INFRARED SPECTROSCOPY OF CHLOROPHYLL 43 



by standing in alcohol. The main product (about 60%) is, of course, a. The infra- 

 red spectra which we have on these compounds check those of Dr. Holt, and his 

 interpretations are in complete agreement with our chemical evidence. 



Some observations on the visible spectra of these derivatives may be of interest. 

 The simple replacement of Cio hydrogen by hydroxy or ethoxy, giving a type "B" 

 compound, has very little effect on the chlorophyll spectrum. However, formation 

 of a type "A' ' compound by insertion of the lactone oxygen into ring V removes the 

 high-energy shoulder from the Soret band of chlorophyll, and shifts the whole 

 spectrum to higher energies. Replacement of ethoxy by hydroxy on Cio has no 

 effect whatever in either type "A" or type "B" spectra. 



Aronofif: ^^'hat I find difficult to conceive is the change in the capacity for 

 hydrogen bonding simply because of the presence or absence of magnesium. I 

 wonder if the mechanism could not possibly involve a racemization of hydrogen 

 at Cio (which, of course, is optically active). 



Rabinowitch : It is well known that the presence of magnesium in the center 

 affects the far corners of the chlorophyll molecule. I think if you have a positive 

 charge in the center it polarizes the ring system, with the outside becoming posi- 

 tive, and this favors enolization of ring V. In a 6 compound, there is a second 

 "sink" at another corner of the ring system; consequently, less positive charge 

 accumulates in ring V. 



It may be collective self-deception, but there is general feeling that there is 

 something important about the lower right corner in the chlorophyll molecule. 

 We ask: why is it chlorophyll a that plays the star role in photosynthesis? Per- 

 haps, you get the proper capacity for reversible changes in ring V by having 

 magnesium in the center, and you take away some of this capacity by having a 

 CO group in ring II. It seems suggestive. 



Weigl: I am a little puzzled about the chlorophyll a chelate. Is there a possi- 

 bility that you had dry carbon tetrachloride in the study of chlorophyll a and a 

 not-quite-as-drj' solvent in the case of chlorophyll h, or vice versa? 



Holt: No! In the chlorophyll b spectrum, there was definitely an OH band, a 

 strong "associated OH" band. 



Becker : You say chlorophyll h shows no chelation? 



Holt : Yes. 



Becker: And the chlorophyll a showed it definitely? 



Holt : It showed an indication of a chelated C — O-group bonded to the ester. 

 Chlorophyll b did not. 



It definitely looks as though water is incorporated into chlorophj-ll b as the OH 

 band is always there. 



Smith : There is a great difference in the ease with which you can pull the mag- 

 nesium out of a and b. It takes 35% acid to pull it out of b, while 25% acid is 

 enough to pull it out of a. In other words, magnesium seems much more strongly 

 bonded in b than in a. Whether this has anything to do with the difference in 

 chelation, I don't know, but it seems a possibility. 



Rabinowitch: I don't know whether we have a theoretical organic chemist 

 here who could say whether it is plausible that a magnesium atom in the center 

 of a molecule would affect the tendency for enolization in ring V. Dr. Becker? 



Becker ; I made the point this morning — and everything since then has borne it 



