OXIDATTOX, ALLOMERIZATIOX AXD REDUCTIOX OF CHLOROPHYLL 1781 



/- 



(37B.3) 



HC — C 



I II 

 O 



Weller and Livingston measured the rate constants of this reaction 

 spectroscopically with seven different amines. They ranged, in the case of 

 chlorophyll a at 20° C, from log /.' = — 1.88 sec.~^ with piperidine, to log 

 k' = —5.38 sec.~' for phenylhydrazine, and < — 8.0 sec.~' for aniline. The 

 order of rate constants parallels (for both chlorophylls a and /;) the order 

 of basicity of the amines (as measured by p/v^ in water). Allomerized 

 chlorophyll a in isobut3damine showed no reaction even after 3 days. 

 Temperature effect on the rate of reaction of Chi a with isobutylamine indi- 

 cated a very small activation energy (<() kcal) ; the (negative) entropy of 

 activation must therefore be large (to account for the relatively low abso- 

 lute rate at room temperature) . 



In weekly basic amines the reaction is so slow that the reversible for- 

 mation of an amine -|- pigment complex can be observed, as described by 

 Krasnovsky and Brin (cf. Chapter 37C, section 2a). 



The phase test reaction with alkali differs from the reaction of chloro- 

 phyll with amines in several respects : (7 ) It goes via a brown intermediate, 

 which is formed at room temperature within less than 1 sec, and disap- 

 pears within 20-30 sec. (in the system chlorophvll a in pyridine -f meth- 

 anolic KOH); {2) the energy of activation is >20 kcal, and (3) the pres- 

 ence of at least a small amount of alcohol is needed for the reaction. The 

 allomerization process has characteristics similar to those of the phase test. 



In analogy with the accepted mechanism of aminolysis of esters, Weller 

 and Livingston suggested the following mechanism of cleavage of ring \ , 

 in its keto form, by amines: 



(37B.4) 



V 



HC 



-C 



II 



o 



+ RNHo 



Transient 



— addition > 



product 



V 



10 9 



HC- CNHo+R 



O 





~* ilO 9 



H2C 



CNHR 

 O 



The basicity of the amine is determined by the charge accumulation on the 



