Interaction of Porphyrins with tRNA. The Influence of Ionic Strength

Previously we have investigated the interaction of meso-tetra-(4N-oxyethylpyridyl)porphyrin (TOEPyP4) and its Zn(II), Cu(II), -metallocomplexes with tRNA at low ionic strength [1]. In this work the influence of ionic strength on the interaction of tRNA from E.Coli with TOEPyP4 and its metallocomplexes is studied by UV/Vis Spectrophotometry and Circular Dichroism methods.The measurements were performed in 0.1 BPSE and 1BPSE buffers (1BPSE = 6 mM Na2HPO4+ 2 mM NaH2PO4 +185 mM NaCl + 1 mM Na2EDTA), respectively μ = 0.02M and μ = 0.2M, pH 6.57.From the absorption datas the Scatchard binding isotherms for porphyrin-tRNA complexes are built and binding parameters are calculated (N - the number of binding sites per molecule of tRNA, and K - the binding constant).In case of low ionic strength for the values of induced CD spectra (at 400-470 nm) for complexes tRNA with TOEPyP4 and CuTOEPyP4 there is an optimum concentration of porphyrins (0.2-0.25) at which the anisotropy of system is maximal. For complexes of ZnTOEPyP4 with tRNA the induced CD spectra are essentially different. The induced CD spectra of complex change a sign and continue to grow (remaining negative) starting from a certain relative concentration. This unusual ICD spectra profile is found for all three porphyrin-tRNA complexes at high ionic strength. It is possible that at high relative concentration of porphyrins the liquid crystal form may exist in the solution.For all porphyrins the binding constants with tRNA at low ionic strength are an order of magnitude greater than in case of binding at high ionic strength. It means that these porphyrins interact stronger with tRNA at low ionic strength.Reference1. Y. Dalyan, I. Vardanyan, A. Chavushyan,G. Balayan. J of Biomol Structure & Dynamics 28, 123-131 (2010).