III. CENTRIFUGATION 97 



single ultracentrifuge, it is very doubtful whether the value can be 

 regarded as significant to within less than 1 or 2% in most cases. 



Boundaries can be detected and their rates measured (with de- 

 creasing accuracy) with concentrations as low as about 0.01% in the 

 case of the larger monodisperse proteins. For molecular weights be- 

 low several hundred thousand, several hundredths of one per cent 

 may be required. The concentrations of fully resolved components 

 can usually be determined from the area under the refractive index 

 curves to within a few per cent when the value is of the order of 1% 

 or more, the accuracy depending on the quality of the optical system 

 and the spread of the boundary. Even when boundaries are not 

 completely separated, their respective concentrations can be esti- 

 mated by a method of interpolation (1). 



It is not possible to make any general statement regarding the 

 accuracy of a molecular weight determination by either the velocity 

 method or the equilibrium method, since so many variables are in- 

 volved. Discrepancies of at least a few per cent are the general rule 

 and one is probably not justified in considering the usual determina- 

 tion significant to within less than 5 to 10%. 



4. Representative Applications 



Thousands of practical applications of the ultracentrifugal method 

 have been reported in the literature and bibliographies covering 

 many of these may be found in the publications of Svedberg and as- 

 sociates (1,5). Less extensive bibliographies have been given by 

 Pickels (2). A few typical publications have been listed in the 

 bibliography at the end of this article. The greatest advantage of 

 the method seems to have been realized in the study of proteins and 

 viruses. Table I summarizes representative findings for some of the 

 serum proteins. 



Of interest in the way of pointing out some of the problems that 

 may be encountered as well as some of the avenues of investigation 

 that may be explored, are descriptions of the following : 



Splitting of hemocyanin molecules by ultrasonic waves (S2) ; dissocia- 

 tion of hemocyanin (S3) ; stability in protein systems (61) ; catalytic effect 

 of papain on thyroglobulin (37) ; interaction of proteins (3S) ; molecular 

 association of hemocyanin produced by X rays (40) ; effect of ultraviolet 

 radiation and X rays on serum albumin (43) ; shape of polystyrene molecules 

 (43) ; splitting of protein molecules by ultraviolet light and X rays (49) ; 

 influence of pH on stability of equine encephalomyelitis virus (57) ; effects of 



