Fluctuating filaments: statistical mechanics of helices

We examine the effects of thermal fluctuations on thin elastic filaments with noncircular cross section and arbitrary spontaneous curvature and torsion. Analytical expressions for orientational correlation functions and for the persistence length of helices are derived, and it is found that this length varies nonmonotonically with the strength of thermal fluctuations. In the weak fluctuation regime, the local helical structure is preserved and the statistical properties are dominated by long-wavelength bending and torsion modes. As the amplitude of fluctuations is increased, the helix "melts" and all memory of intrinsic helical structure is lost. Spontaneous twist of the cross section leads to resonant dependence of the persistence length on the twist rate.