CO-OPERATIVE NON-ENZYMIC BASE RECOGNITION .3. KINETICS OF HELIX-COIL TRANSITION OF OLIGORIBOURIDYLIC OLIGORIBOADENYLIC ACID SYSTEM AND OF OLIGORIBOADENYLIC ACID ALOE AT ACIDIC PH
CO-OPERATIVE NON-ENZYMIC BASE RECOGNITION .3. KINETICS OF HELIX-COIL TRANSITION OF OLIGORIBOURIDYLIC OLIGORIBOADENYLIC ACID SYSTEM AND OF OLIGORIBOADENYLIC ACID ALOE AT ACIDIC PH
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DOI：
10.1016/0022-2836(71)90433-5
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发表时间：
1971-01-01
影响因子：
5.6
通讯作者：
EIGEN, M
中科院分区：
文献类型：
--
作者：
PORSCHKE, D;EIGEN, M
本文研究了中性pH下低聚腺苷·低聚尿苷酸和酸性pH下低聚腺苷酸从双螺旋结构向无规螺旋结构转变的动力学过程。已经观察到两种不同类型的螺旋-线圈转变：(1)以单一弛豫时间为特征的双螺旋-线圈转变（除了长链的情况），(2)具有弛豫过程慢约100倍的三螺旋-线圈转变。在这些过程之后，在不同的A - U比下，紫外光谱发生了特征变化。发现了在不受三股螺旋干扰的情况下可以观察到双螺旋-线圈转变的条件。测定了链长8,9,10,11,14和18的螺旋-线圈转变动力学。螺旋形成的速率常数约为106M−1sec−1，表观活化焓为负，约为−9 kcal./mol。在一定温度下，解离速率随链长的增加而减小。当链长一定时，反应速率随温度升高而升高。链越长，对应的激活焓越大（N= 8，AED= 30 kcal./mol；N= 18，AED= 93 kcal./mol）。在较大的链长时，观察到额外的弛豫过程。这个过程是由于螺旋两端的解拉链平衡，对于温度跳变法的分析来说太快了。用四核苷酸和十八核苷酸进行的一些停止流动测量也表明，螺旋的形成是一个二级反应，重组的速率与负的激活焓有关，约为- 9千卡/摩尔。低聚腺苷酸在酸性pH下的螺旋线圈转变可以用单一的弛豫过程来表征。螺旋的形成是二级的，速率常数为106M−1sec−1，几乎与链长无关。复合速率随温度升高而降低，对应的活化能约为- 6 kcal /mol。用低聚腺苷酸体系对不同链长进行了三度过渡的测量。在一定的过渡度下，所有链长的解离速率都是相同的。温度升高导致解离速率加快；激活焓随寡核苷酸长度的增加而增加。对这些结果作了详细的分析，并说明了基本步骤。结果表明，螺旋核的形成是复合速率的决定步骤。从活化焓值可以看出，三个A·U碱基对形成一个稳定的原子核。根据这一结果，至少需要两个不同的成核参数：第一个碱基对一个，第二个碱基对另一个。碱基对在原子核附近形成的基本步骤的速率常数约为107sec−1。由三个碱基对组成的成核长度的论证对于三元组遗传密码的进化是有趣的。密码子-反密码子相互作用三联体提供了准确识别（稳定配对）和高动态灵活性的组合。
The kinetics of the transition from a double helix to a random coil have been investigated for oligoadenylic · oligouridylic acid at neutral pH and for oligoadenylic acid at acidic pH.The oligoadenylic—oligouridylic acid system has been studied by the temperature-jump and the stopped-flow method. Two different types of helix—coil transitions have been observed: (1) the double helix—coil transition, characterized by a single relaxation time (except in the case of long chains), (2) the triple helix—coil transition with a relaxation process which is slower by a factor of about 100. These processes were followed by characteristic changes in the u.v. spectrum at different A to U ratios. Conditions were found where the double helix—coil transition could be observed without interference from triple-stranded helices. The kinetics of the helix—coil transition have been measured for the chain lengths 8, 9, 10, 11, 14 and 18. The rate constants of helix formation are about 106M−1sec−1and are associated with negative apparent activation enthalpies of about −9 kcal./mole. The rates of dissociation at a given temperature become smaller with increasing chain length. At constant chain length the rates increase with temperature. The corresponding activation enthalpies are larger for longer chains (N= 8,AED= 30 kcal./mole;N= 18,AED= 93 kcal./mole).At greater chain lengths an additional relaxation process is observed. This process is due to the unzippering equilibrium at the helix ends and is too fast for an analysis by the temperature-jump method.Some stopped-flow measurements with the tetradeca- and the octadeca-nucleotides also demonstrate that the helix formation proceeds as a second-order reaction and that the rate of recombination is associated with a negative activation enthalpy of about −9 kcal./mole.The helix—coil transition of an oligoadenylic acid at acidic pH can be characterized by a single relaxation process. The formation of the helix is second order with a rate constant of 106M−1sec−1, the value being almost independent of the chain length. The rate of recombination decreases as the temperature increases corresponding to an activation energy of about −6 kcal./mole. The measurements with the oligoadenylic acid system for different chain length have been performed at three degrees of transition. The rates of dissociation at a given degree of transition are the same for all chain lengths. Increasing temperatures lead to increasing rates of dissociation; the activation enthalpies increase with the length of the oligonucleotide.A detailed analysis of these results is given with an elucidation of the elementary steps. It is concluded that the formation of a helical nucleus is the rate-determining step of recombination. From the values of the activation enthalpies it can be shown that three A · U base pairs form a stable nucleus. According to this result, at least two different nucleation parameters are needed: one for the first base pair and another one for the second pair. The elementary step of base pair formation adjacent to a nucleus is shown to proceed with a rate constant of about 107sec−1. The demonstration of a nucleation length consisting of three base pairs is interesting with respect to the evolution of the triplet genetic code. A codon-anticodon interactionviatriplets provides a combination of accurate recognition (stable pairing) with high dynamic flexibility.