68 



JERRY DONOHUE 



some correlation between N • • • O distance and the deviation of the angle 

 N — H • • • O from 180°, but the available data appears to indicate that a nice 

 smooth function which relates these two parameters probably does not exist. 

 In the case of /3-glycylglycine this fact is obvious, since the shortest hydrogen 

 bond, with N • • • O distance of 2.68 A, shows a deviation of about 10°, but in 

 the case of a second bond which is very nearly linear, a longer N • • • distance 

 of 2.80 A is observed. 



The situation in a still more complex structure, histidine hydrochloride mono- 

 hydrate (Donohue, Lavine, and Rollett, 1956) is shown in Fig. 7. In this crystal 



Fig. 4. The environment of the 

 graphic projection. 



-NH3+ group in DL-alanine shown in stereo- 



FiG. 5. The environment of the 

 graphic projection. 



-NH3+ group in L-threonine shown in stereo 



