Interstellar Space Contains Peptide Bond - Conh - Molecules Generated Mechanism Research

The vast interstellar space is not real vacuum. With the help of radio telescope, a largenumber of clouds, which contain different kinds of molecules and grains, have been found in theinterstellar space. Formamide and acetamide have been detected in the interstellar space, both ofwhich are the only interstellar species with an NH₂group bound to a CO group, the so-calledpeptide bond that provides the linkage for polymerization of amino acids. The peptide bondplays an important role in determining protein structure, stability, and dynamics, therefore,formamide and acetamide are frequently used as models to understand the properties ofpeptide-bond containing material, such as the proton transfer in protein and the hydrolysis of thepeptide bond in biological system. So the discovery of both molecules in the interstellar spaceencourages scientists to study the relationship between life and universe and the formationmechanism of both interstellar molecules. Ices in molecular clouds are dominated by the verysimple molecules H₂O, NH₃, CO, CO₂and probably H₂, HNC, HCOOH and CH₃COOH havealso been found on the surface of the ices. Ultraviolet photolysis of realistic laboratory analogareadily produces the moderately complex organic molecules: R-CN and hexamethylenetetramine.Based on the grounds, the formation mechanisms of formamide and acetamide have been studied,using NH₃, CO, H₂O, HNC, HNCO, HCOOH and CH₃COOH as reactants.The formation mechanisms of formamide and acetamide have been investigated by densityfunctional theory. The geometries of the reactants, intermediates, products and transition stateshave been optimized at the B3LYP/6-311++G（d,p） level of calculation. And the energys havealso been calculated at the B3LYP/6-311++G（d,p） level. Harmonic vibration frequencies havebeen calculated to check whether the obtained species are transition states. To identify the correctconnection between all the critical structures located on the potential energy surface, the intrinsicreaction coordinate （IRC） calculations have been carried out. We have also studied the formationmechanisms of formamide and acetamide with the catalysis of small molecules, such as H₂O, HF,NH₃, etc. All calculations were performed with Gaussian 03 program package.To understand the formation mechanism of formamide, we have selected different pair ofreactants, such as NH₃and CO, H₂and HNCO, H₂O and HNC, HCOOH and NH₃. Quantumchemical calculations have been carried out on the different reaction system. The comparison shows that, the energy barrier of the reaction between HCOOH and NH₃is the lowest. Researchshows that, the energy barrier of the reaction of different channel decreases with the catalysis ofsmall molecules. And the activation energy of the reaction between NH₃and CO with two HFmolecules as catalyst decreases significantly, from 282.9 kJ/mol to 17.1 kJ/mol, and it is verypossible to occur under interstellar condition. The activation energy of the reaction between NH₃and HCOOH with two HF molecules as catalyst reduced to 14.0 kJ/mol. In conclusion, thereaction between HCOOH and NH₃is the main passage to product formamide. The reactivebarrier of the reaction between HCOOH and NH₃under the catalytic role of HF molecules is14.0 kJ/mol, and that makes the formation of formamide from HCOOH and NH₃easily underinterstellar conditions.As for the formation mechanism of acetamide, the pair of CH₄and HNCO molecules andthe pair of CH₃COOH and NH₃molecules have been selected as reactants respectively. Thequantum chemical calculations have been carried out on different reaction system. Thecomparison shows that, the energy barrier of the reaction between CH₃COOH and NH₃is180.1kJ/mol, which is lower. Research shows that, the energy barrier of the reaction of differentchannel decreases with the catalysis of small molecules. And the activation energy of thereaction between CH₄and HNCO with one HF molecule as catalyst decreases significantly, from301.5 kJ/mol to 221.6 kJ/mol. The activation energy of the reaction between NH₃andCH₃COOH with two HF molecules as catalysts decreases dramatically, from 180.1 kJ/mol to20.6 kJ/mol, and it is probably to occur under interstellar condition. In conclusion, the reactionbetween CH₃COOH and NH₃is the main passage to product acetamide. The reactive barrier ofthe reaction between CH₃COOH and NH₃under the catalytic role of HF molecules is reduceddramatically, and that makes the formation of acetamide from CH₃COOH and NH₃easily underinterstellar conditions.