The thermodynamic contribution of the 5-methyl group of thymine in the two- and three-stranded complexes formed by poly(dU) and poly(dT) with poly(dA)
✍ Scribed by Philip D. Ross; Frank B. Howard
- Publisher
- Wiley (John Wiley & Sons)
- Year
- 2003
- Tongue
- English
- Weight
- 168 KB
- Volume
- 68
- Category
- Article
- ISSN
- 0006-3525
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✦ Synopsis
Abstract
To assess the thermodynamic contribution of the 5‐methyl group of thymine, we have studied the two‐stranded helical complexes poly(dA) · poly(dU) and poly(dA) · poly(dT) and the three‐stranded complexes—poly(dA) · 2poly(dU), poly(dA) · poly(dT) · poly(dU) and poly(dA) · 2poly(dT)—by differential scanning calorimetry, and uv optical melting experiments. The thermodynamic quantities associated with the 3 → 2, 2 → 1, and 3 → 1 melting transitions are found to vary with salt concentration and temperature in a more complex manner than commonly believed. The transition temperatures, T~m~, are generally not linear in the logarithm of concentration or activity of NaCl. The change in enthalpy and in entropy upon melting varies with salt concentration and temperature, and a change in heat capacity accompanies each transition. The poly(dA) · 2poly(dU) triple helix is markedly different from poly(dA) · 2poly(dT) in both its CD spectrum and thermodynamic behavior, while the poly(dA) · poly(dT) · poly(dU) triple helix resembles poly(dA) · 2poly(dT) in these properties. In comparing poly(dA) · 2poly(dT) with either the poly(dA) · poly(dT) · poly(dU) or the poly(dA) · 2poly(dU) triplexes, the substitution of thymine for uracil in the third strand results in an enhancement of stability against the 3 → 2 dissociation of ΔΔ__G__° = −135 ± 85 cal (mol A)^−1^ at 37°C. This represents a doubling of the absolute stability toward dissociation compared to the triplexes with poly(dU) as the third strand. The poly (dA) · poly (dT) duplex is more stable than poly(dA) · poly(dU) by ΔΔ__G__° = −350 ± 60 cal (mol base pair)^−1^ at 37°C. Poly(dA) · poly(dT) has 50% greater stability than poly(dA) · poly(dU) as a result of the dT for dU substitution in the duplex. © 2003 Wiley Periodicals, Inc. Biopolymers 68: 210– 222, 2003