MOLECULES AND STRUCTURE FORMATION 9 



modif\'ing the properties of films are clearly shown in studies of the 

 evaporation of water through compressed films of stearic acid or 

 cetyl alcohol; the latter present an extraordinary energy barrier to 

 the penetration of water molecules (Langmuir and Schaefer, 1943). 

 This possibility, namely, that non-polar portions of molecules, par- 

 ticularly lipid molecules, may interact to present a high-energy 

 barrier to the penetration of water, is of considerable biological 

 interest; for, in spite of the relative abundance of water in biological 

 systems, local regions at the particulate level may involve bound- 

 aries between essentially aqueous environments and environments 

 where the interactions of non-polar portions of molecules are so well 

 integrated that water is virtually excluded. A structure of this type, 

 for example, may be necessarv in effecting the key reactions of 

 photosynthesis (Calvin, 1959). 



In this introductory discussion, I would like to summarize some 

 of our experiences with two protein systems in which short-range 

 interactions, coupled with particular types of specificity, lead either 

 to extended fibrils or to spherical micelles, namely, the interactions 

 of insulin (Waugh, 1957) and of casein (Waugh, 1958). 



Mechanism of Fibril Formation. The insulin fibril forms under 

 conditions where the dimer of M ~ 11,000 is the prevalent form, 

 i.e., at pH 2. Heating at 80° to 100° C causes a spontaneous trans- 

 formation into a population of fibrils, the most numerous, and larg- 

 est, of which are about 200 A in diameter and many thousands of 

 Angstroms in length. The reaction goes essentially to completion 

 but is reversible in the sense that under alkaline conditions the fibril 

 disaggregates to yield insulin. Evidence which gives a clue to the 

 mechanism of fibril formation, and which suggests that the insuhn 

 molecule does not undergo extensive unfolding in the process of 

 forming fibrils, comes from experiments in which fibril segments are 

 seeded into insulin solutions at pH 2. While such solutions alone are 

 stable for long periods of time at temperatures of 20° C or below, 

 the seeded fibrils or fibril segments recruit insulin from solution and 

 in the process grow according to first-order kinetics. Structurally, 

 the new portions of the fibrils obtained after fibril growth at lower 

 temperatures appear to be identical with those which are formed at 

 80° to 100° C. 



When a solution of insulin is heated and the transformation of 

 insuhn into fibrils is plotted as a first-order reaction, the resulting 

 curves typically have a lag period which is followed by an essen- 



