It is still debated whether the 'unusual' hydrogen bond between a donor and an aromatic acceptor is a true hydrogen bond. Often called atypical hydrogen bonds or XH-pi (X being the hydrogen donor) interactions, these are usually several kilocalories less strong than a standard hydrogen bond (~ 3 kcal/mol). The most common donors in proteins are generally nitrogen or oxygen in such instances, although theoretically, sulfur could also behave that way and people are now beginning to look at CH-pi hydrogen bonds. These interactions are so weak, it is sometimes difficult to discern between hydrogen bonding and intimate packing.

If I may wax expositionally on a topic of personal interest. Tryptophan is an interesting residue in that it can be both an atypical hydrogen bond donor and an acceptor. The benzyl ring (and potentially the pyrrole ring) of trp are both aromatic and may act as acceptors. The indole sidechain has an aromatic nitrogen which is slightly acidic and can potentially donate a hydrogen bond either to a standard acceptor or to another aromatic ring. When tryptophan donates a hydrogen bond to an aromatic ring, the fluorescence is almost absolutely quenched. This is believed to occur due to the formation of an excited state complex or exciplex. The exciplex undergoes a charge separation, resulting in a strengthening and shortening of the hydrogen bond during the lifetime of the excited state. It is this interaction which depletes the excited state, resulting in low fluorescence intensity. This has been observed in a number of proteins including immunophilins, fibronectin and potentially in homeodomains.