PHOTOREACTIVATION 



465 



h}^ an tMiuilibrium condition involving a dark reaction. The substance 

 involved in such a dark reaction might be the pigment that absorbs the 

 light, or the product of the photochemical reaction. This point will be 

 developed further in the treatment of the effect of interrupted light 

 (Sect. 3-5d). 



3-5c. Effect oj Temperature. Photoreactivation is considerably affected 

 by temperature; this gives further evidence that dark reactions are 

 involved. The temperature coefficient of the process (Qio) is near 2 at 

 temperatures around 37°C and near 8 for temperatures near 0°C. This 

 considerable variation of Qio with temperature shows that more than one 

 dark reaction is involved in the process. 



1. 4' X I0"3 



O 



o. 2 X 10"" 



2 3 4 5 



RELATIVE LIGHT INTENSITY, 



10 



6 7 8 9 



arbitrary units 



Pig. 12-4. Photoreactivation rate as a function of the intensity of the reactivating 

 light. The photoreactivation rate is expressed in reciprocal seconds and the light 

 intensity in arbitrary units. Phage T2 was irradiated with the germicidal lamp, 

 adsorbed on resting bacteria, and illuminated in liquid at 37°C. 



3-5d. Effect of Interrupted Light. The analysis of the effect of inter- 

 rupted light has been carried out by Bo wen (1953). 



One of his most significant observations was that with a fixed dose of 

 light given as long flashes (longer than 1 min) and long dark intervals, the 

 fraction of reactivated particles is similar to that obtained with continuous 

 illumination; by making the flashes shorter, without changing the total 

 dose, a point is reached at which the reactivated fraction increases and 

 tends to a maximum when the length of the flashes tends to zero. 



Another observation was that a fixed dose of light given as short 

 flashes of constant length has a different effect according to the length of 

 the dark period interposed between the flashes. The number of reacti- 

 vated particles increases by increasing the length of the dark periods, and 

 for very long intervals it tends asymptotically to a limit value. 



A practical consequence of these findings is that if the number of par- 

 ticles photoreactivated is determined as a function of the time of exposure 



