Robertson M P, Levy M, Miller S L
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla 92093-0317, USA.
J Mol Evol. 1996 Dec;43(6):543-50. doi: 10.1007/BF02202102.
The reaction of guanidine hydrochloride with cyanoacetaldehyde gives high yields (40-85%) of 2,4-diaminopyrimidine under the concentrated conditions of a drying lagoon model of prebiotic synthesis, in contrast to the low yields previously obtained under more dilute conditions. The prebiotic source of cyanoacetaldehyde, cyanoacetylene, is produced from electric discharges under reducing conditions. The effect of pH and concentration of guanidine hydrochloride on the rate of synthesis and yield of diaminopyrimidine were investigated, as well as the hydrolysis of diaminopyrimidine to cytosine, isocytosine, and uracil. Thiourea also reacts with cyanoacetaldehyde to give 2-thiocytosine, but the pyrimidine yields are much lower than with guanidine hydrochloride or urea. Thiocytosine hydrolyzes to thiouracil and cytosine and then to uracil. This synthesis would have been a significant prebiotic source of 2-thiopyrimidines and 5-substituted derivatives of thiouracil, many of which occur in tRNA. The applicability of these results to the drying lagoon model of prebiotic synthesis was tested by dry-down experiments where dilute solutions of cyanoacetaldehyde, guanidine hydrochloride, and 0.5 M NaCl were evaporated over varying periods of time. The yields of diaminopyrimidine varied from 1 to 7%. These results show that drying lagoons and beaches may have been major sites of prebiotic syntheses.
在模拟益生元合成的干燥泻湖模型的浓缩条件下，盐酸胍与氰基乙醛反应可高产率（40 - 85%）地生成2,4 - 二氨基嘧啶，这与之前在较稀溶液条件下获得的低产率形成对比。氰基乙醛的益生元来源氰基乙炔是在还原条件下通过放电产生的。研究了pH值和盐酸胍浓度对二氨基嘧啶合成速率和产率的影响，以及二氨基嘧啶水解生成胞嘧啶、异胞嘧啶和尿嘧啶的情况。硫脲也能与氰基乙醛反应生成2 - 硫代胞嘧啶，但嘧啶产率远低于盐酸胍或尿素。硫代胞嘧啶先水解为硫脲嘧啶和胞嘧啶，然后再水解为尿嘧啶。这种合成过程可能是2 - 硫代嘧啶和硫脲嘧啶5 - 取代衍生物的重要益生元来源，其中许多存在于转运核糖核酸（tRNA）中。通过干燥实验测试了这些结果对益生元合成干燥泻湖模型的适用性，在干燥实验中，氰基乙醛、盐酸胍和0.5 M氯化钠的稀溶液在不同时间段内蒸发。二氨基嘧啶的产率在1%至7%之间变化。这些结果表明，干燥泻湖和海滩可能是益生元合成的主要场所。
