Quantum Chemical Study on Stability and Reactivity of 1-Aminocyclopropane-1-carboxylic Acid Amine Radical Cation

Abstract

Abstract The stability and reactivity of the 1-aminocyclopropane-1-carboxylic acid (ACC) amine radical cation, which is a key intermediate of ethylene biosynthesis in plants, were examined by quantum chemical calculations. The local interactions to stabilize radical species were treated with a hydrated cluster. Potential energy curves were obtained for two types of reactions from ACC amine radical cation: (1) direct ring opening of ACC amine radical cation, followed by proton abstraction from the amino group, and (2) proton abstraction from the amino group, followed by ring opening of ACC aminyl radical. A remarkable difference was found in the ring-opening processes of reactions (1) and (2). Rate constants (k) of 6.10×1012 and 8.89×109 s−1 at 298.15 K were estimated for the ring openings of ACC amine radical cation and ACC aminyl radical, respectively, at the B3LYP/6-31+G(d,p) level, and were in agreement with experimental results. The ring opening of ACC amine radical cation was 103-fold faster than that of ACC aminyl radical with almost no barrier height. It was thus quantitatively demonstrated that reaction (1) was more favorable than reaction (2) for ethylene synthesis.