The interactions of 2-thiopyrimidine bases with hydroxymercurybenzene sulfonate.

Abstract

The interaction of hydroxymercurybenzene sulfonate (HOHgBzS) with 2‐thiouridine (s2Urd) and 2‐thiocytidine (s2Cyd) by means of absorption‐difference spectroscopy and spectrophotometric titration was investigated. The absorption spectra of the reaction mixtures displayed isosbestic points indicating uniform reactions. Scatchard plots of the titration data revealed the formation of 1:1 complexes with a dissociation constant for s2Urd · HOHgBzS and s2Cyd · HOHgBzS of Kd= 0.55 μM and Kd= 0.54 μM, respectively. The interactions of poly(s2U) and poly(s2c) with hydroxymercurybenzene sulfonate were established by their corresponding absorption difference spectra. The binding of hydroxymercurybenzoate (HOHgBz) to poly(s2C) and poly(s2U) was investigated by means of equilibrium dialysis. The association of HOHgBz to 2‐thiocytosine bases in poly(s2C) follows a simple hyperbolic binding isotherm. The Scatchard plot of the binding data disclosed that only a fraction (n= 0.32) of the s2CMP residues in poly(s2C) was accessible to the interaction with HOHgBz. The dissociation constant of this interaction was found to be Kd= 4.3 μM. The binding of HOHgBz to the helical poly(s2U) is a cooperative process as indicated by the sigmoidal binding cuve. Under saturating conditions all s2UMP residues in poly(s2U) interact with the ligand.