MEASUREMENTS OF ABSORPTION SPECTRA OF PIGMENTS 1803 



mil see below in section 3, fig. 37C. 16-18, that the formation of crystalline 

 particles also produces new bands, which then shift with the growth of the 

 particles. A more convincing argument against Strain's suggestion is that 

 phase changes are not likely to be reversible or lead to an equilibrium. 



In the same paper, Freed and Sancier (1952) described the low-tempera- 

 ture transformation of the spectrum of chlorophyll b' in diisopropyl amine 

 (10%) + propane (45%) -\- propene (45%); the red band, located at 

 645 m/x at -43° C, was replaced, at -103° C, by one at 662 mn. (The 

 transformations in the blue-violet region indicate that in this system, 

 irreversible aminolysis is superimposed on solvation.) 



Freed and Sancier (1954) based the above-mentioned reinterpretation 

 of the temperature effect on its similarity with the effects caused by 

 changes in solvent. Cooling of chlorophyll b solution in (not specially 

 dried) hydrocarbons enhanced the long-wave band components, at- 

 tributable to complexes with water, at the cost of the short-wave com- 

 ponents, attributable to water-free pigment. 



This, however, is only a partial explanation, since a similar trans- 

 formation was found also in dry (nonflu orescent) solutions. It had to 

 be attributed there to thermal excitation of a vibration {AH = 1.4 ± 0.2 

 kcal/m.). A similar AH value (1.32 kcal m., corresponding to 460 cm.-^ 

 was derived from the vndth of the band split in the red; the split of the 

 blue band gave AH = 1.42 kcal/m. In wet solvent (n-propyl benzene), 

 on the other hand, the temperature dependence indicated a AH of only 

 2.5 kcal/m.; this lower value suggested that, in this case, vibrational 

 excitation was coupled with the dissociation of a hydrate. Thermally 

 excitable vibrational states were noted, in addition to chlorophyll a, also 

 in chlorophyll b, and possibly in bacteriochlorophyll, but not in the pheo- 

 phytins, allomerized chlorophylls, or metal porphyrins. By vaiying the 

 concentration of a polar admixture (water, propyl ether, pyridine, diiso- 

 propylamine) in a nonpolar solvent, spectroscopic evidence of the forma- 

 tion of both mono and disolvates could be obtained. With pyridine as 

 admixture, and chlorophyll b as solute, an equilibrium constant K = 

 [ChlPy2]/([ChlPy][Py]) = 10 was calculated (at 2° C). 



Solvations of this type occur also with allomerized chlorophyll, the 

 pheophytins, and metal porphyrins. With isopropylamine as admixture, 

 a second type of solvation can be observed, leading to bro-wn compounds 

 (phase test intermediates, cf. chapter 37B, section 4). If this phenomenon 

 is attributed to interaction of the amine with the enol group in position 9, 

 the first type of solvation could be interpreted as involving the polar group 

 in the center of the molecule (magnesium-nitrogen bonds in chlorophyll, 

 imino groups in pheophytine, metal-nitrogen bonds in metal porphyrins). 



