Nicotinamide adenine dinucleotide consists of two nucleosides joined by a pair of bridging phosphate groups. The nucleosides each contain a ribose ring, one with adenine attached to the first carbon atom (the 1' position) (adenosine diphosphate ribose) and the other with nicotinamide at this position. The nicotinamide moiety can be attached in two orientations to this anomeric carbon atom. Because of these two possible structures, the compound exists as two diastereomers. It is the β-nicotinamide diastereomer of NAD that is found in organisms. These nucleotides are joined together by a bridge of two phosphate groups through the 5' carbons.