Properties of P-form DNA as revealed by circular dichroism

Investigations of DNA using CD spectroscopy show that the P‐form is available in a wide variety of methanol–ethanol mixtures when the water content is low. Increasing the temperature or the ethanol content of a 95% methanol solution causes DNA to undergo a cooperative transition to the P‐form. However, this transition cannot be reversed on cooling, or on adding methanol. Thus P‐form DNA appears to be stable at high methanol concentrations, but it is usually not observed because the DNA is trapped by a kinetic barrier. P‐form DNA will instantaneously assume the native B‐form on addition of water, confirming earlier reports that P‐form DNA is not strand separated [E. Kay (1976) Biochemistry 15, 5241]. CD spectra extended to 190 nm show that there is no base–base interaction in the P‐form. However, the P‐form is extremely stable to heat denaturation in solvents which promote hydrogen bonding between the base pairs. A number of models that can account for the properties of P‐form DNA are discussed.