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.

Quantum Chemical Study on Interstellar Synthesis of Cytosine by the Automated Reaction Path Search

“…The reaction proceeds between neutral species without requiring ion−molecule reactions. 18 First, NH 2 CCHO is the key molecule for cytosine formation, and a barrierless reaction, NH 2 CCH (ethynamine) + O radical, would form NH 2 CCHO (Route 1ss). After that, a radical−molecule reaction of NH 2 CCHO + CN generates NH 2 C(NC)CHO with an 8.5 kJ/mol barrier (Route 1s).…”

“…Furthermore, another quantum chemical study proposed barrierless reactions to form protonated cytosine (as well as protonated thymine and uracil) from interstellar urea with protonated cyanoacetylene or protonated cyanoacetaldehyde. 18,19 In this study, we examine possible cytosine formation pathways with radical−molecule reactions that could proceed at lower temperatures as interstellar space using an unbiased method of transition state (TS) calculations. The conventional TS calculation 20 requires reaction pathways before the evaluation based on experimental results or theory, and in many cases, given paths are possibly too simplified.…”

“…As a result, the cytosine would generate in cold conditions, and C 4 N 3 OH 5 was proposed as a precursor of cytosine. Furthermore, another quantum chemical study proposed barrierless reactions to form protonated cytosine (as well as protonated thymine and uracil) from interstellar urea with protonated cyanoacetylene or protonated cyanoacetaldehyde. , …”

Quantum Chemical Study on Interstellar Synthesis of Cytosine by the Automated Reaction Path Search

“…We propose a barrierless and exothermic formation pathway of cytosine using CN and HNCH under lower temperatures based on Route 1 (Route 1ss → 1s → 1). The reaction proceeds between neutral species without requiring ion-molecule reactions 18 . First, NH 2 CCHO is the key molecule for cyto- Table 1 The summary of whole reactions.…”

“…As a result, the cytosine would generate in cold conditions, and C 4 N 3 OH 5 was proposed as a precursor of cytosine. Furthermore, another quantum chemical study proposed barrierless reactions to form protonated cytosine (as well as protonated thymine and uracil) from interstellar urea with protonated cyanoacetylene or protonated cyanoacetaldehyde 18,19 .…”