Xodo L E, Manzini G, Quadrifoglio F, van der Marel G A, van Boom J H
Department of Biochemistry, Biophysics and Macromolecular Chemistry, University of Trieste, Italy.
Biochemistry. 1988 Aug 23;27(17):6327-31. doi: 10.1021/bi00417a019.
The capacity to assume a left-handed conformation and the thermodynamics of loop formation in concentrated aqueous NaClO4 have been investigated for the following palindromic sequences: d-(CGCGCGAAAAACGCGCG) (A5), d(CGCGCGTTTTTCGCGCG) (T5), d(CGCGCGTACGCGCG) (TA), and d(CGCGCGATCGCGCG) (AT). The results show that (a) each oligomer assumes a Z conformation upon exposure to increasing NaClO4 concentrations; the salt concentration at the transition midpoint is 1.8 M for both A5 and T5 and 3 and 3.5 M for TA and AT, respectively; (b) in high salt the four oligomers exist, over a wide range of nucleotide concentrations (up to 10(-3) M) and of temperature (greater than 0 degrees C), as unimolecular hairpin structures; (c) hairpins TA and AT exhibit, in buffer A, a lower thermal stability with respect to A5 and T5 (delta T about 16 degrees C), contrary to what is observed at low ionic strength; (d) on hairpin formation, the enthalpic term is about -52 kcal/mol for the two 17-mers and -38 kcal/mol for the two 14-mers, while the change in entropy is found to be around -150 eu for A5 and T5 and -115 eu for TA and AT. This thermodynamic picture suggests that a two-residue loop for TA and AT, found at low ionic strength [see preceding paper (Xodo, L.E., Manzini, G., Quadrifoglio, F., van der Marel, G.A., & van Boom, J.H. (1988) Biochemistry (preceding paper in this issue)], is substituted by a longer one including two additional residues from a missing dC.dG base pairing at the top of the stem.
对于以下回文序列，研究了其在浓NaClO₄水溶液中呈现左手构象的能力以及环形成的热力学性质：d-(CGCGCGAAAAACGCGCG) (A5)、d(CGCGCGTTTTTCGCGCG) (T5)、d(CGCGCGTACGCGCG) (TA) 和 d(CGCGCGATCGCGCG) (AT)。结果表明：(a) 随着NaClO₄浓度增加，每种寡聚物都呈现Z构象；A5和T5在转变中点的盐浓度均为1.8 M，TA和AT分别为3 M和3.5 M；(b) 在高盐条件下，这四种寡聚物在广泛的核苷酸浓度范围（高达10⁻³ M）和温度范围（大于0℃）内以单分子发夹结构存在；(c) 与在低离子强度下观察到的情况相反，发夹TA和AT在缓冲液A中相对于A5和T5表现出较低的热稳定性（ΔT约为16℃）；(d) 在形成发夹时，两个17聚体的焓项约为 -52 kcal/mol，两个14聚体的焓项约为 -38 kcal/mol，而A5和T5的熵变约为 -150 eu，TA和AT的熵变约为 -115 eu。这种热力学情况表明，在低离子强度下发现的TA和AT的两残基环 [见前文(Xodo, L.E., Manzini, G., Quadrifoglio, F., van der Marel, G.A., & van Boom, J.H. (1988) Biochemistry (本期前文)] 被一个更长的环取代，该环在茎的顶部包含来自缺失的dC.dG碱基对的另外两个残基。
