50 FRANCIS O. SCHMITT 



The Technical Background 



The determination of protein structure requires a complete and exact 

 knowledge of the amino acid composition and of the configuration of 

 the amino acid residues. To what extent are the physical and chemical 

 techniques so far developed adequate for this purpose and to what 

 extent have these techniques been applied? 



Until recently amino acid data were not only relatively unreliable but 

 also incomplete with respect to the constituents present in small amounts. 

 If proteins are to be dealt with stoichiometrically, the analytical data 

 must be exact. Recent advances in amino acid analysis, particularly the 

 stable isotope and microbiological and adsorption techniques, should 

 help to fill many of the gaps in the cases where really pure preparations 

 can be obtained. Unfortunately few fibrous proteins have been prepared 

 in a state of purity approaching that which is required. 



Fairly good technical and theoretical background exists for the 

 determination of particle size and shape. This includes the methods of 

 streaming double refraction, viscosity, diffusion and ultracentrifuga- 

 tion. But those methods yield appropriate information only when the 

 preparation is truly monodisperse. In the case of myosin, for example, 

 this criterion has not been fully met. The particles are fairly uniform 

 in cross section but have highly variable lengths. Hence the measure- 

 ments led to an average value for particle size, the larger sizes receiv- 

 ing greater weight, rather than to dimensions which could be ascribed 

 to the myosin molecule. 



Electron staining and shadow casting techniques have greatly en- 

 hanced the value of the electron microscope in the investigation of the 

 structure of fibrous proteins. Resolution of the order of 25 A has 

 already been achieved with favorable materials and it is probable that 

 this may be further increased. Small-angle x-ray dififraction techniques 

 permit the recording of reflections corresponding to periodicities of the 

 order of hundreds of Angstrom units, thus widely overlapping the 

 resolving power of the electron microscope. The electron microscope 

 data assist greatly in the interpretation of the diffraction patterns of 

 fibrous proteins such as collagen and paramyosin which manifest a 

 large fiber-axis repeating period. In these cases fine structure within 

 the main period can be demonstrated in the electron micrographs ; this 

 fine structure may be correlated with the x-ray data to deduce the struc- 

 ture. 



Only a few of the fibrous proteins have been studied with all of the 



