Formation constants for the complexes of adenosine di- or tri-phosphate with magnesium or calcium ions

Abstract

The formation of complexes between adenosine tri- or di-phosphate (ATP or ADP) and Mg++ or Ca++ has been studied in aqueous solution at pH 7.9-8.9. A spectrophotometric method was used in which 8-hydroxyquinoline acts as an indicator of free metal ion. Adenosine triphosphate has a much greater affinity for both metals than has adenosine diphosphate. The formation constants (i.e. reciprocals of the dissociation constants) for the MgATP[long dash]and MgADP[long dash]complexes were 38,000 and 2200[image] -1 respectively at 25[degree], or 98,000 and 6900[image] -1 at 64[degree] (I 0.11; 85 m[image] tributylethylammonium bromide plus 25 m[image] triethanolamine hydrochloride). The interpolated values for 37[degree] are 55,000 and 3300[image] -1 respectively. There was no clear indication that bimetallic complexes were formed at concentrations of up to 80 m[image] Mg++ or Ca++. The results also indicate the formation of a ternary complex between Mg, ATP and 8-hydroxyquinoline.