1] FACTORS IN PROTEIN STRUCTURE 21 



of the amide group, as in a peptide, we must take into consideration the 

 contribution of structures in which there is an interaction between the ionic 

 character of the sigma bond and the unshared pair of the oxygen atom lying 

 in the plane of the group. This causes the nature of the carbon-oxygen bond 

 to assume to some extent the character of the pi bonds in carbon dioxide. 

 In carbon dioxide we may, as a first approximation, describe each carbon- 

 oxygen bond as a double bond to which the pz and the py electrons contribute 

 equally (there is also, of course, some contribution of the triply bonded and 

 singly bonded structures, as well). The contribution for the bond in which 

 the double bond has 50% ttz character and 50% Try character, normahzed 

 to the same basis as for Equation 1, is equal to 



pQ7r2+^7rJ=4+2V3'cos d (2) 



It is found by differentiating this expression that the maximum is at 0=0°; 

 that is, along the carbon-oxygen axis. 



In the amide group the ionic structure for the sigma bond between the 

 carbonyl carbon and the nitrogen atom would effectively liberate a p orbital 

 for carbon lying in the plane of the group, and permit a double bond to be 

 formed with use of the py orbital of the oxygen atom and the pair of elec- 

 trons written as an unshared pair in the usual structural formulas. The 

 electron distribution corresponding to a contribution by n% of this 

 structure, with a contribution of (100-«)% of the structure involving the 

 double bond orbitals perpendicular to the plane of the group, is found on 

 differentiating the corresponding expression for p to have the maximum in 

 electron density along the carbon-oxygen axis when the double bond has 

 21% or more -ny character and 79% or less ttz character. It seems 

 not unreasonable to expect that there is approximately this amount of Try 

 character in the double bond in the amide group, and accordingly that the 

 electron distribution for the oxygen atom in the amide group is such as to 

 favor a linear arrangement of the hydrogen bond at this atom, whereas in 

 other compounds containing carbon-oxygen double bonds the electron dis- 

 tribution has its maximum in the plane of the ring, with the angle approxi- 

 mately equal to the tetrahedral angle. 



From these considerations we reach the conclusion that an additional 

 structural principle can be formulated for folded polypeptide chains ; namely, 

 that the stable configurations will tend to be those in which the N — H--0 — C 

 hydrogen bond is not only linear with respect to the three atoms N — H--0, 

 but also with respect to the carbon atom adjacent to the oxygen atom, with 

 the bond angle at the oxygen atom close to 180°. This structural principle 

 may be of value in the search for additional polypeptide-chain structures. 



