Dissociation of DNA/anti-DNA complexes at high pH

A high-avidity human antibody (Ab) to double-stranded DNA (dsDNA) could be virtually completely dissociated from Crithidia luciliae kinetoplast dsDNA at pH 12 and a low-avidity Ab at pH 11. At low pH (pH 2), no dissociation occurs of either Ab. Low-avidity Ab could be dissociated at neutral pH with 1 M NaCl, but even with 5 M NaCl high avidity Ab could not be dissociated. Prolonged exposure to pH 12 did not affect DNA-binding by Ab after dialysis against PBS. A decrease in surface tension of the liquid medium in no case prompted dissociation. Contact angle measurements on DNA showed it to be very hydrophilic. It is concluded that concomitant with the strong negative charge of the antigen and the positive charge of the Ab, both antigenic determinant and antibody-active site are exceptionally hydrophilic, which causes their van der Waals attraction in aqueous media to be negligibly small. This particular antigen-antibody bond thus is mainly electrostatic and can be completely dissociated by abolishing the positive charge of the antibody-active site through a drastic increase in pH.