Four-stranded complexes composed of oligonucleotides 3 '-d(GT)(5)-pO(CH2CH2O)(3)p-d(GT)(5)-3 '

We have shown earlier that oligonucleotides 3'-d(GT)(5)-pO(CH5)(6)Op-d(GT)( 5)-3' covalently connected with hydrophobic hexamethylene linkers form inte rmolecular four-stranded complexes of guanine quartets in physiological sol utions, Here we considered the effect of the oxyhexamethylene linker -pO(CH 2CH2O)(3)p- on the formation and stability of intermolecular complexes of 3 '-d(GT)(5)-pO(CH2CH2O)(3)p-d(GT)(5)-3' (parGT). Polarized fluorescence of e thidium bromide was used to calculate the parGT hydrodynamic volume corresp onding to that of a quadruplex of four d(GT)(5) strands, called (parGT)(2). The CD spectra were indicative of the presence of guanine quartets and par allel orientation of the strands in the (parGT)(2) quadruplex. The amount o f thymine residues not involved in hydrogen bonding in (parGT)(2) was calcu lated from the life time of heterogeneous fluorescence of acridine orange. Sixteen out of 20 thymines were unpaired. The thermodynamic parameters of ( parGT)(2) were determined from UV-melting profiles. The enthalpy per G quar tet was 5.6 +/- 0.4 kcal/mol. Metal ions stabilized the (parGT)(2) quadrupl ex as follows: Mg2+ > Na+ approximate to K+.