TRIPLEX FORMATION AS FUNCTIONS OF VARIATION OF SEQUENCE AND CHAIN-LENGTH OF DEOXYOLIGONUCLEOTIDES AT VARIED CONCENTRATIONS OF NACL AND MGCL2

The stabilities of triplexes with two sequences, A(n) and (AG)(n), were studied by 5'-C(2)A(16)C(2)/5'-T-n (n = 12, 14, and 16) and 5'-(AG)(8)/5'-(CT)(n) (n = 4, 5, 6, and 7) systems by variation of the chain length (of oligopyrimidine sequences), the composition of base triad (T.A.T versus C+.G.C), pH (acidic and neutral), and the concentration of NaCl and MgCl2. No stable C+.G.C base triad was formed at neutral pH. However, C+.G.C is more stable than T.A.T in acidic conditions. The tripler formed from the 5'-C(2)A(16)C(2) and T-12 mixture with 12 T.A.T units has a melting temperature Tm 11 degrees C. The tripler formed by 5'-(AG),/5'-(CT)(4) with four T.A.T and four C+.G.C base triads shows Tm 41 degrees C. Tm of 5'-C(2)A(16)C(2).[T-16](2) tripler increased proportionally to the increased NaCl and MgCl2 concentrations. Tm of 5'-(AG)(8).[5'-(CT)(7)](2) became constant when NaCl and MgCl2 attained 0.5 M and 5 mM, respectively. The properties of triplexes with A(n) and (AG)(n) sequences were discussed.