MANUEL F. MORALES AND JEAN BOTTS 



relative molecular force interactions: solvent-myosin, solvent- 

 solvent, and myosin-myosin. In any event, there is an inter- 

 play between length of the array and array-solvent mixing — 

 especially dramatic in passing from dry to immersed array, but 

 of course equally operative on changing the length of an im- 

 mersed array. From this interplay we can also expect a con- 

 tribution to the free energy of deformation. Another factor, 

 additional to those encountered in the idealized single molecu- 

 lar chain, is that of H-bonding. There is evidence that H-bond- 

 ing plays a role in the aggregation of myosin systems (10), and 

 recent theories of regular folding of polypeptide chains have 

 stressed that H-bonding influences the stability* of various chain 

 configurations. More concretely, if one assumes that, as an 

 array of polypeptide chains is unfolded, intra-ch.a.in, H-bonds 

 give way to inter-chain bonds, then again any change in the linear 

 dimension of the array is accompanied by a free-energy change 

 due to H-bonding, and this effect too has a purely configura- 

 tional factor (24), since a given macroscopic stretch can in 

 general be achieved in several microscopically different ways 

 (e.g., undoing any one of many folds along the same molecular 

 chain). Hill's analysis of the foregoing factors has shown that 

 on the basis of realistic values for certain constants of the model 

 (e.g., number of charges per myosin residue, binding constant 

 for ATP, ionic strength of the medium) one predicts mechanical 

 effects of about the magnitude observed in actual experimental 

 models or in muscle. This result attests to the mechanistic 

 plausibility of our model. The analysis has also turned up at 

 least one effect not hitherto realized in elastomeric theory. For 

 certain plausible choices of the constants specifying the (myosin 

 chain) -(solvent) mixing processes, or for certain choices of the 

 energies of intra- and inter-chain H-bonds, the analysis predicts 

 that as the ATP concentration surrounding a free myosin fiber 



* Some stability calculations, however, neglect the effect of competitive 

 H-bonding by water molecules or else estimate it very roughly. Nevertheless 

 it is certainly plausible that the H-bonding should have an effect and, as we 

 have said, there is evidence so indicating. 



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