THE VISUAL PIGMENTS 



THE CONTRIBUTION OF INTERNAL ENERGY TO THE PHOTO- 

 CHEMICAL BLEACHING OF VISUAL PURPLE 



The fact that the visual purple molecule can be *bleached' either 

 photochemically (by absorption of a quantum) or thermally (by a 

 chance-accession of the energy of activation) led st. george (1952) 

 *to study the transition between photic and thermal activation.' He 

 considered that 'on going from shorter wavelengths to red light a 

 point should be reached beyond which the incident quanta cannot 



lOOO/T 



Fig. 3.9. The influence of temperature on the rate of thermal decom- 

 position of frog's visual purple at various acidities. Abscissae : 1 ,000/r, 

 where Tis the absolute temperature. Ordinates : logioAT, v^here K is the 

 velocity constant (time in seconds, natural logarithms). The upper 

 dotted line is for a dialysed solution of low salt content; the lower 

 dotted line for one containing added salt (2M NaCl). 

 {Lythgoe and Quilliam, 1938) 



supply the entire energy needed to activate the rhodopsin molecule. 

 There should therefore be a critical wavelength beyond which the 

 bleaching by light becomes temperature-dependent.' 



To investigate this possibihty, st. george measured the rates of 

 photochemical bleaching (in digitonin solutions buffered at pH 6-7) 

 at 2°C and at 32°C. The bleaching lights, of various wavelengths 

 between 400 and 750 m/i, were obtained by passing the output from 

 a high-power projection lamp through suitable interference filters. 



The visual purple solutions (prepared both from cattle and from 

 frog retinae) were contained in thermostatted quartz cells of dimen- 

 sions 3x10 mm. The bleaching hght traversed the short dimension 



86 



