18] INSULIN CRYSTALS 343 



only if— the ad hoc hypothesis is made that during crystallization the crystals 

 throw off nuclei or crystal fractions at a rate proportional to their surface 

 area and to their hnear growth rate. This hypothesis finds support in the 

 microscopical appearance of the suspension, since the very small (<4/x) 

 crystals are predominantly found on the vertices or faces of the bigger crys- 

 tals. It was furthermore observed that the nucleation is enhanced if the 

 surface of the crystallization system is increased, and suppressed by Tween-80 

 or by a surface layer of paraffin. The linear rate of deposition of insulin on 

 the crystal faces was approx. 1 /x/minute at the maximum. It was shown to 

 be a function of the insulin concentration. In the absence of added halo- 

 genide, it was proportional to the third power of the supersaturation and 

 in 7% NaCl to the second power. 



The deposition of insulin on the various crystal faces was studied by 

 letting the crystals first grow in a buffer containing Indian ink and then 

 in a non-coloured buffer. The Indian ink became incorporated in the crystal 

 bodies, and after some further growth in the colourless medium, it was 

 observed that insulin is only deposited on the three faces meeting in one of 

 the obtuse vertices. The same phenomenon was also observed in seeding 

 experiments with non-coloured crystals. Since the unit-cells are identically 

 orientated in the crystals, an 'up and down' with respect to growth in the 

 unit-cell can be concluded. 



If it is true that the rate of deposition on a crystal face is the same on the 

 mirror image of that face, then it can be concluded from these observations 

 that the unit-cell of the rhombohedral insulin crystals has no center of 

 symmetry. This conclusion is in agreement with the lack of centro symmetry 

 in the unit-cell of the rhombohedral insulin crystal. 



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4. K. HALLAS-M0LLER, K. PETERSEN and J. SCHLICHTKR ULL, 5c/ence, 116, 394 



(1952). 



5. J. SCHLICHTKRULL, J. Acta Chem. Scand., 10, 1455 (1956). 



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7. J. SCHLICHTKRULL, Acta Chem. Scand., 10, 1459 (1956). 



8. D. CROWFOOT and d. riley, Nature, 144, 1011 (1939). 



