Confronting the Water Problem: The Lifetime of Aqueous Cyanide and its Role in Origins of Life

Several chemical routes are proposed to understand the synthesis of biologically significant molecules on early earth. However, the research has just began to discover the feasibility of these reactions under prebiotic conditions and the physical factors influencing their efficiency in a planetary context. Hydrogen cyanide (HCN) has emerged one of the key molecules for the synthesis of major building blocks of life – lipids, sugars and nucleotides – through a series of chemical reactions, some mediated by ultraviolet light along with other scenarios. The amount of cyanide available in a natural environment can be predicted from the rate at which it is produced in an environment plus the rate it is introduced to the environment, versus the rates it is destroyed and leaves the environment. One of the most important mechanisms of HCN loss is hydrolysis. The rate of hydrolysis is known to depend on the physical and chemical conditions of the environment.

I will present the most comprehensive study thus far of the hydrolysis of HCN. I explore hydrolysis across a range of temperatures, pH and in the presence of salts like sulphite, sulphide and phosphate (for two example measurements. We determine the degradation rate for this range of conditions and predict the rate constants for acid-catalyzed hydrolysis and base- catalyzed hydrolysis of cyanide, along with uncertainties. These uncertainties are critical for comparing our results to similar studies, and applying our results to environmental conditions. We find that the hydrolysis rates are significantly influenced by pH and temperature with significant variations observed with salts. The activation energy for acid-catalyzed and base-catalyzed hydrolysis of HCN is found to be 75.7 ± 6.7 KJ/mol and 49.7±4.0 KJ/mol, respectively. This result is consistent within the errors to the literature but are systematically faster than the literature values and favours maximum lifetime around pH-7 rather than pH-4. We apply our results, in comparison to a variety of cyanide sources found in the literature to provide new predictions of cyanide availability in natural waters. Our results are critical for constraining the prebiotic environment where the prebiotic synthesis of amino acids, nucleotides and phospholipids could have occurred.