68 MACROMOLECULAR COMPLEXES 



lization sites in the collagen fibrils. Preliminary studies by low- 

 angle x-rav diffraction, in which the changes in the intensities of the 

 diffraction spacings were measured, have also indicated that there 

 are specific sites in the collagen fibrils which act as nucleation cen- 

 ters.^ 



Obviously, the next step in understanding the nucleation process 

 would be to discover which amino acid side-chain groups interact 

 with which mineral ions, the nature of the intermolecular forces be- 

 tween collagen and the mineral ions, and the kinetics and atomic 

 sequence of the nucleation process itself. Unfortunately, even in 

 very simple systems, little is known about such phenomena, and 

 more attention has been paid to nucleation rates than to how the 

 nuclei themselves are formed. However, since it is possible that 

 the initiation and sequence of nucleation may be enzymatically 

 controlled and regulated in biological mineralization, such informa- 

 tion is of crucial importance. 



As to the specific organic groups and mineral ions involved in 

 calcification, most investigators in the past have felt that the initial 

 step involved the combining of an anionic organic group with cal- 

 cium ion (Rubin and Howard, 1951; DiStefano et ah, 1953; Sobel 

 and Burger, 1954; Sobel, 1955). However, there are some consider- 

 ations which cast doubt on the hypothesis of the primary role of 

 calcium-binding in crystal induction. 



The case for the importance of the phosphate ion has been sum- 

 marized excellently by McLean ( 1958 ) . To such points as the im- 

 portance of organic phosphate compounds for energv transfer 

 ( high-energy phosphate bonds ) and the possible enzymatic control 

 of such reactions, might be added some structural considerations as 

 well. Namely, the apatites are essentiallv phosphate salts with struc- 

 tural characteristics primarily attributable to the phosphate groups 

 and not to calcium atoms; the latter can be replaced bv a number of 

 other cations, such as strontium and lead, without changing the 

 major features of the crystal structure or svmmetrv. In addition, 

 if cellular-controlled, enzymatic transfers of mineral ions to specific 

 groups in the collagen are involved in the nucleation process, phos- 

 phate is again the most likely candidate. No calcium-splitting 

 enzymes or calcium transferases have as yet been described, al- 



^ Many of the low-angle x-ray diffraction studies were carried out in cooperation 

 with Dr. A. Posner, American Dental Association, Research Division, National Bureau 

 of Standards, and with Dr. L. Bonar, Department of Biophysics, Massachusetts In- 

 stitute of Technology. 



