PROTEIN STRUCTURE 



the nth into the (n + l)th, the resulting structure is a heHx, The 

 residues in a helical structure all have identical intra 

 chain environments. Once the limiting conditions for strong 

 hydrogen bonding are described for one residue they are 

 defined for all. In the /3 structure, where inter-chRin hy- 

 drogen bonds are used to provide configurational stabil- 

 ity, simple symmetry operations may relate one residue to 

 the next succeeding one along the same chain, or they may 

 relate one residue to another in a neighboring chain. Corey and 

 Pauling (26) have defined a strong hydrogen bond as one in 

 which the O....H-N distance is approximately 2.79±0.12A 

 and where the vector from the nitrogen atom to the hydrogen- 

 bonded oxygen atom lies not more than 30° from the NH 

 direction. 



They also considered the favorable orientation of groups 

 about the HN-CHR and HRC-CO bonds which should lead to 

 maximum bond stabiUty (78). Van der Waals forces between 

 nonbonded atoms also aff'ect the over-all stability of a con- 

 figuration. The importance of van der Waals forces and the 

 semiquantitative estimates of the instability resulting from de- 

 viations from the initial criteria set up by Pauling and Corey will 

 be discussed later. 



One helical structure — the a-helix — completely satisfies all 

 the chemical criteria for configurational stabiHty (79). It also 

 fits the physical requirements for an appropriate configuration. 

 The structure is shown in Figure 4. Each succeeding residue 

 Ci(CO.NH)C2, C2(CO.NH)C3, etc., lies in one of a zone of 

 vertical planes parallel to the helical axis. Figure 5, which 

 shows a similar helix structure, perhaps demonstrates this 

 relationship more clearly. The plane which contains the residue 

 Ci(CO.NH)C2 is parallel to the helical axis (vertical). It pro- 

 jects as a line on the plane normal to the helical axis. The 

 planes containing the C2 - C3, C3 - C4, C4 - C5, etc., residues, 

 are all parallel to the helical axis. All intersect the plane normal 

 to the helical axis as shown in Figure 5. These are some of the 

 planes in this zone. When the helix repeats, the plane of the 



397 



