THE VISUAL PIGMENTS 



purple protein. Assuming the impurities (20 per cent or more) to be 

 phospholipins, with a nitrogen content of 1-8 per cent, broda, 

 GOODEVE and lythgoe therefore arrived at the value 26,500 for an 

 upper limit to R. 



In these calculations, the molar extinction coefficient of the 

 chromophore was taken to be 23,000, a value derived from the photo- 

 sensitivity results on the assumption that y = 1 . As we have seen, 

 however (p. 79 et seq.), there are grounds for beUeving that y was 

 0-5 or thereabouts in the photometric curve experiments. 



If WALD and brown's value 40,600/? for the extinction per chromo- 

 phore is used, the carrier weight (upper hmit) recalculated from 

 BRODA, GOODEVE and lythgoe's data, is 45,000/?. Thus the 

 molecular weight of visual purple (upper limit) as indicated by this 

 method, is 45,000/? . «, where p is the number of Cgo units (retinene 

 or vitamin A) in a chromophore and n is the number of chromo- 

 phores in a molecule. 



MOLECULAR WEIGHT 



A theoretical estimate. In 1949, we ale suggested that the molecu- 

 lar weight of a dissolved substance could be estimated from the shape 

 and position of its main absorption band and its optical density in a 

 solution of known concentration. Starting from an equation 

 developed by houstoun (1909), weale (1949a) showed that the 

 molecular weight, Af, was given by 



, , r , W 1 A max 



M = K .p .- . — . 



V D . r A^ max — •^max 



where K was a known universal constant, /?, the number of electrons 

 (per molecule) responsible for the band, wjv, the concentration and 

 D and r the optical density and refractive index, respectively, at the 

 maximum. 



For visual purple, Amax and A^max are, respectively, 5,020 and 

 5,480^°. Assuming /? = 1, r = 1-33 and using broda, goodeve 

 and lythgoe's wjv and D data for their best solution, weale 

 calculated the molecular weight of visual purple to be 45,600. 



weale's method was criticized by collins and morton (1949) 

 and more recently by hubbard (1954) mainly because of the 

 arbitrary choice of unity for /?, the number of electrons responsible 

 for the band. It is not possible, at present, to assess the value of the 

 method but, as weale (1949b) pointed out, 'the empirical device of 



92 



