THE PHYSICAL CHEMISTRY OF VISUAL PURPLE 



is a ratio) shall be dimensionless. An extinction coefficient is, there- 

 fore, an area and may be regarded as the product of the physical 

 area of the chromophore and a probabiUty factor expressing the 

 chance that a quantum within this area is absorbed. The maximum 

 value of the probability term is unity, and consequently the maximum 

 possible extinction is given by the chromophore area. This is Hkely 

 to be of the order 10-^^-10-^^ cm^. Thus the extinction of visual 

 purple is of the same order as the theoretical maximum. 

 The molar extinction coefficient, e, is given by the relation 



logio 7 = ^cl 



where the concentration, c, is in number of gram-molecules per litre. 

 Conversion to these units is effected by multiplying a by 6-1 x 10^3 

 (Avogadro's number), by 0-43 (natural to common logarithms) and 

 by c. 10-^ (cubic centimetres to litres). If the maximum photo- 

 sensitivity is expressed in these units we have ey == 24,000«, i.e. if 

 y = 0-5, e = 48,000/2, where n is the number of chromophores in 

 each visual purple molecule. 



An independent estimate of the molar extinction coefficient was 

 obtained by collins and morton (1950a) from the relations between 

 the density spectra of visual purple and of its derivatives indicator 

 yellow and retinene. The molar extinctions of the latter substances 

 could be directly measured, collins and morton found that £max 

 for visual purple = 48,000/? . n, where p is the number (now known 

 to be one) of C20 (i-e. retinene or vitamin A) units in the visual purple 

 chromophore, and n, as before, the number of chromophores per 

 molecule, wald and brown (1953) obtained the value 40,600« in 

 a similar investigation in which possible comphcations, arising from 

 the cis-trans isomerization of visual purple derivatives, were 

 ehminated. 



hagins (1954) measured the photosensitivity of visual purple in 

 situ in the excised eyes of rabbits. He obtained the value 27,000, 

 indicating (if y = 0-5) that £max = 54,000rt. This high result was 

 expected since the visual purple molecules as oriented in the retinal 

 rods have an extinction about 50 per cent higher than in solution. 

 hagins found that digitonin solutions of rabbit visual purple, 

 soUdified as agar gells but otherwise treated in the same way, had a 

 much lower photosensitivity (17,000). 



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