Cation Hosts Flashcards
How do crown ethers behave in solution?
Crown ethers equally soluble in aqueous and organic solvents with a lipophilic outside and polar inside. Twisting and solvation of lone pairs i.e. conformation change, can change the properties.
How does phase transfer catalysis work?
Reaction occurring over two phases, with a phase transfer catalyst to take guest species from one phase to the other.
What is the naked anion effect?
Naked anion = anion without counter-cation. More exposed so reactivity promoted. Host can sequester away cationic guest to give the naked anion.
Why is membrane transport important?
Cells keep a transmembrane electric potential with high conc. of potassium inside the cell and high conc. of sodium outside the cell.
By what mechanisms does ion transport work to maintain a transmembrane electric potential within the cell?
Channel pore, gated channel, carrier or pump mechanisms
How does a channel pore work?
Lipophilic region to pass in and out of cell. A channel pore is functionally a hole in the membrane to allow ion transport without loss of solvation.
How does a gated channel work?
Channel allows ion transport as the gate is open and the ion can pass through the selectivity filter - selective for potassium over sodium.
How does a carrier work in ion transport?
Carrier (aka ionophore) encapsulates alkali metal cation, stripping away solvation sphere. Complex then moves across the membrane, releasing ion back into aqueous solution on opposite side. Works in one direction due to concentration gradient.
How does a pump work in ion transport?
Active transport. Change in conformation allows transmembrane movement.
How does selectivity of natural ionophores vary?
Most ionophores anti-selective for sodium rather than selective for potassium. Natural products, so unlikely to encounter Rb and Cs.
How does the potassium ion channel work?
Gating done by slightly negative charged amino acids, and opened by neurotransmitters or hormones. The ion is solvated throughout. Selectivity filter comes from aromatic residues - cation-pi interactions allow movement of cation past. Unable to shrink down for sodium, so limited interaction. Very fast throughput due to limited desolvation and mutual repulsion of potassium ions.
How is the ion channel model using hydraphiles built?
Crude mimic giving rise to similar patterns. Entry portals/headgroups given by azacrown ethers with a crown ether central relay. Hydrophobic sidearms and connectivities (alkyl groups) lead to cell-membrane type interactions.
