67 



another are two 0~'s of the dissociated OH groups of the phos- 

 phate chain, and two N's of the purine. The two 0"'s are those 

 lying on either side of the terminal ^P, which has to be split and 

 give up its (£), while the two N's of the purine would be, in all 

 probability, the N of the NHo group at position 6 (since this N 

 seems to be most involved in the reaction occurring in muscle 

 contraction) , and its second neighbor at position 7. 



However, free rotation does not mean free motion. Such a C — C 

 bond does not permit the molecule to bend in any way it pleases, 

 for the valency angles have to be kept constant. The motion is not 

 freer than that of the relative motion of two wheels mounted on 

 the same axis. So even with three rotating links the freedom of the 

 ATP molecule is a very limited one and, statistically, the chances 

 that the NH2 and N7 would be able to meet the two 0"'s and 

 make a close fit, are very remote. So if the four could meet and 

 make a close fit, it would be probable that this is not mere chance, 

 but has a functional meaning and that the ATP molecule is made 

 that way and does not happen to be that way. Whether such a 

 meeting is possible can be decided by building up the molecule of 

 an atomic model which keeps account of atomic radii and limita- 

 tions of freedom, the rigidity and small flexibility of bond angles. 

 Such a model is the Courtauld atomic model." Figure 15 shows an 

 ATP molecule built up of this model, in its conventional linear 

 form corresponding to Fig. 12. (The molecule is here in its dis- 

 sociated form, with 0"'s instead of OH's on the phosphates.) 



If the molecule is now rotated around the C — C, C — N, and 

 C — O bonds, then the phosphate-end can be folded back so that 

 the O's of the terminal and middle phosphate just touch the N's 

 mentioned. This situation is shown in Fig. 16. If some sort of a 

 link is formed now between the 0~'s and N's, then the P — O — P 

 link, which has to be broken in contraction and supply the energy 



' Produced by Griffin and Tatlock, London, available in U.S. through the 

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