Lipids and Membranes 4 Flashcards
Why is mixing expected to be favoured by entropy?
ΔG = ΔH - TΔS
If the new bonds formed due to mixing between two mixed substances A and B are stronger than A-A and B-B, the mixture is at a lower energy than the initial separated molecules and excess energy is released as heat (exothermic). For an exothermic reaction ΔH<0, so the contribution of this to ΔG will be negative, so mixing is favoured.
This predicts that the tendency to mix should increase with T and degree of mixing should depend on the enthaplies of mixing and the strength of the bonds broken and formed.
What is the central property of hydrophobic compounds?
In hydrophobic compounds, entropy favours de-mixing.
What is the hydrophobic effect?
The observed tendency of nonpolar (hydrophobic) molecules to aggregate (and self-assemble into structures) in an aqueous solution and exclude water molecules in order to minimise the interface surface between the hydrophobic molecules and water in order to increase the entropy of the system and minimise its free energy.
An entropic effect caused by the energy cost of disrupting many hydrogen bonds by a non-polar molecule and the requirement to minimise it.
What is the basic driving force behind self-assembly and why does this occur?
The minimisation of free energy/entropy. Self-assembly of hydrophobic molecules (e.g. lipid bilayers) decreases the entropy of the hydrophobic molecules but increases the entropy of the solvent water molecules and the system as a whole increases in entropy (decreasing the free energy of the system).
At what temperature do hydrophobic compounds tend to undergo self-assembly the most?
At minimum solubility which is at the temperature range where proteins normally work and life exists.
Why does the hydrophobic effect occur?
A water molecule in tetrahedral arrangement with 4 nearest neighbours can arrange itself in 6 different positions.
If you replace one of the 4 surrounding water molecules with a hydrophobic molecule unable to H-bond, it reduces the possible configurations from 6 to 3 (i.e. configurational entropy is halved).
So mixing causes the entropy of water molecules at the water-hydrophobic interface to decrease (so water becomes more ordered), so to counteract this the system minimises the interface between the hydrophobic molecule and the water (causing hydrophobic molecules to aggrogate) to minimise the free energy. The less water surface area that is exposed to hydrophobic molecules, the fewer water molecules have to suffer the loss of conformational entropy.
Why may non-polar solvents be viewed as cavities in water?
As the volume taken up by a non-polar solvent is a region where H-bonds cannot occur.
What is the ΔG due to introducing a small non-polar solute to water? What is the ΔG due to introducing a large non-polar solute to water?
ΔG due to small solute ∼ Volume of solute
ΔG due to large solute ∼ Area of solute
Why is there a specific temperature at which the hydrophobic effect is strongest?
Because the temperature dependence of entropy and enthalpy are not the same.
ΔG = ΔH - TΔS
In this context, T↑ = H↑ from 0 at room temperature towards a maximum
and
T↑ = S↑ from negative at room temperature towards 0.
There is a temperature at which the value of ΔH and ΔS cause ΔG to be minimised the most. This is the temperature at which the hydrophobic effect is the strongest.
Why does the hydrophobic effect decrease in strength with temperatures below the optimum temperature?
The decrease in strength of the hydrophobic effect with decreasing temperatures is probably due to reduction in overall entropy. The structure of water at low temperatures (ice) is more stable and ordered as it approaches 0K, so the cost of adding a hydrophobic molecule which should normally increase water order by decreasing configurational and rotational entropy is minimised (as there is almost no configurational or rotational entropy left to decrease at low temperatures) so de-mixing occurs less or not at all.
Why does the hydrophobic effect decrease in strength with temperatures above the optimum temperature?
At high temperature, a decrease in entropy by mixing is negligible as the water has so much energy/entropy.
Describe the forces acting on a cell membrane.
Cell membranes are held together by van der Waals interactions and strongly attractive capillary force (created by air bubbles (picked up by hydrophobic patches) that bridge two approaching surfaces) acting to compact the cell membrane whilst the electrostatic and steric repulsion within the chains and head groups act to expand the cell membrane. These forces balance each other.
What are the effects of the forces acting on a cell membrane on its properties and function?
As a result of the strong, opposing forces acting on the cell membrane, the hydrophobic chains within a cell membrane are under a lot of pressure and are very dense. This makes it hard for things to push through the cell membrane. Despite the liquid-like flow of molecules within the cell membrane, it responds to forces acting upon it similar to a solid due to its high pressure and density. High lateral pressure within cell membranes creates strong forces that close any punctures within the cell membrane.
What is chemical potential, μ?
The potential energy due to chemical bonds in a system that can be absorbed or released.
A measure of how readily a chemical transformation (reaction), phase change or migration of particles takes place.
The amount by which the energy of a system would change if an additional particle were introduced, with the energy and volume fixed.
