The Influence of the Form of tRNA on Complex Formation with Porphyrins

Abstract

In the current work we have investigated the interaction of meso-tetra-(4N-oxyethylpyridyl) porphyrin (TOEPyP4) and its Zn(II), Cu(II), -metallocomplexes with tRNA which has 2 forms: hairpin structure and spatial reversed “L” structure. The interaction of tRNA from E.Coli with porphyrins is studied by UV/Vis Spectrophotometry and Circular Dichroism methods.The measurements were performed in 0.1 BPSE and 1BPSE buffers (1 BPSE = 6 mM Na2HPO4+ 2 mM NaH2PO4 +185 mM NaCl + 1 mM Na2EDTA), correspondingly μ = 0.02M and μ = 0.2M, pH 6.57. (tRNA has hairpin form at μ = 0.02M and reversed “L” structure, when μ = 0.2M).From the spectrophotometric titration 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 tRNA's hairpin structure 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 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 in case of tRNA's reversed “L” structure. It is possible that at high relative concentration of porphyrins the liquid crystal form may exist in the solution.The binding constants with tRNA in case of hairpin form are an order of magnitude greater than in case of reversed “L” structure. It means that this porphyrin interacts stronger with tRNA when it has hairpin form.