Flow dichroism of T7 DNA as a function of salt concentration

Abstract Measurements have been made on the flow dichroism of T7 DNA as a function of NaCl concentration. In high salt, our results are compatible with an optical factor of −1.4 and a persistence length of 470 Å. The former is in agreement with expectations from the x‐ray diffraction structure of fibrous B–DNA, and the latter is in the midrange of recent determinations. As salt concentration is decreased, the persistence length increases. The relation of our study to other recent investigations and with current theories of the electrostatic contribution to persistence length is discussed. We note that the separation of the electrostatic expansion factor into long‐ and short‐range effects is somewhat arbitrary and might affect the interpretation of different experimental results in different ways. Finally, our hydrodynamic factors are consistent with a chain which is partially free‐draining. This goes against the traditional interpretation but is in agreement with two recent observations.