Print out these three pictures on card stock, cut them out and glue them together. The final model will look like a box with one open side, with both aromatic structures (the adenine and the niacin) lying in parallel plans because their pi-electron rings intermesh.
NIACIN nucleoside: in this the active site is indicated by a "D", which stands for the stable heavy isotope of hydrogen, deuterium. When NAD+ is converted to NADH, this deuterium is not randomized with the incoming H: because the incoming H: pushes the D toward the interior of the molecule. When the NADH is oxidized such as when it enters the electron transport system, it is the H: that leaves and never the D: Thus the niacin ring can only be "attacked" from one side, because the other side is toward the inside of the "box".
