404 



LIGHT AND LIFE 



TABLE I 

 Characteristics of Negative Absorption Shift at 700 m/i 



Wave length maximum effect 



Band shape 



Magnitude 



Ratio P700: Chlorophyll a 



i.e 



Velocity of bleaching 

 Return reaction 

 Quantum yield of bleaching 

 Bleaching sensitized by 



v^v703 mix 



Often asymmetric 



A ^ 0.01 OD if OD 675 ~ 4 



.^1 :400 



Instantaneous: Photochemical 



In dark, t 1/., > 5 msec 



Order of unity 



Chlorophyll a, carotenoid 



10-** sec) bleaching was induced, as well as other evidence, indicates 

 a direct photochemical event. During dark periods ol suitable length, 

 a restoration of the absorption occurs. As will be discussed later, 

 the rate of this back reaction is influenced by various agents. In 

 steady-state flashing light (dark time ^60 msec.) we measured (in 

 whole algae) a half restoration time of some 7 milliseconds. 



In such a steady-state sequence of flashes and dark periods one can 

 determine: (a) the size of the cyclic absorbance shift; (b) the num- 

 ber of flashes per unit time; and (c) the amount of absorbed actinic 

 light per unit time. If we now assume that the involved pigment has 

 a molar extinction like chlorophyll (10-^ M'^cm-'^) then the rate of 

 pigment turnover and the quantum yield can be calculated. Al- 

 though with our present apparatus we were able to make only rough 

 estimates (with Auacystis and relatively weak actinic light of 670 

 myu,), a quantum yield of the order of unity was indicated. This in- 

 dicates that at least a substantial fraction of the light quanta absorbed 

 by chlorophyll a contribute to the bleaching of "P 700." 



Finally, we tried the effect of varying the wavelength of the ac- 

 tivating light, that is, we determined an activation spectrum of this 

 negative shift. It appeared that under normal conditions the effect 

 was sensitized mainly (but hardly ever exclusively) by chlorophyll a 

 and probably also by carotenoids (cf. Fig. 6) . However, in aged or 

 poisoned preparations, accessory pigment can also contribute con- 

 siderably to this sensitization (see below) . 



In conclusion, these data fit the picture that "Pigment 700" is an 

 efficient trap, collecting excitations from surrounding chlorophyll a 

 molecules, and undergoing a temporary bleaching each time. 



The Photochemicallv Induced Formation of "P 700" 



In the previous section we have dealt only with instantaneously in- 

 duced, quickly reversible jihcnomena ("fast effect") . However, we 



