Cell Constitution 



53 



THE PARACRYSTALLINE (MESOMORPHIC) 



STATEj TACTOIDS, COACERVATES AND 



LONG-RANGE FORCES 



The crystal is possessed of perfect, non- 

 statistical order. However, symmetry may 

 occur in certain types of materials in one or 

 two dimensions as well as in three dimen- 

 sions. There are, in fact, various transitions 

 of ordered arrangement between that in a 

 crystal and the lack of order characteristic 

 of a liqviid. Hermann showed that there are 

 18 possible transitional states, which have 

 been called paracrystalline or mesomorphic. 

 Two of these, the nematic and smectic states, 

 are of great importance in the microstructure 

 of protoplasm and will be briefly character- 

 ized. The nematic state concerns the fibrous 

 systems discussed in the preceding and fol- 

 lowing sections. The smectic state concerns 

 more importantly a subsequent section on 

 lamellar, membranous systems. However, for 

 the sake of clarity, both states are considered 

 jointly. 



THE NEMATIC STATE 



This state is characterized by arrays of 

 thin elongate particles which are constrained 

 to remain oriented parallel to a preferred 

 axial direction. The particles are free to 

 rotate about their axes or be translated later- 

 ally or axially. Such systems are birefring- 

 ent and show evidences of the parallel orien- 

 tation of the particles in x-ray patterns. The 

 particles may be regularly spaced laterally, 

 the interparticle distance depending on the 

 concentration, the charge distribution on 

 the particles, the ionic strength and the pH 

 of the aqueous medium. 



The conditions which determine the state 

 of a nematic system were investigated in 

 some detail in the case of solutions of 

 tobacco mosaic virus (TMV) by Bernal and 

 Fankuchen ('41) and by Oster ('50). This 

 will be discussed below in connection with 

 tactoid theory. EM studies have greatly en- 

 hanced our knowledge of such systems. 



THE SMECTIC STATE 



In smectic systems there is one degree of 

 freedom less than in nematic systems. The 

 elongate molecules, though oriented in a 

 common direction, are constrained to lie 

 in planes perpendicular to the direction of 

 molecular orientation. The system has thus 

 a layered, planar structure. The molecules 

 may have freedom of rotation and of lateral 

 translation but are constrained to remain 



within their own planes (like people stand- 

 ing on floors of a building but unable to 

 walk between floors). 



The lipids compose the chief type of 

 smectic system encountered biologically. In 

 such systems the lipid molecules usually oc- 

 cur as bimolecular layers, the polar ends of 

 the molecules being located at the aqueous 

 surfaces. The distance between double layers 

 depends upon the water content, ionic 

 strength, specific ions and pH. In some cases, 

 e.g., cephalin, lipid double layers may be 

 separated by water layers thicker than the 

 double layers themselves (Palmer and 

 Schmitt, '41). Because such layered lipid and 

 lipid-protein systems are of great importance 

 biologically, as in the nerve myelin and in 

 cell membranes, it is important to gain an 

 understanding of the forces and conditions 

 which determine their stability. 



Most cellular lipid double layers are com- 

 posed of mixed lipids: phospholipids, cerebro- 

 sides and steroids. The ability to incorporate 

 water extensively between double layers of 

 mixed lipids is determined primarily by 

 certain of the lipids, notably the cephalins, 

 which present negative charges at the aque- 

 ous surfaces. When positively charged ions, 

 particularly multivalent cations such as Ca*"^ 

 or histones and protamines, are added the 

 water is expelled from between the double 

 layers, causing the system to precipitate and 

 lose its characteristic colloidal texture. This 

 illustrates the great sensitivity of such sys- 

 tems to changes in the ionic environment. 



Both nematic and smectic systems show 

 birefringence which is positive with respect 

 to the optic axis which parallels the direc- 

 tion of orientation of the molecules. The 

 sign of the birefringence depends upon the 

 nature of the molecules and on the distance 

 between layers (extensive solvate layers may 

 produce lamellar form birefringence which 

 is uni-axially negative). 



TACTOIDS AND LONG-RANGE FORCES 



Mesomorphic systems, both of the nematic 

 and the smectic types, frequently form tac- 

 toids and coacervates (Kruyt, '49) in which 

 phases form spontaneously depending on the 

 concentration of the material and the na- 

 ture of the environment. The phases may 

 separate microscopically or macroscopically 

 in vitro. These are illustrated in the case of 

 TMV suspensions. The threadlike macromole- 

 cules aggregate laterally to form birefrin- 

 gent lens- or spindle-shaped droplets. These 

 are called positive tactoids (Bernal and 



