HAIRPIN-DIMER EQUILIBRIUM OF A PARALLEL-STRANDED DNA HAIRPIN - FORMATION OF A 4-STRANDED COMPLEX

The 24mer deoxyoligonucleotide 3'-d(T)(10)-5'-5'-d(C)(4)-d(A)(10)-3' ( psC4) with an uncommon 5'-p-5' phosphodiester linkage was designed to enable the formation of a hairpin structure with unusual parallel-stra nded stem, As reference hairpin structure with an antiparallel-strande d stem, the 24mer 5'-d(T)(10)-d(C)(4)-d(A)(10)-3' (apsC4) was chosen, The behaviour of these oligonucleotides at different temperatures, DNA and salt concentrations was characterised by a combination of UV melt ing, CD, CD melting, infrared and Raman spectroscopy, infrared melting and analytical ultracentrifugation. The parallel-stranded hairpin str ucture was found to be formed by psC4 only under conditions of low DNA concentration and low salt concentration, Increase of the NaCl concen tration beyond the physiological level or high DNA concentration suppo rts the formation of intermolecular multi-stranded structures, The exp erimental data are in agreement with a four-stranded complex formed by two molecules of psC4. The base pairing model of this asymmetric four -stranded complex is based on the pyrimidine motif of a triple helix w ith two bifurcated hydrogen bonds at the O4 of the thymine each direct ed towards one of the amino protons of both adenines, In contrast, the reference oligonucleotide apsC4 forms only an antiparallel-stranded h airpin under all experimental conditions.