In our discussion of flexible polymers of finite length, we have seen that finite-size effects essentially influence the formation of “crystalline” structural phases. We will now investigate how the arrangement of monomers in this crystalline regime changes, if the flexibility is successively reduced by bond–bond correlations. The class of linear macromolecules that effectively fills the gap between purely flexible and stiff chains is called semiflexible polymers. The most prominent representatives in nature are DNA and RNA, but also many proteins possess an effective stiffness. Examples are myosin fibers and actin filaments, which are sufficiently stiff to prevent self-interaction. In these cases, the continuum wormlike-chain model (1.22) is reasonable to investigate thermal fluctuations. However, if the persistence length is small enough, self-interaction occurs, which can cause conformational transitions, depending on environmental conditions. For the basic introduction to semiflexible polymers and their modeling, the reader may consult Section 1.6.

Meanwhile, many examples of single- and double-stranded nucleic acids are known, where the structural behavior deviates from the one predicted by the wormlike-chain model. In these cases, the persistence length is not constant, but depends on chain length and external conditions such as temperature and salt concentrations [171–173]. Consequently, the persistence length is no longer an appropriate length scale for the description of structural behavior.