MANUEL F. MORALES AND JEAN BOTTS 



proper thermodynamic question. Another instructive by-product 

 is the conclusion that several current molecular models are 

 "thermodynamic homeomorphs." For example, the afore- 

 mentioned theory of the Huxleys, wherein ATP-binding is said 

 to allow relaxation and ATP removal to permit contraction, is 

 subject to exactly the same analysis except for changes in alge- 

 braic sign at obvious places. Likewise the experiments of Pry or 

 (61) on tendons, and the subsequent ones of Laki and Bowen 

 (48) on myosin fibers, wherein adsorption of I~ and of SCN~ 

 causes a contraction which can then be abolished by rinsing, 

 are susceptible to the same analysis, the only difference being 

 that in those cases the contracting adsorbate is removed not by 

 chemical conversion into a noncontracting adsorbate, but by 

 dilution.* Foreshadowing a later suggestion, we note that any 

 exergonic reaction which removes ATP (split or unsplit) from 

 the myosin sites, or which cancels the electrostatic effects of 

 bound ATP, can be invoked to construct a contractin^-relaxing 

 model of the primary event. 



The second theoretical consideration we have in mind has 

 to do not with thermodynamics but with molecular events. 

 We have said above that in our theoretical myosin model the 

 abolition of extensile electrostatic repulsions through ATP 

 adsorption leads, "for certain reasons," to contraction. It has 

 been pointed out that for an idealized molecular chain in a 

 structureless medium one of these reasons is that (configura- 

 tional) entropy is gained (free energy is lost) on passing iso- 

 thermally from an extended to a semicurled state. In a more 

 realistic model the factors to be considered are many more, and 

 they cannot, strictly speaking, be separated one from the other, 

 or even resolved into purely energetic or purely entropic contri- 

 butions. But in a qualitative fashion it is nonetheless valuable to 

 consider what are apt to be first-order effects (33-35,55). To 

 be specific, let us consider one of several theoretical models 

 analyzed by our colleague. Dr. Hill, viz., a cylindrical array 



* This was, of course, recognized by Pryor, who developed a thermo- 

 dynamic treatment appropriate to his experiment. 



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