Question

True story: a young graduate student is told by his advisor to simulate helical polypeptides (which are semiflexible polymers…sort of like weakly bending rods) in a computer. The advisor specifically wants to know the molecular weight dependence of the persistence length. Comment on the advisor’s request.

Answer 1

The persistence length is a useful measure of the degree of structural rigidity of a polymer chain and the energy cost of deforming it. A continuing stream of persistence-length data reflects the importance attached to an understanding bendability by proteins.

Persistence length behaves as a rodlike polymer at the air−water, and with an increase in the molecular weight the chains undergo chain entanglements to give smaller extrapolated area. Such a molecular weight dependence is in good agreement with the fluorescence images at the air−water interface.