The terminal phosphate in d(pGpG) reduces self-association

An investigation of the self-association behavior of 2'-deoxy[5'-phosphate- guanylyl-(3'5'-guanosine] (d(pGpG)) in the presence of Na+ and K+ ions has been carried out by H-1 and P-31 NMR and FTIR spectroscopy. A comparison ha s been made of the self-association behavior of d(pGpG) with that of the re lated dinucleotide d(GpG), which has been shown to form extended structures based on stacked G-tetrads. Chemically, d(pGpG) monomer differs from d(GpG ) only by the addition of a phosphate at the 5'-OH of the sugar residue. It was found that the addition of the second phosphate interferes with self-a ssociation. A suitable counterion is all that is required by d(GpG) to indu ce the formation of large super structures, but for d(pGpG) a large excess of salt is needed to produce the same effect. However, once self-associatio n occurs, d(pGpG) forms similar structures to d(GpG) and has nearly the sam e properties. For both compounds, the K+ ion induces a more stable structur e than the Na+ ion. The P-31 NMR chemical shift ranges of d(pGpG) were cons istent with the reported data for a phosphodiester and terminal phosphate. The small change in the chemical shift of the terminal phosphate with incre asing temperature suggests that no major change in the terminal phosphate c onformation occurred upon self-association. It was concluded that the termi nal phosphate did not result in steric hindrance to self-association, but t hat interference to self-association was due to electrostatic repulsion eff ects.