STRUCTURE 



135 



is vitamin B P , a substituted cyano-cobalt amide of molecular weight 1357, 

 used in treating pernicious anemia, growth failure in children, etc. The 

 complete chemical composition was disclosed in 1955, and culminated with 

 X-ray diffraction analysis of structure three years later. 



Dissolved Macromolecules 



Unfortunately, when crystals such as those described in the previous sec- 

 tion are dissolved in water, the molecules are subjected to a number of new 

 and powerful forces of hydration and of polarization by ions, and the con- 

 figurations of many molecules change. Since water has a diffuse diffraction 

 pattern of its own, the X-ray technique used in crystals cannot be employed 

 to advantage on solutions of these molecules, and other methods which indi- 

 cate structure must be sought. All those to be described are useful, but each 

 has its limitations. 



The problem in solution is complicated by three other facts: (1) The 

 molecule will not usually, unless it is globular-shaped, have a unique molec- 

 ular weight; but rather will its molecular weight vary, some molecules in the 

 solution having weights above, and some below an average value. The dis- 



cs 

 o 

 o 



o 

 o 



o 



o 

 o 

 o 



rO 



o o o 



o o o 



o o o 



»fr If) tD 



I I I I 1 1 I I 1 1 I I 1 1 I 



1 1 1 1 1 1 1 



L ( Angstroms ) 



(b) 



Number average 2450 



Weight average 2820 



Light scattering 3000 



Flow birefringence 

 2600 — 2950 



Intrinsic viscosity plus 

 molecular weight 2970 



Figure 6-6. Ichthyocol (a Protein Food Supplement from Fish): Direct Measure- 

 ments of Molecular Size by Electron Micrographs, Compared with Results from 

 Indirect Methods. Number (N) with length (L) multiplied by L gives total amount of 

 protein with particles of length L Molecular weights, M n and M w , are derived from 

 data plotted. (Data of Hall and Doty, J. Amer. Chem. Soc, 80, 1 269 ( 1 958).) 



