reactions of Pt(II) Complexes(Springer, 2014) Kinunda, Grace; Jaganyi, DeogratiasA kinetic and mechanistic study of chloride substitution by thiourea nucleophiles, namely thiourea, N-methylthiourea, N,N-dimethylthiourea and N,N,N′,N′-tetramethylthiourea in the complexes chlorobis-(2-pyridylmethyl)amineplatinum(II) (Pt1), chloro N-(2-pyridinylmethyl)-8-quinolinamineplatinum(II) (Pt2), chloro N-(2-pyridinylmethylene)-8-quinolinamineplatinum(II) (Pt3) and chlorobis(8-quinolinyl)amineplatinum(II) (Pt4) was undertaken under pseudo-first-order conditions using UV–visible spectrophotometry. The study showed that lability of the chloro leaving group is dependent on the strength of π-interactions between the filled dπ-orbitals of the metal and the empty π*-orbitals of the chelating ligand in the following manner: Pt1 > Pt3 > Pt2 > Pt4. Introduction of the quinoline moiety within the non-labile chelated framework of the Pt(II) complexes results in a more electron-rich metal centre which retards the approach of the nucleophile through repulsion. Moreover, the net σ-effect of the ligand moiety plays a significant role in controlling the reactivity of the complexes. The experimental results are interpreted with the aid of computational data obtained by density functional theory (B3LYP(CPCM)/LANL2DZp//B3LYP/-LANL2DZp) calculations. The mode of substitution remains associative as supported by negative entropies and the dependence of the second-order rate constants on the concentration of entering nucleophiles.