PARALLEL PURINE-PYRIMIDINE-PURINE TRIPLEX - EXPERIMENTAL-EVIDENCE FOREXISTENCE

Oligonucleotides 5'-d(CT)(5)-L-d(AG)(5)-L-d(GA)(5)-3' and 5'-d(GA)(5)L -d(TC)(5)L-d(GA)(5)-3' [L = pO(CH2CH2O)(3)p] were studied by thermal d enaturation, chemical modification and binding of fluorescent dyes. Bo th oligonucleotides are shown to fold back on itself twice forming at pH 7 a sufficiently stable tripler ether with antiparallel-oriented ol igopurine strands (the first compound) or parallel-oriented oligopurin e strands (the second compounds). The parallel tripler is significantl y less stable than the antiparallel one. On the basis of conformationa l modeling, possible types of base tripling in the triplets are propos ed. Thus our data provide the first convincingly evidence for the exis tence of a purine-pyrimidine-purine tripler with parallel orientation of identical strands.