UNIT 20 Self Assembly and the Molecular Structure of Membranes

The spontaneous orientations of molecules in phase boundaries are of particular interest for questions of self-organization and stability of biological membranes. Let us first consider the behavior of molecules in the boundary between two phases, one being hydrophilic (water), the other hydrophobic (for example oil). A protein tends to orient its polar groups as much as possible toward the aqueous phase, and its non-polar, i.e. hydrophobic groups toward oil. This leads to an orientation of the molecule at the phase boundary. If the polar groups of the molecule are distributed homogeneously over the whole molecule, then it could become unfolded by orientation in such a boundary. The protein, now being a fibril will spread over this interface.

The most important constituents of biological membranes are phospholipids. This is a group of compounds which consists of two fatty acids and a phosphate group, all attached to a polyhydric alcohol such as glycerol. The phosphate group is usually etherified with a nitrogen-containing alcohol. Two long hydrophobic fatty acid chains and a single phosphate group etherified with an ethanolamine residue are linked to a central glycerol molecule. The phosphate and the amino-group of the ethanolamine residue represent the hydrophilic part of the molecule.

The large variety of biological lipids is realized by the diversity of the fatty acids as well as by their different head groups. In the case of phosphatidyl-choline, the head group is polar, but does not carry net charges under physiological conditions. The negative charge of the phosphate is compensated by a positive charge of the amino group. Phosphatidyl-serine, in contrast to this, contains an additional negative charge, namely that of a carboxyl group. This is the reason why it significantly influences the surface charges of biological membranes.

Phospholipids strongly orientate themselves at interfaces. In this case the hydrophobic chains of fatty acids are orientated parallel to each other and perpendicular to the interface. In this way, monomolecular layers, so-called monolayers, are formed.

In case of homogeneous aqueous phases, phospholipids reach a state of minimal surface tension if they orientate their hydrophobic portions toward one another.

Define the following words using a dictionary

Hydrophilic, hydrophobic, etherified, aqueous, a residue, a monolayer

Complete the sentences

1. The spontaneous orientations of molecules in phase boundaries are of particular interest…
2. A protein tends to orient its polar groups…
3. If the polar groups of the molecule are distributed homogeneously over the whole molecule,…
4. Phospholipids are a group of compounds…
5. The large variety of biological lipids…
6. The negative charge of the phosphate is compensated…
7. In case of homogeneous aqueous phases,…