SHAPE AND SIZE OF THE CHLOROPHYLL MOLECULE 449 



The narrow sides of the chlorophylHn plates have areas of 3.66 X 

 15.48 = 56.6 A^; but, because of the 55° incUnation, each molecule 

 requires an area of 69.2 A^ on the basal plane. A monomolecular layer 

 consisting of such obliquely stacked chlorophyllide molecules is 12.8 A 

 thick. In the crystal, a second layer, situated on top of the first, has all 

 its molecules rotated around the c-axis by 120°; a third consists of 

 molecules rotated by 240°, while the fourth layer is a repetition of the 

 first (thus, the height of the elementary cell is three times the thickness 

 of a single layer). In other words, the crystal contains a threefold 

 screw axis. 



The area of 242 A^, deduced from x-ray measurements, is more than 

 twice the 100-110 A^ estimated above for the nonsubstituted porphin 

 system. This difference shows that, although the side chains probably 

 stick out from the plane of the porphin plate, they nevertheless increase 

 considerably the area occupied by the molecule in this plane. 



Experiments on ethyl chlorophyllide monolayers on water (Hanson 

 1937, 1939) showed that the extrapolated surface which such a layer 

 occupies at zero osmotic pressure (at pH 5.4) is 70 A^ This value agrees 

 well with the value of 69 A^ derived above from the x-ray analysis for 

 the area taken by an ethyl chlorophyllide molecule in the basal plane of 

 the crystal. Hanson concluded that films of ethyl chlorophyllide on 

 water are crystalline monolayers, composed of flat molecules stacked 

 obliquely to form an angle of 55° with the surface of water. He suggested 

 that the active, hydrophilic part of the molecule is the cyclopentanone 

 ring V, with its easily enolizable keto group (c/. page 444). 



In the case of waxy chlorophyll (as distinct from chlorophyllide) the 

 only result which could be derived from x-ray experiments was the 

 occurrence of a periodicity of 4.2 A, which must be interpreted as the 

 thickness of the chlorophyll molecule. Experiments with chlorophyll 

 surface films showed that the surface requirement of chlorophyll increases 

 with increasing pH, probably because of a progressive enolization and 

 consequent hydration of the cyclopentanone ring. At pH 4.1 (the 

 lowest pH at which experiments can be performed without converting 

 chlorophyll into pheophytin), the extrapolated surface requirement at 

 zero osmotic pressure is 106 A^. It appears that hydration is negligible 

 at this low pH. Thus, phytol increases by 37 A^ the area occupied by 

 the chlorophyll molecule in the surface film. At the higher pH, the 

 surface requirement of chlorophyll is considerably larger, but the film 

 is also more compressible; both effects can be ascribed to hydration. 

 (Hanson calls the layer formed at pH 4.1 "dry" or "crystaUine," and 

 that formed in alkaline media — e. g., at pH 7.6 — "viscous.") 



