Theoretical Study On Adenine - Water Molecule Clusters

We say vitamin B4 is just adenine in our daily life, DNA and RNA is an essential part of the human genome composition, and adenine is essential component for DNA and RNA. It is necessary to study of adenine for exploring the secret.Cluster is a transition state, which the size is between atoms and molecules. It generally exists in the physical and chemical reaction process in people’s daily life, which is closely related to the change of nature.Hydrogen bond is a kind of weak interaction, which has a variety of forms, exists between adenine molecules and water molecules. Water is the source of all things, so the combining of adenine and water molecules will offer great help for understanding the hydrogen bonding.There are a lot of study for adenine, we use density functional theory methods(DFT) in quantum chemistry for studying infrared vibration spectra and structure of Adenine and water molecule clusters. This article is divided into three parts. The first part describes the nature and purpose of adenine molecules and has carried on the introduction to clusters, recounted the function of hydrogen bond and the principle of infrared spectrum. The second part presents the calculation method of quantum chemistry, the Gaussian software, density functional theory, the AIM and PED software. In the third part, the C5H5N5?(H2O)m(m=1~3) molecule was optimized and vibrational frequency was calculated using density functional theory(DFT) at the B3LYP/6-311++G(d, f) level, getting six kinds of stable structure of clusters, energy difference is very small between isomers of the clusters. With the increasing of the number of water molecules, the stability structure of the energy is reduced of adenine and water molecule clusters. AIM program was used to calculated critical points’ topological parameters of there most stable structures. The results show that strong red-shifted O—H…N hydrogen bond is attributed to the decrease of the O—H electron density and stretching vibrational frequency, meanwhile the red-shifted N—H…O hydrogen bond is attributed to the decrease of the N—H electron density and strenth and stretching vibrational frequency. We may also be make covalent, which is stronger then hydrogen bond. The IR spectra of C5H5N5?(H2O)m(m=0~3) clusters were assigned by veda4 software and part of the vibration frequency were compared. The results show that the blue-shifted hydrogen bond O—H…N affects the stretching vibration frequency is better than the stretching vibration frequency of the corresponding adenine itself; Meanwhile the red-shifted N—H…O hydrogen bond Influences the stretching vibration frequency is smaller than of the stretching vibration frequency of corresponding adenine itself. The clusters of C5H5N5(H2O) m(m=1~3) in water solvent has carried on the infrared spectrum analysis of the vibration frequency. The structures of adenine molecules were optimized and their vibrational frequencies were calculated using density functional theory(DFT) at the B3LYP/6-311++G(d, f) level and the PBE1PBE/aug-cc-pvtz basis set level by Gaussian09 W program. With theoretical values and experimental values comparative analysis of the previous, vibrational frequencies are calculated with different basis sets are compared with the experimental values are the most likely value is formed by different vibration mode of groups.