title: The mode of Mg++ binding to oligonucleotides. Inner sphere complexes as markers for recognition?
abstract: Large changes of UV absorbance and CD spectra as well as specific relaxation processes with time constrants around 50 mus are found for the association of Mg++ with A(pa)n. The Mg++ binding constants strongly increase with increasing n. The relaxation data demonstrate that a large fraction of Mg++ bound to short A(pA)n forms inner sphere complexes (ISC), with H2O molecules from the inner hydration sphere of Mg++ exchanged against some site (s) of the oligomer. This fraction decreases from about 85% for A(pA)4 to less than 10% for A(pA)17. A parallel decrease is observed in the relative change of CD spectrum upon Mg++ binding from 77.5% for A(pA)4 to 13.4% for (pA)17. The rate of ISC formation decreases with increasing n suggesting some (probably sterical) hindrance effect at high n. The data support the conclusion that Mg++ favours the formation of outer sphere complexes with linear polynucleotides and require a special chain folding for ISC. Measurements of Mg++ binding to C(pC)5, U(pU)5, I(pI)5 and d[A(pA)5] did not give evidence for the formation of ISC, indicating that both specific base and sugar residues are required for ISC. These results suggest the possibility that Mg++ISC ARE USED FOR SPECific recognition of nucleic acid sequences.
mesh_terms: D000225:Adenine; D055598:Chemical Phenomena; D002621:Chemistry; D002942:Circular Dichroism; D007700:Kinetics; D008274:Magnesium; D008433:Mathematics; D009690:Nucleic Acid Conformation; D009841:Oligonucleotides; D009843:Oligoribonucleotides; D013056:Spectrophotometry, Ultraviolet
chemical_list: D009841:Oligonucleotides; D009843:Oligoribonucleotides; D008274:Magnesium; D000225:Adenine