PROBING CONFORMATIONAL ISOMERIZATIONS OF DOUBLE-STRANDED POLY(DA-DT) BY A SUBSTITUTION OF MINOR AMOUNTS OF THE THYMINE METHYLS WITH BULKY HYDROPHOBIC ISOPROPYL GROUPS

We probed conformational polymorphism of a synthetic DNA poly(dA-dT) b y introducing various small amounts of bulky spherical hydrophobic iso propyl groups into the polynucleotide primary structure. For this purp ose, three mixed copolymers of poly(dA-dT,ip(5)dU) were synthesized in which 2.6%, 8.6% or 14.2% of the polynucleotide pyrimidine bases has the isopropyl group in position 5. The isopropyls made the formation o f both A-form and X-form incomplete, and this effect increased with th e increasing isopropyl amount in the polynucleotide. However, the poly nucleotide isomerization into the A-form was hindered by the isopropyl s while the isomerization into the X-form was rather promoted. This ob servation indicates that, unlike the A-form, the X-form has the base p airs shifted towards the double helix major groove. Z-form was also pr omoted by the lowest concentration of the isopropyl groups while the m ost isopropylated poly(dA-dT) aggregated under the Z-form inducing con ditions.