Prebiotic Cytosine Synthesis from Urea in Interstellar Space: A Computational Mechanistic Study

Abstract

The potential energy surfaces for the formation of cytosine (Cyt) and a protonated cytosine (CytH(+)) from reactions of urea with cyanoacetylene (CA), cyanoacetaldehyde (CAA), or their protonated ions, CAAH(+)and CAH(+)with or without H2O, have been determined from quantum chemical calculation using the CBS-QB3 method. The overall activation energies of the formation of Cyt or CytH(+)from urea + CA, urea + CAA and, urea + CAH(+)are 127, 211, and 31 kJ mol(-1), respectively, which are too high for the thermal reactions to occur in the interstellar medium (ISM). The barrierless reaction pathways have been proposed for the formation of CytH(+)from urea + CAH(+)+ H2O and urea + CAAH(+). A kinetic analysis shows that the synthesis of Cyt through the formation of CytH(+)from urea, CAH(+), and H2O would be possible in the ISM.