The structure of metaphase chromosomes: I. Electrometric titration, magnesium ion binding and circular dichroism

Abstract

Isolated mammalian metaphase chromosomes suspended in ion-free or 10 −3 m-NaCl solution undergo an irreversible, pH-dependent structural change, as reflected by electrometric titrations, pH-stat experiments and measurements of circular dichroism spectra. Unmasking of titratable groups with p K's lower than the isoionic pH of chromosomes in ion-free water, pH 5.6, accompanies the change in conformation. The structural change is inhibited in aqueous solutions of 0.001 m-magnesium or calcium ion. Adding magnesium or calcium ions to a final concentration of 10 −3 m results in a lowering of the pH of the water-washed chromosome solution from 5.6 to 4.9. The binding constant for magnesium ion to chromosomal sites was measured to be 1.1 × 10 5 (moles/liter) −1 and the number of binding sites is 1.6 × 10 −5 millimole per milligram of chromosomes.