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Metal ligand interactions under physiological conditions
188
Abstracts
B133
METAL LIGAND INTERACTIONS UNDER PHYSIOLGGICAL CONDITIONS J J Powell*, P Taylor b, R P H Thompson. ?he Rayne Institute, St Thomas’ Hospital, London SE1 7EH, UK. bChemistry, UCL, London WC2H OPP, UK.
Ligand interactions with hydrolytic metal-ions are difficult to study under physiological conditions and are presently, therefore, determined under simplifying conditions of lower pH which risk omitting species that only predominate at near neutral pHs. In addition, equilibrium calculations may be inappropriate under physiological conditions, particularly when the metal-ion ligand exchange reactions are slow, as for Al(II1). We have used several techniques to study the influence of different ligands on the precipitation of Al(II1) in MOPS buffered saline (pH=7.0). Using laser light scattering, in the absence of added ligand, solutions of aluminium (0.1-0.75mM) formed particles within seconds of being introduced to neutral pH and then changed little for at least a further 15h. Particle sizes were particularly affected by sheer forces, for example, with OSmMAl, continuous stirring decreased particle size (10-45~m;20~m) (90% range; modal) compared to unstirred systems (30-55pm;42km). Bicarbonate (5OmM) slowed the rate of precipitation (from a few seconds to 300 seconds, for completion) and reduced particle size (3-12pm;5pm). This may be relevant since bicarbonate promotes the formation of hydroxy-aluminosilicates in solution. The ability ot hgands then to compete witn tms anmumum aydroxwe precipitation was studied using quantitative empirical measurements. After centrifugation the concentration of aluminium in solution was measured by plasma emission spectrometry. Some ligands maintained more aluminium in solution than expected, for example, surprisingly, malt01 (3-hydroxy-2 methyl4 pyrone) maintained up to 2.3 mole aluminium/mole malt01 in solution for > four weeks, suggesting that at least at metal-ion:ligand ratios > I, oligomeric hydroxy-bridged aluminium ligand complexes are formed. Above a critical Ahligand ratio, there was catastrophic precipitation of aluminium. Thus, at 6: 1 (aluminium:maltol) total aluminium in solution was near zero for > four weeks. High resolution NMR showed the ligands became incorporated within the precipitate, suggesting that at the pre-critical aluminiunnligand ratios, soluble species are dynamic metastable hydroxy-bridged aluminium ligand oligomers that then polymerise to critical size for precipitation, The maximum binding capacity of ligands for aluminium was decreased in the order, citrate > malt01 > malate > oxalate > lactate = ascorbate. Succinate bound no aluminium, confirmed using laser light scattering, The techniques have been applied to more complex solutions, namely, pancreatic juice, bile, gastric juice and distal small bowel contents and the latter three all appreciably bound aluminium (0.55, 0.17 and 4.7mM maximum, respectively), largely due to their soluble mucin content , shown in further experiments.
Abstracts
B133
METAL LIGAND INTERACTIONS UNDER PHYSIOLGGICAL CONDITIONS J J Powell*, P Taylor b, R P H Thompson. ?he Rayne Institute, St Thomas’ Hospital, London SE1 7EH, UK. bChemistry, UCL, London WC2H OPP, UK.
Ligand interactions with hydrolytic metal-ions are difficult to study under physiological conditions and are presently, therefore, determined under simplifying conditions of lower pH which risk omitting species that only predominate at near neutral pHs. In addition, equilibrium calculations may be inappropriate under physiological conditions, particularly when the metal-ion ligand exchange reactions are slow, as for Al(II1). We have used several techniques to study the influence of different ligands on the precipitation of Al(II1) in MOPS buffered saline (pH=7.0). Using laser light scattering, in the absence of added ligand, solutions of aluminium (0.1-0.75mM) formed particles within seconds of being introduced to neutral pH and then changed little for at least a further 15h. Particle sizes were particularly affected by sheer forces, for example, with OSmMAl, continuous stirring decreased particle size (10-45~m;20~m) (90% range; modal) compared to unstirred systems (30-55pm;42km). Bicarbonate (5OmM) slowed the rate of precipitation (from a few seconds to 300 seconds, for completion) and reduced particle size (3-12pm;5pm). This may be relevant since bicarbonate promotes the formation of hydroxy-aluminosilicates in solution. The ability ot hgands then to compete witn tms anmumum aydroxwe precipitation was studied using quantitative empirical measurements. After centrifugation the concentration of aluminium in solution was measured by plasma emission spectrometry. Some ligands maintained more aluminium in solution than expected, for example, surprisingly, malt01 (3-hydroxy-2 methyl4 pyrone) maintained up to 2.3 mole aluminium/mole malt01 in solution for > four weeks, suggesting that at least at metal-ion:ligand ratios > I, oligomeric hydroxy-bridged aluminium ligand complexes are formed. Above a critical Ahligand ratio, there was catastrophic precipitation of aluminium. Thus, at 6: 1 (aluminium:maltol) total aluminium in solution was near zero for > four weeks. High resolution NMR showed the ligands became incorporated within the precipitate, suggesting that at the pre-critical aluminiunnligand ratios, soluble species are dynamic metastable hydroxy-bridged aluminium ligand oligomers that then polymerise to critical size for precipitation, The maximum binding capacity of ligands for aluminium was decreased in the order, citrate > malt01 > malate > oxalate > lactate = ascorbate. Succinate bound no aluminium, confirmed using laser light scattering, The techniques have been applied to more complex solutions, namely, pancreatic juice, bile, gastric juice and distal small bowel contents and the latter three all appreciably bound aluminium (0.55, 0.17 and 4.7mM maximum, respectively), largely due to their soluble mucin content , shown in further experiments.