Properties of DNA modified with N-methyl-N-nitrosourea

The initial effect of N-methyl-N-nitrosourea (MNU) on double-stranded calf thymus DNA was studied by means of derivative pulse polarography, kinetic formaldehyde method and spectrophotometric denaturation curves. In the first 30 min of the reaction it was found that, at a molar ratio MNU : DNA P = 20 : 1, changes in the pulse-polarographic behaviour of DNA took place, while no changes in Tm were detected. A higher reactivity of DNA with formaldehyde was observed in the same time interval, when a higher ratio MNU : DNA P (50 : 1) was used. The increase of the pulse-polarographic peak that was observed at room temperature and slightly acid pH, or at neutral pH and elevated (premelting) temperature was explained by a local destabilization of the double helix.