The binding of Violamycin BI to poly-C

The binding constant Kst of Violamycin BI (VBI) to poly-C at small and medium values of the concentration ratio p (0-12) is determined using the procedure of Schwarz under two different conditions: at constant C(VBIo) and variable C(po), and at p = C(po)/C(VBIo) = const. The average value obtained for Kst is 3.3 +/- 0.1 x 10(4) M-1, whereas the cooperativity parameter q of 13 characterizes a moderate cooperative interaction between adjacent bound ligands. In contrast, at large values of p (12-355) the formation of isolated bound dimers on the poly-C chain is observed. At pH 7, VBI dimerizes in solution with a dimerization constant strongly dependent on ionic strength: Kd = 732 +/- 20 M-1 and (9.3 +/- 0.2) x 10(3) M-1 at I = 0.02 M and 0.2 M respectively. The lower and upper boundaries for the binding constant of the dimer to the polynucleotide at large values of p are 1.0 x 10(-5) M-1 and 6.25 x 10(-6) M-1 respectively.