Relaxation kinetics of the interaction between RNA and metal-intercalators: The Poly(A)·Poly(U)/platinum-proflavine system

The interactions of Poly(A)·Poly(U) with the cis-platinum derivative of proflavine [{PtCl(tmen)}2{HNC13H7(NHCH2CH2)2}]+ (PRPt) and proflavine (PR) are investigated by spectrophotometry, spectrofluorimetry and T-jump relaxation at I = 0.2 M, pH 7.0, and T = 25 °C. Base-dye interactions prevail at high RNA/dye ratio and binding isotherms analysis reveals that both dyes bind to Poly(A)·Poly(U) according to the excluded site model (n = 2). Only one relaxation effect is observed for the Poly(A)·Poly(U)/PRPt system, whereas two effects are observed with Poly(A)·Poly(U)/PR. The results agree with the sequence D + S ⇆ D, S ⇆ DSI ⇆ DSII, where D,S is an external complex, DSI is a partially intercalated species, and DSII is the fully intercalated complex. Formation of DSII could be observed in the case of proflavine only. This result is interpreted by assuming that the platinum-containing residue of PRPt hinders the full intercalation of the acridine residue.