School of Chemistry - Theses
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ItemParallel DNA assembly by recombination( 2011)The development of synthetic biology requires the construction of much more sophisticated genetic systems; however, mathematical models are too simplified to produce accurate predictions. Therefore, successful constructions have to rely on trial and error. In order to simplify the constructions process and develop cost-efficient and convenient technology for batch construction tests, a new efficient parallel DNA assembly technology was established after testing several strategies. Recombination was chosen as a robust attachment assembly system as it does not require extreme conditions and can ensure efficient single copy integration and extraction. An experimental demonstration of assembly of five DNA fragments into a host genome was performed. A software platform for design and management of molecular cloning experiments was developed to manage construction of the assembly system. Promising technologies that are useful for recombination-based DNA assembly were evaluated. A DNA assembly theory was set up from the study of different strategies and provides a basis for design of DNA assembly systems and a guideline for future studies. This new family of methods has the potential advantages over all other methods, such as less or no in vitro operations, lower costs and shorter turnover period. This study is the first one to set up the theory and to attempt to extract the full potential of this technology based on the theory.
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ItemTowards the crystal structure of β-silyl carbenium ions with higher electron demand( 2011)Synthesis towards the silanes, 2.03 and 2.22, which were seen as immediate precursors to bicyclic β-silyl carbenium ions was carried out. However, the crucial ring forming step in both cases failed to yield detectable quantities of the silane precursors, and instead polymeric material was obtained. The enthalpy and entropy costs of forming the strained medium rings in 2.03 and 2.22 appeared to be insurmountable. The introduction of ring fusion into the medium ring in target 2.30 was expected to alleviate both the entropy and enthalpy hurdles; however synthesis towards 2.30 was also unsuccessful. Attention was then turned towards precursors of monocyclic β-silyl carbenium ions,3.37, 3.41, 3.44 and 3.47; and their precursor silanes, 3.03-3.06, became the synthetic target. The precursors silanes, 3.03-3.06, were successfully prepared in 6 steps beginning with ring opening of tetrahydrofuran. While trityl tetrakis(pentafluorophenylborate), Ph3C+ [(C6F5)4B]-, required for the conversion of the precursor silanes into the monocyclic β-silyl carbenium ions was successfully prepared in 3 steps from pentafluoroiodobenzene. Attempts to generate β-silyl carbenium ions, 3.37 and 3.41, were unsuccessful due to competing intramolecular hydrosilylation which occurred due to the affinity of the β-silyl carbenium ions to undergo hydride abstraction from the Si-H of silanes, 3.03 and 3.04. Silane, 3.06, also failed to convert to a β-silyl carbenium ion again due to catalytic intramolecular hydrosilylation. The dicyclopropyl substituted β -silyl carbenium ion, 3.44, was successfully generated in the presence of the non-nucleophilic counterion tetrakis (pentafluorophenylborate, however this ion could only be characterized in the solution phase as it did not yield crystals of sufficient quality for X-ray crystallography. Attempts at using a different counterion, Ph3C+ [CHB11Cl11]-, kindly supplied by Professor Christopher Reed did not yield any β-silyl carbenium ion. Acyclic β-silyl carbenium ions were then targeted. Thus b-(tert-butyldimethylsilyl) diphenylcarbenium ion, 4.06, and the b-(triisopropylsilyl) diphenylcarbenium ion, 4.09, were successfully prepared in the presence of tetrakis(pentafluorophenyl)borate, however these could only be characterized in the solution phase as they did not yield crystals of sufficient quality for X-ray crystallography. Attempts at preparing alternative tert-butyldimethylsilylmethyl- and triisopropylsilylmethyl- substituted carbenium ions (cyclopropylphenylium and dicyclopropylium), 4.07, 4.08, 4.10 and 4.11, were unsuccessful presumably due to the rearrangement or fragmentation of the cyclopropyl moiety.
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ItemEffect of sonication treatment on the physico-chemical properties of starch-based suspensions( 2011)Starch, one of the most abundant polysaccharides in nature, is used in a wide range of applications, particularly in Food Industry. Ultrasound treatment has been identified as a potential Food Processing Technology. The aim of my thesis is to investigate how the application of ultrasound affects the physico-chemical properties of starch-based suspensions. Acoustic cavitation-induced structural modifications were achieved by varying ultrasonication parameters (temperature, time, sonication power and frequency) and the characteristics of the starch dispersion (botanical origins, amylose/amylopectin ratio, granular or macromolecular states and granule surface properties).
