COLLOIDAL AND ADSORBED CHLOROPHYLL 049 



which it is converted by the presence of even traces of an alcohol or water. 

 Evstigneev et al. (19490 suggested that in the absence of polar molecules 

 chlorophyll is dimerized, and that the dimer is dissociated by oxygen 

 molecules, dimerization being due to unsaturated magnesium valencies, 

 which can also be saturated by oxygen. Their subsequent results (19492) 

 made this interpretation of ox.ygen action unnecessary. 



In solvents of intermediate type, or in mixed (or simply not extremely 

 purified) solvents, both types of interaction may occur simultaneously. 



Changes in the absorption spectra of chlorophyll can be caused, accord- 

 ing to the observations of Livingston and co-workers (1948, 1949), also 

 by small ciuantities of admixtures other than polar solvents. Examples are 

 iodine, bromine, ferric and eerie salts (Rabinowitch and Weiss, 1937), and, 

 in the case of chlorophyll h (but not chlorophyll a!) , phenylhydrazine. Some 

 effects of this type might be similar to those of polar solvents, i.e., they may 

 be caused by reversible formation of molecular complexes. Mostly, how- 

 ever, they are due to irreversible chemical changes such as allomerization 

 (or, more generally, oxidation), or reduction (which is likely in the case of 

 chlorophyll h and phenylhydrazine); these phenomena do not belong 

 under the heading of "effects of the medium on the absorption spectrum of 

 chlorophyll," but rather under that of "chemical reactions of chlorophyll, 

 revealed by spectroscopic measurements" {cj. pages 450-467 in Vol. I, and 

 chapter 37 in this volume). 



2. Absorption Spectrum of Colloidal and Adsorbed Chlorophyll 



Colloidal aqueous solutions of chlorophyll are obtained by mixing a 

 molecular solution of the pigment (in alcohol or acetone) with water. The 

 spectrum of the resulting solution depends on the conditions of mixing; 

 this is the reason that earlier investigators could not agree on the position 

 of the band maximum of colloidal chlorophyll. Herlitzka (1912), Will- 

 statter and Stoll (1918) and Baas-Becking and Koning (1934) reported 

 that this maximum coincides with that of chlorophyll in leaves, i. e., lies 

 close to 680 m^- Ivanovski (1907, 1913) and Hubert (1935), on the other 

 hand, found the maximum at 668 m/x, i. e., in the same region as in many 

 true solutions. Hubert noticed, however, that the position of the maxi- 

 mum was affected by the degree of dispersion of the colloidal sj^stem : Ad- 

 dition of magnesium chloride, which caused a growth of the particles (and 

 finally led to flocculation) shifted it by as much as 8 mn — from 668 to 

 676 m/z. Later, Wakkie (1935) and K. P. Meyer (1939) found that the es- 

 sential factor is not the .size of the colloidal ])articles, but their internal 

 density, i. e., the concentration of chlorophyll molecides in them. 



