Depolarized Light Scattering from a Helix–Coil System

The spectral distribution of the depolarized component of light scattered from a dilute solution of a macromolecule which exists in dynamic equilibrium between helix and coil forms is computed. The helix is treated as a cylindrically symmetric, optically anisotropic rigid rod, and the coil as a freely jointed chain of cylindrically symmetric, optically anisotropic segments. The duration of the helix–coil transition (as opposed to the “lifetime” for the process) is taken as short compared to characteristic times for molecular translation and rotation. It is assumed that a given segment on undergoing a transition loses all “memory” of its initial orientation—this assumption leads to a spectrum which consists of two Lorentzian lines whose widths are determined by the translational and rotational diffusion constants and by the lifetimes toward interconversion for the helix and coil forms. Under certain conditions, the low-angle scattering spectrum is a single Lorentzian whose width is primarily determined by the lifetime for disappearance of the helix form.