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ItemTowards the synthesis of multipotent antihypertensive and antioxidant nitroxide containing sartans( 2011)In recent years, it has been recognised that reactive oxygen species play an important role in the pathophysiology of hypertension. Consequently, tethering an additional antioxidant pharmacophore to a conventional antihypertensive might generate a multipotent compound with improved blood pressure lowering ability. Sartans (selective AT1 receptor antagonist) are known to be the most direct and widely used class of antihypertensives that antagonise the vasoconstrictive angiotensin II at the AT1 receptor. Nitroxides, on the other hand, due to their ability to scavenge free radicals, are able to protect against oxidative damage in biological systems. It was therefore proposed that incorporating a nitroxide into a sartan would generate a dual-acting compound with both antihypertensive and antioxidant activity. Initial work was directed towards the generation of a series of nitroxide analogues 43, 44 and 82-87 of milfasartan. This approach involved reacting a series of differently substituted pyrimidones (49, 96-98) with two (bromomethyl) nitroxides (43 and 44). Furthermore, in order to investigate whether the N-O• moiety plays a role in interacting with the AT1 receptor, nitroxide analogue 43 of milfasartan was reduced to its amine derivative 114. Also prepared were nitroxide analogues 122 and 123 of eprosartan in which the decisive synthetic transformation comprised of an aldol condensation between imidazole 126 and either nitroxide 139 or 149. The syntheses of the nitroxide derivatives 158 and 159 of losartan then completed the collection of nitroxide-sartan analogues. The losartan analogues 158 and 159 were obtained via Steglich esterification between the protected losartan 156 and nitroxides 61 and 72. Pharmacological evaluations have suggested that both nitroxide analogues 43 and 44 of milfasartan display excellent AT1 receptor antagonist activities. Results from the CHO cell assays also suggested that the N-O• moiety on the nitroxide-containing sartans might play a role in enhancing the compounds’ ability to interact with the AT1 receptor. In addition, results from both the in vitro and in vivo assays have demonstrated that nitroxide-containing sartan 44, in particular, exhibited promising antioxidant, protection against vascular injury and cardioprotective properties. The AT1 receptor antagonism potencies of nitroxide-containing sartan 122/123 and 158/159 on the other hand, were found to be lower than their parent eprosartan (16) and losartan (8), respectively.
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ItemDevelopment of superhydrophobic coatings( 2011)Ultrarough surfaces with variable architecture were synthesised and examined using a combination of atomic force microscopy, synchrotron small angle X-ray scattering and contact angle goniometry. Using silica nanoparticle sol-gel technology, hierarchical superhydrophobic surfaces with water contact angles > 150° and hysteresis < 10° were fabricated. The addition and subsequent removal of 400 nm latex particles facilitated in carefully controlled, micro-pore formation, resulting in, amongst other things, a transparent superhydrophobic thin film. Varying nanoparticle size from 7 - 40 nm within the coating, pseudo-fractal dimension measured using SAXS (Small Angle X-ray Scattering) was shown to be critical in optimising superhydrophobicity. Combined with characterisation using Mie light scattering models, surfaces with RMS roughness < 200 nm and fractal dimension > 2.6 was found to exhibit both superhydrophobicity and optical transparency. The randomly aggregated sol-gel coating presenting an ideal platform to study the wetting of hierarchical rough surfaces similar to those in nature. Using in-situ SAXS measurements, combined with contact angle goniometry, the liquid/solid interface of a superhydrophobic surface at various stages of wetting was directly probed. For the first time, an intermediate wetting state whereby a surface is wetted at the macroscale but unwetted at the nanoscale, was observed in situ. The behaviour of superhydrophobic surfaces in practical applications, icing and marine biofouling, were examined. Superhydrophobic surfaces, due to its de-wetting properties, drastically reduced the adhesion strength of ice formed from liquid water. The coating further acted as an heat insulator, inhibiting frost formation from cold humid air. In marine anti-fouling applications, hierarchical superhydrophobic surfaces show considerable promise in resisting the initial attachment of marine organisms. However, field biofouling experiments suggest that micro-roughness and surface chemistry are dominant factors in antifouling.
