HYDROGEN BONDING 67 



All three hydrogen atoms of the — NH3+ group can be brought, by rotation of 

 this group about the C — N bond, into positions such that the angles between 

 N — H and N • • • CI are agreeably small. There is here, as in the case of hydra- 

 zine dihydrochloride, a fourth close Cl~ neighbor for which hydrogen bond for- 

 mation is not possible because of the unacceptably large value for the angle 

 C — N • • • CI. This "head-on" sort of contact occurs not infrequently, as we 

 shall see below. 



In hydroxylammonium chloride (Jerslev, 1948), HO — NH3+-C1~, the situa- 

 tion, shown in Fig. 3, is quite similar in that the — NH3+ group forms three 

 hydrogen bonds to neighboring chloride ions, and that there is a fourth chloride 

 ion contact head-on. It is noteworthy that this last chloride ion is closer to the 

 nitrogen atom than two of the other three, so the distance criterion breaks 

 down here also. 



This treatment works well on the amino acids which have been investigated. 

 In Fig. 4, the hydrogen bonding in DL-alanine (Levy and Corey, 1941 ; Dono- 

 hue, 1950) is shown, and in Fig. 5, that in L-threonine (Shoemaker, Donohue, 

 Schomaker, and Corey, 1950). Another example of the head-on contact occurs 

 in L-threonine. It is remarkable in these complicated structures, such as l- 

 threonine, where there are four hydrogen atoms available for hydrogen bond 

 formation (three on the — NH3+ group, and one on the hydroxyl group on the 

 7-carbon atom) that geometrically acceptable hydrogen bonds are formed by 

 all of them. 



A more complex structure, that of the dipeptide |3-glycylglycine (Hughes and 

 Moore, 1949), is shown in Fig. 6, where it is seen that the environment of the 

 — NH3+ group is quite satisfactory. It might be expected that there would be 



Fig. 3. The environment of the — NH3+ group in hydroxylammonium chloride 

 shown in stereographic projection. The chloride ion at 3.23 A projects onto the cir- 

 cumference of the projection, indicating that the angle O— N ■ • -CI is 90°. 



