Solution structure and energy calculation of bis-intercalation of homodimeric thiazole orange dye derivatives in DNA: Effects of modifying the linker

We have used two-dimensional H-1 NMR spectroscopy obtained at 750 MHz to determine a high-resolution solution structure of the double-stranded DNA oligonucleotide d(5'-CGCTAGCG-3')(2) complexed with the bis-intercalating dye 1,1'-(5,5,9,9-tetramethyl-5,9-diazatridecamethylene)-bis-4-[3-ethyl-2,3-dihydro(benzo-1,3-thiazolyl)-2-methylidene]quino-linium tetraiodide (TOTO11Et). The determination of the structure was based on a complete relaxation matrix analysis of the NOESY cross-peaks followed by restrained molecular dynamics calculations. Forty final structures were generated for the TOTO11Et complex from A-form and B-form dsDNA starting structures. The root-mean-square (rms) deviation of the coordinates for the 40 structures of the complex was 0.52 Angstrom. A conformational analysis of the deoxyribose rings based on coupling constants obtained from selective DQF-COSY spectra revealed that all ring conformations were almost pure S-type. The structure of the TOTO11Et complex was compared with the structure of a similar DNA complex with a dye containing a shorter linker (TOTOEt). Substantial differences were observed between the two structures because of the difference in the length of the linker. Most prominent was a large difference in the degree of unwinding of the dsDNA part in the two complexes. Unwinding of 73 degrees and 22 degrees relative to the free dsDNA was observed for the complexes with TOTOEt and TOTO11Et, respectively. The AMBER94 force field together with the GB/SA solvation model was used for energy calculations on both of the two complexes. In the calculations, the complex formation was divided into two steps: (i) unwinding of the free oligonucleotide and (ii) association of the bis-intercalators to the unwound oligonucleotide. The complex formation was in favor of TOTO11Et, mainly because the dsDNA is distorted less in the complex with TOTO11Et than in the complex with TOTOEt.