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ItemNanobubbles: their role in the hydrophobic force and wetting behaviour of surfaces( 2011)The work presented in this dissertation elegantly demonstrates for the first time, via SAXS, that nanobubbles exist on hydrophobic self-assembled monolayers in a binary ethanol/water system. Atomic Force Microscopy was utilized to measure the force interactions occurring between three smooth hydrophobic surfaces created using the self-assembled monolayers 11-bromo-1-undecanethiol (water contact angle 90°), 1-octadecanethiol (water contact angle 109°) and 1H,1H,2H,2H-perfluoro-1-decanethiol (water contact angle 114°). Small Angle X-ray Scattering (SAXS) was used as a non-invasive technique to detect the formation of nanobubbles. The AFM data demonstrates a localized force effect attributed to nanobubbles on an immersed hydrophobic surface. The SAXS data reveal an electron density depletion layer at the hydrophobic interface that can only be attributable to nanobubbles forming on the interface providing solid evidence that the hydrophobic force is due to nanobubbles. SAXS was also used to investigate the role of nanobubbles on the wettability of rough superhydrophobic surfaces compared to rough superhydrophilic surfaces. The SAXS data suggests that nanobubbles or trapped air plays a pivotal role on the wettability of the immersed superhydrophobic sol-gel interface. When the nanobubbles are removed from the interface, the superhydrophobic surface has the same wetting behaviour as a superhydrophilic surface. The results presented in this dissertation greatly contribute to the theoretical and experimental knowledge of the hydrophobic force, and the role of air on the wetting behaviour of such surfaces.
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ItemSol-gel template synthesis of porous metal oxide beads for sequestration of radionuclides( 2011)Meeting growing energy demands and the need to seek for alternative, cleaner and sustainable sources of energy in the fight to reduce the negative environmental impacts of greenhouse gas emissions (from fossil fuel-related energy schemes) could potentially present itself in the form of nuclear energy. However, several challenges remain regarding the management of the waste generated from nuclear power plants. One of the current practices adopted for the management of spent nuclear fuel is through immobilization in borosilicate glass matrices, which then remain at existing storage facilities. The future of nuclear waste containment is now tending towards the long-term disposal of the immobilized waste in deep geological sites. Durable and stable host matrices are required towards maximizing the life-span of repositories but also towards reducing risks of leaching of the radionuclides. Studies have shown the limitations of borosilicate glass matrices for such purposes, hence urging research towards non-siliceous, inorganic materials which have shown better prospects. The life-span of the repository can be further improved if recycling of the spent fuel is practised. Partitioning of radioactive materials allows reduction in volume of the waste and toxic radionuclides. Most importantly, it allows the recycling of fissile materials, which can be re-used for energy production. In this thesis, hierarchically porous inorganic (TiO2 or mixed TiO2/ZrO2) millimeter-sized beads were synthesized. Their suitability as radionuclide (selective) adsorbents was probed. The synthesis of structurally defined inorganic beads was achieved via a sol-gel templating technique which involved the inorganic coating of preformed template beads, with defined structural properties, and their subsequent removal by calcination. A range of polymeric beads were developed in-house and subsequently examined as sacrificial templates for the synthesis of the inorganic beads. Porous poly(vinylidene fluoride-co-hexafluoropropylene) beads were produced via phase separation processes. This involved the dripping of a polymeric solution into a coagulation bath. Polymeric beads of variable pore architectures were attained by altering the nature of the baths: Aqueous-based baths at room temperature or a liquid N2 bath yielded cellular or radial tubular porous structures, respectively; a combination of both structures was obtained for baths at 2-5 °C. Rigid bead structures were obtained at higher polymer concentrations. Despite the structurally sound scaffold of the polymeric beads, they were not suitable as templates. The resulting inorganic beads were mechanically weak. Robust hierarchically porous carbon-metal oxide beads were obtained by carbonizing the beads in an inert atmosphere. The porous structure of the initial scaffold was preserved in the final beads. Porous agarose beads