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

Isolated mammalian metaphase chromosomes suspended in ion-free or 10d3 YN&l 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 pK’s lower than the isoionic pH of chromosomes in ion-free water, pH 66, accompanies the change in conformation. The structural change is inhibited in aqueous solutions of O-001 Mmagnesium or calcium ion. Adding magnesium or calcium ions to a final concentration of 10m3 Y results in a lowering of the pH of the water-washed chromosome solution from 56 to 4.9. The binding constant for magnesium ion to chromosomal sites was measured to be 1-l x 106 (moles/liter)-r and the number of binding sites is 16 x lows millimole per milligram of chromosomes.