A Simple Complex on the Verge of Breakdown: Isolation of the Elusive Cyanoformate Ion

Abstract

Why does cyanide not react destructively with the proximal iron center at the active site of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase, an enzyme central to the biosynthesis of ethylene in plants? It has long been postulated that the cyanoformate anion, [NCCO2]–, forms and then decomposes to carbon dioxide and cyanide during this process. We have now isolated and crystallographically characterized this elusive anion as its tetraphenylphosphonium salt. Theoretical calculations show that cyanoformate has a very weak C–C bond and that it is thermodynamically stable only in low dielectric media. Solution stability studies have substantiated the latter result. We propose that cyanoformate shuttles the potentially toxic cyanide away from the low dielectric active site of ACC oxidase before breaking down in the higher dielectric medium of the cell.

Pathogen-Responsive MPK3 and MPK6 Reprogram the Biosynthesis of Indole Glucosinolates and Their Derivatives in Arabidopsis Immunity

“…We found that the total levels of cyanide/ thiocyanate measured using the isonicotinic-barbituric acid method were comparable with and without formaldehyde treatment, indicating that the chemical we detected using the isonicotinic-barbituric acid method in the liquid media samples is thiocyanate ion, not cyanide. This is consistent with the previous finding that cyanide from the biosynthesis of ethylene can be rapidly removed (Miller and Conn, 1980;Yip and Yang, 1988;Murphy et al, 2014). It was reported that cyanide plays a role in rice resistance against blast fungus (Seo et al, 2011).…”

1-aminocyclopropane-1-carboxylic acid (ACC) in plants: more than just the precursor of ethylene!

“…The key aspect in isolating ACO was the addition of ascorbic acid (vitamin C) to the extraction media, as was first reported by Ververidis and John (1991) who isolated ACO from melon tissue and quantified in vitro ACO activity. Although the exact role of ascorbic acid for protein stability/activity remained uncertain for a long time (Rocklin et al, 2004), it was recently clarified that ascorbic acid participates in the ring opening of ACC, by providing a single-electron to the active site (Murphy et al, 2014). This catalytic reaction releases ethylene and a cyanoformate ion [NCCO 2 ] - , which is subsequently decomposed into CO 2 and CN - (Murphy et al, 2014).…”

Structurally simple complexes of CO2

“…45 Both of these complexes were prepared through direct exposure of the fluoride or cyanide precursor salts to CO2. 154 cyanoformate (middle) 162 and nitryl cyanide (right) 163 ions. Crystallographically determined bond lengths (Å) for fluorocarbonate and cyanoformate are included.…”

“…154 Murphy et al observed the same tendency toward hydrolysis, isolating crystals of tetraphenylphosphonium bicarbonate hydrate. 162 The cyanoformate ion has been shown, by both experiment and computation, to be on the verge of dissociation. In situ infrared analysis revealed that cyanoformate fragments into its constituents in polar media (e.g., acetonitrile), while in non-polar media (e.g., toluene) it was stable for a longer time.…”