Unearthing the Mechanism of Prebiotic Nitrile Bond Reduction in Hydrogen Cyanide through a Curious Association of Two Molecular Radical Anions

Abstract

HCN is clearly associated with the prebiotic chemical evolution of life. It has been known for decades that the radiolysis of HCN solutions produces sugars, amino acids and nucleobases. Remarkably, recent experimental studies have shown that the photolytic reduction of aqueous HCN by a photoredox reagent [Cu(CN)3](2-) specifically yields sugars, which are the essential building blocks of RNA. Although a mechanistic understanding of such reductions with solvated electrons is poor, the general consensus is that they involve neutral free radicals. We show herein through the use of electronic structure studies and molecular simulations that the reduction of the nitrile bond of HCN is initiated through the formation of a molecular dipole-bound anion from the photoredox reagent. Our theoretical studies show how HCN binds to the photoexcited reagent and then extracts an electron from the reagent and is ultimately detached as a dipole-bound anion. The dipole-bound anionic form of [HCN](-) can easily convert into a solvated valence-bound form of [HCN](-). After the formation of solvated [HCN](-), an extraordinary chemical event ensues through a counter-intuitive coupling of two valence-bound anions to form a solvated molecular dianionic intermediate, [HCN]2(2-). Finally, a proton-coupled electron transfer occurs within the dianionic entity to complete the reduction. This mechanistic scenario is applicable to the reduction of other prebiotic nitrile species and avoids neutral radical-based pathways, thereby preventing the proliferation of reactive species and preserving chemical selectivity. Furthermore, we show how such similar nitrile reduction pathways operate to yield the sugar precursors.

Influence of the UV Environment on the Synthesis of Prebiotic Molecules

“…This suggests UV irradiation is required for this pathway. Banerjee et al (2014) suggest an alternate mechanism underlying the photoredox synthesis of simple sugars discovered by Ritson and Sutherland (2012). They conduct theoretical calculations to suggest that instead of photoionizing electrons from tricyanocuprate (I) to reduce HCN, the UV input excites the transition metal complex from its ground S 0 state to its excited S 1 state.…”

“…The Ritson and Sutherland (2012) and Banerjee et al (2014) photoredox pathways differ significantly in their wavelength dependence on the input UV energy. The Ritson and Sutherland (2012)…”

“…Empirically, we know λ c ︎ ≥254 nm, since the reaction proceeded under action of 254 nm radiation. Alternately, Banerjee et al (2014) suggest that the pathway is driven by UV excitement of tricyanocuprate (I) from the S0 to the S1 state. The energy difference between these states is calculated to correspond to 265 nm; consequently, 265 nm radiation should be much more efficient than 254 nm radiation at promoting this transition, and the Ritson and Sutherland (2012) experiments may underestimate the reaction rate.…”

Mass Spectrometry of Analytical Derivatives. 1. Cyanide Cations in the Spectra of N-Alkyl-N-Perfluoroacyl-α-Amino Acids and Their Methyl Esters

“…The decomposition of alkyl-perfluoroacyl-amino acids and their derivatives with the formation of nitrilium cations may be particularly useful to a better understanding of the origin of toxic cyanides in the environment 40,41 and in prebiotic chemistry . 42,43…”

Solvated-electron production using cyanocuprates is compatible with the UV-environment on a Hadean–Archaean Earth

“…2). Two different mechanisms have been proposed for this process: (1) the Ritson-Sutherland mechanism 10 postulates a photoionization as the underlying photoprocess, suggesting that any photon with energy greater than the activation energy of the process should be sufficient, and (2) Banerjee et al 2014 33 use theoretical calculations of electronic structures and a mechanism involving dipole-bound forms of HCN to predict a Gaussian efficiency centered at 265 nm. Our results do not support this mechanism, given that the relative quantum yield at 265 nm is roughly a tenth of the maximum value at 245 nm.…”