Effects of pH and temperature on dimerization rate of glycine: Evaluation of favorable environmental conditions for chemical evolution of life
Effects of pH and temperature on dimerization rate of glycine: Evaluation of favorable environmental conditions for chemical evolution of life
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DOI：
10.1016/j.gca.2010.08.032
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发表时间：
2010-12-01
影响因子：
5
通讯作者：
Yokoyama, Tadashi
中科院分区：
文献类型：
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作者：
Sakata, Kasumi;Kitadai, Norio;Yokoyama, Tadashi
为了评估有利于生命化学进化的环境条件，我们研究了pH和温度对甘氨酸（Gly：NH（2）-CH（2）-COOH）二聚化速率的影响，甘氨酸是最简单的氨基酸之一。Gly二聚形成甘氨酰甘氨酸（GlyGly），GlyGly进一步反应形成二酮哌嗪（DKP）。将pH范围为3.1至10.9的Gly溶液在140 ℃下加热1-14天，并评价Gly、GlyGly和DKP的浓度变化。在pH 9.8下，在120、140、160和180 ℃下进行实验。Gly的二聚化速率在pH 3-7下几乎恒定，并且随着pH从7增加到9.8而增加，然后随着pH的进一步增加而降低。为了阐明这种pH依赖性的原因，我们评估了Gly的三种解离状态（阳离子状态：Gly（+），两性离子状态：Gly（+/-）和阴离子状态：Gly（-））的作用。当pH >6时，主要形式为Gly（+/-）和Gly（-），随着pH的升高，Gly（+/-）的摩尔分数降低，Gly（-）的摩尔分数升高。Gly（+/-）与Gly（-）之间的反应速度最快，其反应速率常数分别是Gly（-）与Gly（-）之间的10倍和Gly（+/-）与Gly（+/-）之间的98倍。在pH 9.8时，Gly（+/-）和Gly（-）的摩尔分数基本相等，二聚反应速率最大，二聚反应速率随温度升高而增大，活化能为88 kJ mol（-1）。基于这些结果和先前关于氨基酸在水热条件下的稳定性的报道，我们确定Gly在约150 ℃的碱性pH（类似于9.8）下最有效地二聚化。(C)2010爱思唯尔有限公司版权所有。
To evaluate favorable environmental conditions for the chemical evolution of life, we studied the effects of pH and temperature on the dimerization rate of glycine (Gly: NH(2)-CH(2)-COOH), one of the simplest amino acids. Gly dimerizes to form glycylglycine (GlyGly), and GlyGly further reacts to form diketopiperazine (DKP). Gly solutions with pH ranging from 3.1 to 10.9 were heated for 1-14 days at 140 degrees C, and changes in concentrations of Gly, GlyGly, and DKP were evaluated. At pH 9.8, the experiments were conducted at 120, 140, 160, and 180 degrees C. The dimerization rate of Gly was nearly constant at pH 3-7 and increased with increasing pH from 7 to 9.8 and then decreased with further increases in pH. To elucidate the reason for this pH dependency, we evaluated the role of the three dissociation states of Gly (cationic state: Gly(+), zwitterionic state: Gly(+/-), and anionic state: Gly(-)). For pH >6, the dominant forms are Gly(+/-) and Gly(-), and the molar fraction of Gly(+/-) decreases and that of Gly(-) increases with increasing pH. The dimerization rate was determined for each dissociation state. The reaction between Gly(+/-) and Gly(-) was found to be the fastest; the rate constant of the reaction between Gly(+/-) and Gly(-) was 10 times the size of that between Gly(-) and Gly(-) and 98 times that between Gly(+/-) and Gly(+/-). The dimerization rate became greatest at pH 9.8 because the molar fractions of Gly(+/-) and Gly(-) are approximately equal at this pH. The dimerization rate increased with temperature, and an activation energy of 88 kJ mol(-1) was obtained. Based on these results and previous reports on the stability of amino acids under hydrothermal conditions, we determined that Gly dimerizes most efficiently under alkaline pH (similar to 9.8) at about 150 degrees C. (C) 2010 Elsevier Ltd. All rights reserved.