Dr. Susan Hanlon and co-workers at the University of Illinois-Chicago developed a method for specifically modifying G/C base pairs in B-DNA. The reagent, an N-butyl amine:formaldehyde mixture does NOT react with A/T or I/C containing base pairs. They have shown that the modified residues in DNA are positively charged. To probe the effect of modification on DNA structure, they perform two experiments and obtain the following results.

Result 1. Exposing linear DNA to reagent followed by circularization of this linear modified form gives circular products that are of higher mobility than circularized, unmodified DNA. Treatment of the unmodified form with ethidium bromide increases its mobility.

Result 2. Exposing closed circular relaxed DNA to the reagent results in a form of modified circular DNA which has a higher mobility in agarose gels than does the unmodified starting DNA. Addition of increasing amounts of ethidium bromide. A low amounts of ethidium, the mobility of the modified DNA is decreased. At higher ethidium concentrations, the mobility of the modified form increases again.