448 



CHLOROPHYLL 



CHAP. 16 



5. Shape and Size of the Chlorophyll Molecule 



Formulas 16.III and 16.1 show that chlorophyllin forms a large, flat, 

 almost square, "head" of the chlorophyll molecule, to which phytol is 

 attached as a long flexible "tail." From the standard nuclear distances 

 and bond angles, one can estimate that the approximately square porphin 

 "disc" has an area of 100-110 square angstroms; the "hole" between the 

 four nitrogen atoms, where the magnesium is located, is approximately 

 2.5 A in diameter. The covalent diameter of magnesium in crystals is 

 2.4 A, so that it fits snugly into the hole. 



X-ray experiments by Ketelaar and Hanson (1937) and Hanson 

 (1939) showed that the actual area of chlorophylhn, with all its sub- 

 stituents, is much greater than the value estimated above. Because of 

 the waxy nature of chlorophyll, these experiments were carried out with 

 ethyl chlorophylUde. Its crystals are trigonal hemiedric (symmetry class 

 C3). The elementary cell has a length of 38.4 A (along the c-axis), and 

 a volume of 2660 Al The pycnometrically determined density is 1.28. 

 This shows that the elementary cell contains three molecules — an 

 assumption which leads to a calculated density of 1.23. Each molecule 

 thus occupies 887 A^ The planes of maximal density are 2116, 1216, 

 and 1126, inclined by about 55° to the basal plane of the elementary cell. 

 These planes must be occupied by the porphin plates. The latter are 

 thus stacked obliquely, like books on a partly empty shelf (Fig. 49a). 



aV^ 



a b 



Fig. 49. — Crystal structure of chlorophyllide (after Hanson 1939). 



Each plate is, however, displaced against its neighbor by one-half of its 

 width (as shown in Fig. 49b). The frontal area of the porphin plates 

 is 15.48 X 15.62 = 242 A\ This gives 887/242 = 3.66 A for their 

 thickness.* 



* Hanson gives 3.87 A for the thickness of the ethyl chlorophyllide molecule, but 

 does not explain the discrepancy between the volume of the molecule, 15.48 X 15.62 X 

 3.87 = 936 A', calculated from this thickness, and the smaller value (887 A') which 

 follows from density. 



