6 R. LIVINGSTON 



TABLE I. Values for r" of Chlorophylls a and b 



Authors RpferencR Method t° (a) (sec.) t" (6) (sec.) 



Rabinowitch vol. 2a Extinction values n'rins' 



data) 8 X lO^' X lO"" 

 Forster 4 lilxtinction v.-ilucs (Zscheilcs' 



data) 2 X 10-« 4 X lO"* 



Extinction values 1.3 X 10-» 1.6 X 10-« 



Extinction values 1 .5 X IQ-* 2.3 X 10 -« 



Polarization measurements 3 X 10~' 4 X 10~« 



Polarization measurements 1.6 X 10~* 



" These new values were presented by Dr. Rabinowitch at the Gatlinburg 

 meeting (1955). 



It is well established (4,10,11) that energy of electronic excitation 

 can be readily exchanged between chlorophyll molecules, separated 

 by distances as great as 50 A. This process, which is called induced 

 resonance, leads to self quenching and concentration depolarization 

 when it occurs between like molecules, and to fluorescence quenching 

 and sensitized fluorescence when it occiu-s between unlike molecules. 

 For chlorophyll solutions, self quenching is a quadratic function of 

 concentration (4,10), v'max.V = 1 + km"^- It becomes appreciable 

 at rn = 2 X 10~' and reaches its half value (v'max./v = 2) at about 

 1.5 X lO"'^ m. It is not a function of the viscosity of the solvent, being 

 practically identical in benzyl alcohol (4), acetone (10), and ethyl 

 ether (10). Concentration depolarization follows a first-order law 

 (4), p°/p = 1 + k'm. The relative polarization, p/p°, equals one 

 half at a concentration of 6 X 10~^ m. By a slight extrapolation of 

 these data one would predict that <p and p should equal 0.5% and 1%, 

 respectively, in a 0.1 m randomly oriented solution of chlorophyll in 

 a highly viscous solvent. Since the concentration of chlorophyll is 

 approximately 0.1 m in the grana it is interesting to compare these 

 calculated values to the corresponding quantities which have been 

 observed for chloroplasts and algae; 3% (Latimer) and 2% (Arnold), 

 respectively. This near agreement suggests that the chlorophyll 

 molecules are randomly oriented (at least in two dimensions), which 

 is surprising in view of the evidence indicating that the pigment 

 molecules in the grana exist in some ordered orientation. 



In solutions which contain chlorophyll a and b, the fluorescence of 

 6 is quenched by a (10) and the fluorescence of a is sensitized by b 



