Aggregate structure, protein molecular

If preparative or instrumental artifact is ruled out, the universal occurrence of red-shifted Cotton effects with a-helical character in all the membranes studied points to a common property of the proteins in biological membranes. The ORD results from lipid-free mitochondrial structural protein and erythrocyte ghost protein are consistent with assigning the red shift in these membranes to aggregated protein. It is, therefore, reasonable that similar protein-protein association may occur in all membranes. Ionic requirements for membrane stability could then reflect in part the requirements for protein-protein association. To some extent the molecular associations which stabilize membranes, therefore, may be protein-protein as well as lipid-lipid in nature. 

In contrast to typical chiral thermotropic constituent molecules, protein molecules have a huge number of chiral centres, and the twist between assembled proteins is typically much larger (-1/10 of a revolution). We can expect to find a range of similar liquid crystalline phases in protein aggregates, although with significantly smaller lattice parameters (compared with typical protein dimensions). Indeed, the aggregation processes of (e.g. structural) proteins may be driven by their chirality as much as by their molecular shape (and amphiphilicity). This is discussed in more detail in Chapter 6. 

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