PRIMARY EVENT IN MUSCLE ACTION 



of roughly parallel myosin molecular chains, at either end fixed 

 to transversely rigid end plates (this might be crudely thought of 

 as a metameric unit, say 400 A long, in a muscle filament), and 

 immersed in a solution containing ions, including ATP. Sup- 

 pose the model to be at its equilibrium* length for a specified 

 tension and chemical potential of ATP, and suppose that the 

 solution concentration of ATP is increased. If the myosin array 

 is initially (+)-charged, it will adsorb ATP and do work be- 

 cause the system as a whole thereby loses free energy. How- 

 ever, this loss is now the net result of several important processes, 

 some already mentioned and others not. There is, by hypothe- 

 sis, a specific attraction between ATP*^" and any one adsorbing 

 site on the myosin, and since generally there may be more vacant 

 sites than ATP ions which will be adsorbed, the binding process 

 per se has a configurational term. As in tlie idealized molecular 

 chain the entropy of the individual chains will increase on 

 shortening. Also as before, the electrostatic energy of the system 

 of charges fixed to the chains is altered, both because the average 

 distance between charges is lessened and because the number of 

 net ( + )-charges is reduced; however, computation of the 

 electrostatic "energy" (now really a free energy) must now take 

 into account the fact that the forces between charges fixed to 

 the myosin chains are attenuated by "atmospheres" of pre- 

 dominantly oppositely charged mobile ions (e.g., K"*", Cl~) 

 which form around each myosin-fixed ion. Recognition that 

 the solvent really consists of molecules which may interact with 

 the material of the chains introduces an entirely new factor. 

 To see this, we may consider what happens when a roughly 

 parallel array of tightly packed "dry" myosin chains is immersed 

 in pure solvent. Clearly, maximization of the entropy is served 

 by intermingling of the solvent molecules with the chain mole- 

 cules ; the resultant swelling would in turn cause linear deforma- 

 tion of the array. This tendency to intermingle may be en- 

 hanced or off'set, however, depending on the strengths of the 



* Here we temporarily assume that adsorbed ATP does not go on to be 

 split. 



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