Hydrogen-Bond Analysis in Protein Crystal Structures

In helices all main-chain atomic groups and hydrogen bonds are oriented in one direction along the helix axis. Therefore, a significant dipole moment is created, with positive pole in the direction of the amino terminus. The potential at about 5 A distance from the N-terminus of an a-helix of 10 A length (about 2 turns) was estimated to be about 0.5 eV [600, 601]. 

Other kinds of turns which are much more rarely encountered are the a-turn with l -5 hydrogen bond which represents one single turn of an a-helix, or the three-residue y-turn with a l -3 hydrogen bond. This latter turn produces a kink rather than a hairpin-like reversal in a polypeptide chain [596, 603], see Thble 19.5. 

2 Hydrogen-Bond Analysis in Protein Crystal Structures 

A deficiency in all the protein crystal structures investigated thus far at high resolution 1.8 A is that the data/parameter ratio is insufficient to permit least-squares 

1 In fact, six different types of turns were characterized, but this is of no importance in our discussion (see [588] and Thble 19.5). 

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