were prepared using a non-conventional method whereby a polymeric solution (agarose dissolved in dimethyl sulfoxide) was dripped into liquid N2. The beads featured an interconnected macroporous fibrous structure and mesopore sizes which varied with the content of agarose. The regularity (shape and size) of the beads improved at higher agarose contents. Similar characteristics were imparted in the mixed TiO2/ZrO2 beads. The addition of a secondary metal oxide (ZrO2) component to TiO2 influenced the properties of the beads: The highest surface areas were achieved at a 25 wt% Zr regardless of the agarose content; increasingly denser porous structures were obtained at higher Zr contents and the pure oxides displayed larger interparticulate mesopores than the mixed oxides. Yet, the inorganic beads lacked sufficient mechanical stability. Mechanically robust inorganic (TiO2 or mixed TiO2/ZrO2) beads were produced using alginate beads as templates. The variable structural properties of the alginate beads (obtained by altering the bead synthesis, e.g. concentration of sodium alginate, NaAlg, and calcium ions, Ca2+, in the gelation bath) were used to control the properties of the TiO2 beads. The surface area of the TiO2 beads reached maxima using template beads prepared in a 0.27 M Ca2+ bath. A 1 wt% NaAlg-templated system yielded anatase beads whereas semi-crystalline TiO2 beads were obtained for the 2 wt% NaAlg-templated system. The TiO2 (anatase) beads displayed high surface areas of 170 ± 10 m2 g-1 (calcined at 450 °C) and beads calcined at 700 °C remained anatase with a decrease in surface area to 46.1 ± 0.5 m2 g-1. The functionalization of the TiO2 beads was studied using an amino-bisphosphonate molecule. The solution pH and surface area of the TiO2 beads influenced the loading. Hierarchically porous TiO2/ZrO2 beads were prepared with varying parameters (such as surface area, pore size and surface hydroxyl groups), attained through varying Ti/Zr composition (22, 36 and 82 wt% Zr) or crystallinity (amorphous or crystalline), achieved by varying calcination temperatures (500 or 700 °C). The effect of the above parameters on the uptake a uranyl, and the hydrolytic and radiolytic properties of the beads were assessed. Higher adsorption capacities were obtained for the crystalline beads. This was attributed to the higher surface hydroxyl density, the presence of limited microporosity and the larger mesopores in the crystalline beads, relative to the amorphous beads. The pore hierarchy of the crystalline beads resulted in improved uranyl adsorption rates over reported exclusively mesoporous beads and comparable rates to reported xerogel powders. The crystalline beads displayed superior hydrolytic properties than the amorphous beads (22 wt% Zr sample showed the highest resistance to matrix leaching in HNO3 solutions). The crystalline beads were stable to amorphization when subjected to γ-irradiation. TiO2/ZrO2 beads were functionalized with amino-bisphosphonates and investigated for the selective uptake of radiotracers: actinide (239Pu) and lanthanides (152Sm, 153Eu, 158Gd) in HNO3 media. The two amino-bisphosphonate molecules studied both comprised of a bisphosphonic group, used as anchor points to the metal oxide support, and a pendant group (either -NH2 (Alen) or -N(CH3)2 (Alenmod)). Both amino-bisphosphonate molecules displayed higher affinities towards Pu, thereby affording separation from the lanthanides. However, Alenmod-grafted molecules displayed superior extraction performance. The nature of the porous bead support onto which the organic moieties were grafted also influenced the adsorption process. The uptake of the radiotracers was more selective towards the functionalized crystalline TiO2/ZrO2 beads than towards the functionalized amorphous TiO2/ZrO2 beads. The larger porous structure of the crystalline matrix was believed to facilitate the access of the radionuclides to the grafted organic moieties. The organic loading did not direct the selective uptake process. The hydrolytic stability of the Ti or Zr matrix was further improved under the presence of the grafted organic moieties.
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ItemAdventures with peroxyl radicals as precursors for α-oxo carbenes and related reactive intermediates( 2011)The development of free radical methodology for the synthesis of carbene and/or oxirene intermediates has been a new area of interest in our laboratory. Chapter two describes our attempts to form carbene and/or oxirene intermediates by the reaction of peroxyl radicals and alkynes in solution. Two systems were used to prove the formation of carbenes or/and oxirene intermediates by utilising characteristic reactions, such as Wolff-rearrangement or insertion into C-H bonds, as markers.
