Aromatic compounds are crucial in biochemistry. NAD+, and the purines and pyrimidines of nucleic acids are examples of important aromatic species in biochemistry, with their aromatic nature playing a significant roll in the biochemical behavior. For example, the oxidized form of nicotinamide adenine dinucleotide (NAD+) contains an aromatic pyridinium ring, while the reduced form (NADH) does not. The aromaticity of NAD+ serves to decreases the oxidation potential of NADH, making the substance an ideal electron donor and receiver. With the purines and pyrimidines in nucleic acids, their aromaticity is also very significant because their aromaticity causes the molecules to be planar, which is very helpful for base-pairing. Pyrimidines are aromatic. The two nitrogens are of the sp2 hybridized 'pyrolle type'.