770 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



at the bottom of Table 23.IIA. In this case the approximate equation 

 (23.4D) should be replaced by the exact equation: 



(23.4E) K = 



[Ac]y, -0.5[Chl]o 



Livingston used, however, the simplified equation for all the systems 

 studied by him. Furthermore, he calculated the constants K, not from 

 the value of [Acl^/^, but from the average slope of the activation curve. 

 He obtained in this way the K-values given in Table 23.IIIA, which differ 

 somewhat (although not in the order of magnitude) from values that could 

 be calculated by means of equation (23.4E). (For example, the latter 

 would give, for piperidine, K = 2.2 X 10^ instead of 1.6 X 10^). 



Deviations from straight line in the plot of log [(Fo/F) — 1] against 

 [Ac]o were noted by Livingston and co-workers at low concentrations of 

 the activator, and were ascribed by them to the presence of an "adventi- 

 tious" activator, Ac' (probably water). They held the latter responsible 

 for the weak fluorescence still noticeable even at [Ac]o = {e.g.,F ^ 0.03Fo 

 in purest benzene). By using the same assumptions as before («chiAc = 

 achi for excitation light and [Ac']o » [ChlAc']), and assuming the adventi- 

 tious activator to be water, Livingston and co-workers obtained theoretical 

 curves fitting well the experimental results. 



From the equilibrium constant K and its change with temperature, 

 values of AF, AH and AS were calculated for three systems shown in 

 Table 23.IIIB. 



Table 23.IIIB 



Thermodynamic Constants for the Association of Chlorophyll a 

 WITH "Activators" in Benzene 



Since Fo seems to be independent of the nature of the activator, Livings- 

 ton suggested that the essential effect of association is isomerization (or, 

 rather tautomerization) of chlorophyll, e. g., conversion of the keto form 

 into an enol. If — as assumed by Livingston — this is the transformation 

 discussed in Volume I, page 459, allomerized chlorophyll should be inca- 

 pable of it, and should therefore remain fluorescent in pure hydrocarbons. 

 This consequence remains to be tested. 



Livingston attributed the postulated stabilization of the keto form by 



