Metrical Analysis of Hydrogen Bonds in Proteins

In protein X-ray structure determinations hydrogen atoms cannot be located, and their positions have to be calculated from the known coordinates of the C, N, O atoms. Their positions are an order of magnitude less well defined than with small molecule crystal structures. Nevertheless, general trends can be derived and yield valuable information. 

Main-chain to main-chain hydrogen bonds in a-helices are relatively uniform and well defined, with only small deviations from average values (Thble 19.4). This regularity is also reflected in a narrow spread of torsion angles 0, IF which cluster around -63°, -40°. 

2 In the peptide bond, y can have positive and negative values because the Ca atoms distinguish between each side of the peptide plane. This is not so in the small molecule survey where a number of different types of molecules were considered. Therefore, absolute values should be compared, 18(9)° in proteins and 30° to 80° in small molecule crystal structures. 

Main-chain to main-chain hydrogen bonds in /i-pleated sheets. The most striking features of these N-H -0=C hydrogen bonds are that (1) their metrical 

Why do antiparallel /1-sheets occur much more frequently than parallel ones 

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