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ItemMolecular characterization of copper tolerance and resistance proteins and enzymes in gram-negative bacteria( 2011)Copper is an essential trace element required by all living organisms. However, it is potentially toxic when in excess or in its “free” form due to its redox activity and highly competitive binding affinities. Specific metabolic pathways exist to safeguard its transport within the cells. A disruption at any point in these pathways in humans is manifested in several fatal diseases (eg. Wilson, Menkes) and contributes to symptoms of neurodegenerative diseases such as Alzheimer. Bacterial cells protect themselves from toxic environmental copper by expressing resistance defense, similar to antibiotic/drug resistance. The cue system in E. coli is part of the chromosomally encoded copper homeostatic system that enables the survival of bacterial cells at micromolar copper concentrations. This system involves two proteins: CopA, a P-type ATPase which removes excess CuI ions from the cytoplasm to the periplasm where they are oxidised by CueO to less toxic CuII ions. CueO is a multicopper oxidase (MCO). MCOs are a large family of enzymes capable of coupling four one-electron oxidation steps of substrates to the four-electron reduction of one molecule of dioxygen to water. Their active sites feature at least four copper atoms which are traditionally classified into three categories designated T1, T2 and binuclear T3 Cu sites. The T1 site catalyses substrate oxidation while a trinuclear cluster T2/T3 site catalyses the dioxygen reduction. However, CueO is unusual: it displays an extra methionine rich -helix insert which covers the T1 site and which accommodates a labile CuII binding site. This work demonstrates that this labile binding site is actually a specific CuI substrate docking/oxidation (CuI-SDO) site. As a cuprous oxidase, this site must be empty to allow catalytic turnover of CuI. As a phenol oxidase, this site must be occupied by a CuII ion to act as an electron-transfer mediator between the buried T1 site and substrates which are not able to dock directly at the specific CuI-SDO site. Using novel approaches, the affinities of this site for both CuI and CuII have been determined. The combined data provide compelling evidence that CueO is a cuprous oxidase in vivo and not a phenol oxidase. The bacterium Cupriavidus metallidurans CH34 is resistant to high environmental concentrations of many metal ions, including copper. The resistance to copper is attributed primarily to the presence of a large plasmid pMOL30 that includes a cop cluster composing of 21 genes. Expression of the three soluble periplasmic proteins CopK, CopC and CopA is highly induced by the presence of copper indicating their important roles in the copper resistance. This work has expressed and characterised these three proteins. CopK is unique to this system and exhibits intriguing copper binding chemistry. It is a weakly associated dimer that has little affinity for CuII (KD > 10-6 M), but can¬ bind CuI with modest affinity (KD = 2 x 10-11 M¬). However, CuI binding induces structural change that leads to dimer dissociation and generation of a high affinity CuII binding site (KD = 3 x 10-12 M) and the high affinity CuII binding in turn enhances the CuI¬ binding affinity by a factor of 102. Such strong binding cooperativity between CuI and CuII is unprecedented in copper binding proteins. The molecular basis for this unusual property has been explored and uncovered via biochemical studies (including site-specific mutagenesis) and structural approaches (including X-ray crystallography and NMR spectroscopy). CopC shares high sequence similarity with two well-characterised protein homologs, CopC from Pseudomonas syringae and PcoC from E. coli. All potential protein ligands for CuI and CuII are also highly conserved among these three proteins. The latter two proteins bind CuI and CuII specifically at two separate sites. Indeed, CopC from C. metallidurans expressed and isolated in this work features similar CuI and CuII binding chemistries. CopA has not been isolated and studied previously. Its primary sequence derived from the copA gene predicts it to be a MCO and to contain a Met-rich sequence motif in a position equivalent to that in CueO that accommodates the specific CuI-SDO site. Therefore, CopA is assumed to take a similar role as a cuprous oxidase in vivo. However, preliminary studies in this work indicate that CopA is not only a robust cuprous oxidase, but also a robust phenol oxidase. The former function is buffer-dependent, but in contrast to CueO, the reaction buffers have only minimum effect on the phenol oxidase activity of CopA. Preliminary data suggest that, relative to the enzyme functions of CueO, the new enzyme properties of CopA are arisen from the fact that CopA may feature a CuI-SDO site that can bind either CuI or CuII with higher affinities than those of the equivalent site in CueO. This work also demonstrates that CopA catalyses air-oxidation of CuICuII-CopC efficiently, but not of CuICuII-CopK. Although these two proteins bind CuI with similar affinities, their CuI binding sites are quite different in aspects such as solvent-exposure, ligand number and ligand composition. All these differences are likely to affect their specific interactions with the CopA enzyme and emphasise that specific molecular recognition and interactions are important in copper homeostasis in biology.
