HYDROGEN BONDING 



69 



the — NH3+ groups have two different sorts of hydrogen bond acceptor neigh- 

 bors, but everything appears to be quite normal. 



The very first amino acid whose structure was determined was glycine (Al- 

 brecht and Corey, 1939). In this crystal, the amino group was found to have 

 four close oxygen neighbors, but the angular relations were such that none of 

 these was of the head-on type. The orientation of the — NH3+, as postulated by 

 Albrecht and Corey, with its neighbors, is shown in Fig. 8. As can be seen, 

 when the — NH3+ group is rotated to minimize the angular deviation in the 

 case of the two bonds of length 2.76 A and 2.88 A, the third hydrogen atom 



Fig. 6. The environment of the — NH3+ in /3-glycylglycine shown in stereographic 

 projection. 



Fig. 7. The environment of the — NH3+ group in histidine hydrochloride mono- 

 hydrate shown in stereographic projection. 



