Circular dichroism spectra of DNA oligomers show that short interior stretches of C.C+ base pairs do not form in duplexes with A.T base pairs.
Circular dichroism spectra of DNA oligomers show that short interior stretches of C.C+ base pairs do not form in duplexes with A.T base pairs.
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DNA 寡聚物的圆二色光谱表明，C.C 碱基对的短内部延伸不会与 A.T 碱基对形成双链体。
DOI：
10.1021/bi00414a033
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发表时间：
1988
期刊：
影响因子：
2.9
通讯作者：
Gray,DM
中科院分区：
文献类型：
--
作者：
Edwards,EL;Ratliff,RL;Gray,DM
用圆二色谱(CD)对一系列DNA低聚体进行了研究，以确定质子化的胞嘧啶-胞嘧啶(GC+)碱基对是否可以与腺嘌呤-胸腺嘧啶(·)碱基对共存：(1)当pH降至6或5时，d(AACC)5和d(CCTT)5寡聚体在pH降至6或5时，出现以282-284 nm为中心的长波长CD带，表明在不存在·碱基对的情况下，GC+碱基对发生变化。结果表明，d(CCTT)5在pH为5，0.5M Na+，2 0℃时，可能采用以GC+碱基对和环状胸腺嘧啶为中心的结构.在相同的条件下，d(AACC)5中似乎也形成了GC+碱基对，但腺嘌呤仍在螺旋内。随着温度的降低，这些低聚物中的每一个都表现出形成CC+碱基对的协同转变，在d(CCTT)5中形成GC+碱基对的温度高于d(AACC)5中。·碱基对在d(AACC)5和d(CCTT)5等摩尔混合物中形成，通过250 nm CD带负值的增加来监测。然而，混合物的CD谱在约285 nm处并没有出现较大的增加，这表明即使在相同的条件下，d(AACC)5-d(CCTT)5双链中也没有GC+碱基对的堆积。因此，在这个双链中，·碱基对阻止了相邻的内部GC+碱基对的形成。(2)d(A10C4T10)的CD测量也是如此。(3)在pH为5的条件下，发现d(A6C6A6)和d(T6C6T6)能够形成GC+碱基对，但在1：1的混合液中只有·碱基对存在，即使在pH低于C-C+碱基对形成的Pala时也是如此。因此，在所研究的条件下，在这些齐聚物的络合物中，2、4或6个GC+碱基对的短内伸展与·碱基对不相容。1在接近中性的pH条件下，一个质子由两个胞嘧啶共享，可以在含胞嘧啶的DNA序列中形成(Inman，1964；Gray等，1980,1984,1988；Brown等，1985)。聚[d(C)]的酸性自络合物中堆积的GC+碱基对的形成导致了特征性的CD光谱变化，包括在长波长(>；280 nm)处显著增加的正CD值，其中胞嘧啶的吸收随着质子化而增加(Gray&Bollum，1974；Marck等人，1978；Gray等人，1988)。对于聚[d(C)]，GC+碱基对在0.05M Na+时形成PXA为7.4(Inman，1964)。CD实验表明，混合序列聚合物质子化时也可以形成GC+碱基对。CD和光化学研究表明，聚[d(CT)]在0.05M Na+时能形成pof为6.2的自复合体；这种自复合体由堆积的GC+碱基对和中间胸腺嘧啶环进溶液的中心核心组成(Gray等人，1980；Brown等人，1985)。在聚[d(CG)]的情况下，表示GC+碱基对的CD带在形成结构的过程中出现在pH=3(0.1M Na+)的结构中
Circular dichroism (CD) experiments were carried out on a series of DNA oligomers to determine if short internal stretches of protonated cytosine-cytosine (GC+) base pairs could coexist with adenine-thymine (·) base pairs.(1) GC+ base pairs didform in the absence of· base pairs in the individual oligomers d (AACC) 5 and d (CCTT) 5, as indicated by the appearance of a long-wavelength CD band centered at 282-284 nm, when the pH was lowered to 6 or 5 at 0.5 M Na+. A comparison of measured with calculated spectra showed that d (CCTT) 5 at pH 5, 0.5 M Na+, 20 C, likely adopted a structure with a central core of stacked GC+ base pairs and looped-out thymines. Under the same conditions, it appeared that GC+ base pairs also formed in d (AACC) 5, but with theadenines remaining intrahelical. Each of these oligomers showed a cooperative transitionfor formation of CC+ base pairs as the temperature was lowered, with GC+ base pairs forming at a higher temperature in d (CCTT) 5 than in d (AACC) 5.· base pairs formed in equimolar mixtures of d (AACC) 5 plus d (CCTT) 5 as monitored by an increase in the negative magnitude of the 250-nm CD band. However, a large increase did not appear at about 285 nm in CD spectra of the mixtures, showing that there were no stacked GC+ base pairs in the d (AACC) 5-d (CCTT) 5 duplex even though they formed under the same conditions in the individual strands. Thus, in this duplex,· base pairs prevented the formation of neighboring internal GC+ base pairs.(2) CD measurements were also made of d (A10C4T10). At pH 5 there was evidence for formation of a duplex with· base pairs, but no GC+ base pairs were detected.(3) Finally, we found that the individual oligomers d (A6C6A6) and d (T6C6T6) were able to form GC+ base pairs, but only· base pairs were present in 1: 1 mixtures of the two strands, even at pH values below the pAla for C-C+ base pair formation in the individual strands. Thus, short internal stretches of 2, 4, or 6 GC+ base pairs were incompatible with· base pairs in complexes of these oligomersunder the conditions studied. i^^ otonated cytosine-cytosine base pairs (GC+ base pairs), 1 in which one proton is shared by two cytosines, can form in cytosine-containing DNA sequences at close to neutral pH (Inman, 1964; Gray et al., 1980, 1984, 1988; Brown et al., 1985). Theformation of stacked GC+ base pairs in the acid self-complex of poly [d (C)] results in characteristic CD spectral changes, including greatly increased positive CD values at long wavelengths (> 280 nm) where the absorption of cytosine in-creases upon protonation (Gray & Bollum, 1974; Marck et al., 1978; Gray et al., 1988). For poly [d (C)], GC+ base pairs form with a pXa of 7.4 at 0.05 M Na+(Inman, 1964). CD experiments have shown that GC+ base pairs may also form upon protonation of mixed sequence polymers. CD and photochemical studies showed thatpoly [d (CT)] can form a self-complex with a pof 6.2 at 0.05 M Na+; this self-complex consists of a central core of stacked GC+ base pairs with the intervening thymines looped out into solution (Gray et al., 1980; Brown et al., 1985). In the case of poly [d (CG)], CD bands indicative of GC+ base pairs appearat pH values< 3 (0.1 M Na+) during formation of a structure in which the