Average hydrogen bonds, the all-important intermolecular interaction, are thought to be weaker than this. Another surprise is that water can bind to benzene. So far, much of the research has focused on positively charged nitrogen containing molecules. Examples of these are protonated amines and quaternary amines, as shown in Figure 4. One reason scientists have focused on nitrogen cations is their relationship to some very important biological molecules.
Several neurotransmitters have a positively charged nitrogen portion in their chemical structure.
Serotonin, a neurotransmitter involved in processes such as migraine, depression, and anxiety, contains a methylammonium portion. Other molecules include dopamine, epinephrine, and acetylcholine, the last of which contains a quaternary nitrogen. If we're interested in biological examples of -cation attraction, we are not going to look at benzene, an extremely carcinogenic material that is not present in our cells. Certain amino acids, however, contain aromatic rings, including phenylalanine, tyrosine, tryptophan, and histidine. Several research groups have shown that charged nitrogen compounds are attracted to the aromatic portion of these amino acids.