386 CHEMICAL DYNAMICS 



1-leucyl ( — 13 degrees), glycin (0 degrees) and d-alanin (+2 

 degrees). If the first link to be broken, therefore, is that be- 

 tween the glycyl and alanyl groups the positive rotation of the 

 original solution should increase until it reaches a maximum. 

 If this link were the only one attacked then this maximum rota- 

 tion should be that of a mixture of 1-leucyl-glycin and d-alanin, 

 i.e., over three times that of the original solution. If the first 

 link to be broken were that between the leucyl- and the glycyl- 

 groups the original positive rotation should decrease and finally 

 become negative. If the two links were split simultaneously 

 the positive rotations should decrease until it became very slightly 

 negative. These possibilities are made clear by the following 

 diagram: 2° 



1-leucyl-glycyl-d-alanin 

 -13° 0° 4-2° 

 4-172°^ 



-73° 



The experimental result, employing trypsin, is that the rota- 

 tion at first increases to an extent of about 40 per cent. Hence 

 1-leucyl-glycin must be liberated and the point of first attack 

 (or, at least, most rapid attack) must be the bond between the 

 glycyl and the alanyl groups. After attaining a maximum 

 positive rotation only 40 per cent in excess of that of the original 

 solution, however, the rotation declines again, owing to the 

 hydrolysis of the 1-leucyl-glycin. Hence the enzyme does not 

 complete the decomposition of the one bond before attacking 

 the other. In other words the two reactions proceed side by 

 side but at different velocities, that of the splitting of the glycyl- 

 alanin bond being the most rapid. The leucyl-glycin bond, 

 however, does not appear to be attacked at all while it is bound 

 up in the tripeptid molecule, there being no evidence of the 

 formation of glycyl-d-alanin. The progress of the reaction is 

 therefore, it would appear, the following, first the glycyl-alanin 

 link is broken and 1-leucyl-glycin and alanin are split off. The 

 velocity of this reaction is so much greater than that of the other 

 possible reactions (splitting of the leucyl-glycyl link) that the 

 latter reaction does not occur to any appreciable extent. The 

 1-leucyl-glycin which is thus formed is immediately attacked by 



