Introduction 



Remarkable advances in the knowledge of protein structure have been 

 made during the last decade, and although many aspects of protein struc- 

 ture still raise some difficult problems, essential features are now clarified. 

 Protein molecules are made of one single polypeptide, or of a few poly- 

 peptidic chains associated together in a specific manner. The backbone of 

 polypeptides is a quite regular structure in which one nitrogen and two 

 carbon atoms alternate ; one of these carbons carries an oxygen, the other 

 one, which is asymmetrical, carries a hydrogen and a radical R. 



-NH-CH-CO-NH-CH-CO-NH-CH-CO-NH-CH-CO- 



I I I I 



Ri R2 R3 R4 



Fig. 1 



There are twenty common varieties of R radicals, and the sequence of 

 these confers their individuality to the polypeptides, which would other- 

 wise be all alike. Hydrolysis splits the peptide bonds — CO— NH— and the 

 chains thus break down into a mixture of some twenty a-amino acids. All 

 belong to the l series; this means that the relative positions of — NH2,— 

 COOH, — R and — H around the asymmetric carbon is the same whatever 

 the nature of the R radical. A very important consequence of the identical 

 steric configuration of all the amino acids is that certain regular types of 

 folding of the chains tend to form, whatever the nature of the R radicals. 

 Among the possible types of folding a few are privileged ; especially a cer- 

 tain type of helical folding, the a helix (Pauling et ah, 1951), which is found 

 in many proteins. In this structure, each —CO— of one spire is hydrogen 

 bonded with an — NH— in the next one ; this confers a great stability and 

 some rigidity to the framework. 



Protein molecules, however, should not be visualized as straight and 

 perfectly regular helices. Some R side chains may interact and thus cause 

 deformations of the helices. Proline, one of the natural protein constituents, 

 is actually an a-imino acid, and its presence causes the helix to bend sharply. 

 Besides, extensive regions of polypeptides often are not in the form of 

 perfect helices. As a result, the folded polypeptide is not contained within 

 a straight cylinder as would be a perfect a helix; it is actually much dis- 

 torted, and it must look like an oddly contorted piece of tubing, as depicted 



1 



