Persistence length: What are the beginning and end point in an polymer?

Juqon

Hello,

can you tell me where the starting point and the ending point of the (parts of the) persistence length in a polymer are?
I thought the persistence length was the greatest length that only just is not bent. This, however, cannot be as every part of the molecule down to the beginning is bent at least a little (maybe invisible). You would have to say, e.g.: "Every part of the molecule that is bent less than 3% belongs to the persistence length."

Mike H

I might be misunderstanding your question here, but you are not going to worry about the detailed molecular structure of the polymer, as that will inevitably not be perfectly linear in many cases. For example, when people discuss the the persistence length of DNA, they figure to treat the backbone and nucleotide as a tube. See this page discussing DNA elasticity for more detail.

Juqon

I was already figuring the DNA as one tube.
In your article it says "the elastic cost of bending is totally negligible", but what is the limit for this negligence?

Mike H

Well, persistence lengths will vary depending on the polymer, of course. I used to work with actin, and its persistence length is ~ 15 μm. As noted in that article, DNA has a much shorter persistence length (more than two orders of magnitude, in fact).

An alternate way to look at persistence length is that it is the distance where a bend or twist at one point of the polymer does not affect a different point of the polymer. So, for example, if you bend a DNA strand at a point 5 μm from another point, you're not going to notice any correlation between the direction of their tangents, as the persistence length is only ~ 0.05 μm. But if you did that with an actin strand, you would definitely notice the correlation.