Cooperativity of the helix–random coil transition determined from hydrodynamic data

The approximations in the equation relating the viscosity of partially helical molecules to the cooperativity of the helix–random coil transition are investigated. Of the several approximations considered, only the neglect of long‐range intramolecular interactions in the random coil leads to significant error. This is a result of the inverse fourth‐power relationship between the assumed expansion factor and the cooperativity determined from analysis of experimental data. Upon approximately accounting for the random coil expansion, the experimental data of Ptitsyn et al. yield an ionic strength‐independent cooperativity in agreement with other methods.