T. Antony et al., SPECTROSCOPIC STUDIES ON THE BINDING OF METHYLENE-BLUE TO POLY(RIBOADENYLIC ACID), Journal of biomolecular structure & dynamics, 11(1), 1993, pp. 67-81
The binding of methylene blue (MB) to poly(rA) was studied using UV an
d CD spectroscopy at neutral and acidic pH, in which poly(rA) exists a
s a single strand and a duplex respectively. UV spectroscopic studies
and analysis of equilibrium binding data show that salt increases the
cooperativity parameter but reduces the binding between the cationic d
ye and the negatively charged phosphate groups. The binding constant f
or the single strand-MB complex decreased from 1.15 X 10(5) M-1 to 7.6
2 X 10(3) M-1 while the cooperativity parameter increased from 9 to 71
, when salt concentration was increased from 1 mM to 200 mM. At neutra
l pH, changes in the induced circular dichroism of MB-poly(rA) complex
with ionic strength show predominance of electrostatic binding to the
phosphate group externally on the poly(rA) chain, a partial intercala
tion between the adenine bases is also indicated. A red shift in the v
isible absorption band and the ICD profile of the double stranded poly
(rA)-MB complex suggest an electronically coupled intercalation mode
for the dye binding at pH 4.2.