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ItemGas and solution phase investigations into the beta, gamma and delta effects of the Group 14 metals utilizing trialkyl-pyridinium ions( 2011)Gas phase chemistry of N-methyl 2- and 4-trimethylsilylmethylpyridinium ions 53 and 54 and the deuterated derivatives 104 and 105 were investigated using electrospray ionization (ESI) and tandem mass spectrometry. Collision induced ionization (CID) of the derived ions m/z 180 from both 53 and 54 gave rise to Me3Si+ at m/z 73 in addition to ions at m/z 108 and m/z 106 arising from competing proton transfer and hydride transfer pathways from within the initially formed ion-molecule-complexes. CID of the derived ions m/z 189 from the deuterated derivatives 104 and 105 gave rise to (CD3)3Si+ at m/z 82, in addition to ions at m/z 109 and m/z 106. Formation of the m/z 108 ions from 53 and 54 provides evidence to support the proposed mechanism for desilylation of β-silyl carbenium ions in solution. Computational studies at B3LYP/Gen (where 6-311g(d,p) was applied to C, H and N atoms, while SDD was applied to the metals: Si,Ge, Sn and Pb) support the MS studies as it was found that the favoured pathway is the proton transfer from the Me3Si+ ion to the basic enamine intermediate, giving rise to N-methyl-picoline and silylethene, followed closely by dissociation of the ion-molecule complex, with the least energetically favored pathway being hydride abstraction. Crystals of the cation 105 as its triflate salt, when heated at 160°C in a sealed tube for 2 days, gave rise to a mixture of N-methyl 4-CH3-pyridinium ion and N-methyl 4-CH2D-pyridinium ion, indicating that the proton transfer step also occurs in the condensed phase. A crystallographic and computation study on 2- and 4-trialkylgermylmethyl pyridinium ions 161 and 162 provided evidence for strong hyperconjugation between the Ge-CH2 bond and the π-deficient aromatic ring. Isodesmic equations show that the trimethylgermyl-substituent stabilizes the pyridinium cations by 20-26 kJ/mol relative to the germanium-free analogs. Natural bond orbital analysis reveals that the major contributor to this stabilization is hyperconjugation between the Ge-CH2 bond and the aromatic π-system, and that the strength of this interaction is greater for the 2-substituted ions 169 compared to the 4-substituted ions 170. Crystallographic analysis of the 2- and 4-tert-butylgermylmethyl- substituted ions x and x provides the first structural evidence for carbon-germanium hyperconjugation, thus the Ge-CH2 bonds are significantly longer (0.03-0.04 Å) than standard values and the CH2-Ar bond distance is shorter. Investigation of the gas phase unimolecular chemistry of these ions, formed via electrospray ionization (ESI) and subjected to collision induced dissociation (CID), reveals that the principle fragmentation of these ions involves cleavage of the weak CH2-GeR3 bond giving an ion molecule complex between an R3Ge+ and a pyridine-enamine, which then undergoes further rearrangement. Manifestation of the γ-effect can be seen in the crystals and theoretical structures by an increase in the Si-CH2 bond and a decrease in the CH2-CH2-C(Ar) bond angle of a range of trimethylsilylethyl-pyridinium ions (M = Si, Ge, Sn, Pb). The stabilization of the Group IV metals was found to be stronger at the β-position than at the γ-position. It was discovered that due to the tertiary nature of the cations, stabilization via a metal at the γ-position is not as pronounced as it would be if the stabilization of the primary or secondary cation were present. Computational studies on the trimethylsilylpropyl-pyridinium ions 394, 395 and 414-419 (M = Si, Ge, Sn, Pb) provide evidence for significant through-bond (double hyperconjugative) interaction between the M-CH2 bond and the low-lying π* orbital of the pyridinium ion. The strength of this interaction increases in the order Si < Ge < Sn < Pb in line with the σ-donor abilities of the C-M bond. The through-bond interaction for M = Si has been studied in solution using 13C and 29Si NMR studies, however the effect is small. Fragmentation pathways followed by these ions 394 and 395 in the gas phase under CID, is strongly influenced by the through-bond interaction and extrusion of ethylene occurs resulting in the formation of trimethylsilylmethyl-substituted pyridinium ions, 53 and 54, as the major fragmentation pathway.