THE PHYSICAL CHEMISTRY OF VISUAL PURPLE 



consists of one or more chromophores in association with a molecule 

 of protein. 



If M is the molecular weight of a chromoprotein, and n is the 

 number of chromophores in each molecule of protein then i?, the 

 *molecular' weight of that fraction of the chromoprotein molecule 

 which contains one chromophore, is given by, 



n 



The fraction R may be called the carrier weight, for it is approxi- 

 mately equal to the weight of protein required to 'carry' one chromo- 

 phore. Strictly speaking, the carrier weight should be defined 

 through the molecular weight of the protein itself (i.e. without 

 chromophores), but the difference is very small in practice since the 

 chromophore is always very much lighter than the protein. 



The value of R for visual purple was determined by broda, 

 GOODEVE and lythgoe (1940) in the following way. The optical 

 density, D, of a solution is given by, 



D = logio - = eel 



where e is the molar extinction coefficient, c is the concentration in 

 gram-molecules per htre and / is the optical path length in centi- 

 metres. From the photosensitivity measurements the value of e per 

 chromophore was known (assuming 7=1) and therefore the mole- 

 cular concentration of a visual purple solution could be calculated 

 from its optical density. Consequently, by measuring the actual 

 concentration of a solution, in grams per cubic centimetre, the 

 carrier weight could be calculated. 



BRODA, GOODEVE and LYTHGOE prepared visual purple solutions 

 from the frog, and, after measuring the optical densities at 500 m/z in 

 a 1 cm cell, estimated the dry weights of the visual purple in known 

 volumes. Impurities (digitonin, salts and phospholipins) were either 

 eliminated from the solutions by exhaustive dialysis and electro- 

 dialysis, or subsequently separated from the gross evaporation 

 residues by the use of selective solvents. 



The best experiment yielded the value 34,500 for the carrier 

 weight, R. However, the solid residue had a nitrogen content of only 

 12 per cent. Since pure proteins contain 15-19 per cent N, the residue 

 can have contained, at the most, only 80 per cent of pure visual 



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