An exploratory ab initio conformational analysis of selected fragments of nicotinamide adenine dinucleotide (NAD+). Part I: 5-deoxyribose nicotinamide N-glycoside

Ab initio molecular orbital computations were carried out on selected fragments of Nicotinamide Adenine Dinucleotide (NAD+) at the RHF/3-21G and RHF/6-31G(d) levels of theory. The focus of this work was on the nicotinamide-ribose moiety. In this model study, the ribose was replaced by 5-deoxyribose. Key points of examination included the rotation of the carboxyamide functionality, rotors between the nicotinamide and ribose rings as well as ring puckering in the ribose sugar. The structural definitions of the adenosine fragment and diphosphate chain were defined herein, facilitating future theoretical investigation. A preliminary attempt was made to refine the results at the elevated RHF/6-31G and RHF/6-31g(d) basis sets. The former showed results in agreement with the RHF/3-21G data sets, whereas the inclusion of preliminary polarization showed deviance in the χi1 dihedral of the a[g−a] conformer.