Neiu Theory on Prinmry Mode of Auxin Aetion 453 



that all the highly active auxins do have such a partition coefficient 

 (8). Ester and nitrile forms of auxins are more fat-soluble than the 

 acid forms and subsecjuently they often have a higher auxin activity. 

 Chlorination also improves fat solubility of molecules and this is un- 

 doubtedly one of the reasons (although a minor one) for the high ac- 

 tivity of chlorinated auxins. 



(2) Once the auxin molecule has arrived at the site of action, its 

 ring must sink into the membrane of the cytoskeleton to such a depth 

 that the polar group of the auxin molecule fits into the system of H- 

 bonds at the membrane surface. If it sinks in too deeply or not far 

 enough, the proper contact between the polar group of the auxin 

 molecule and the H-bond network is not made (Figure 2). These con- 

 siderations make it obvious that bulky groups on both ortho positions 

 broaden the molecule to such an extent that the ring will not sink 

 deeply into the cytoskeleton; thus with the phenoxyacetic acids the 

 side-chain is relatively long, and in molecules with bulky groups on 

 both ortho positions, the polar group will stick out above the H- 

 bond network (Figure 2). This, then, is the reason for the inactivity 

 of the 2,6-dichlorophenoxyacetic acid (II). On the other hand, the 

 compounds with relatively short side-chains such as benzoic and 

 phenylacetic acids benefit by the presence of two bulky ortJw groups 

 (1), as it locates the polar group exactly in the right position relative 

 to the H-bond network. It is obvious that methyl groups in the ortho 

 positions would be just as effective as chlorine atoms in preventing 

 the molecule from sinking into the membrane too deeply. 



(3) In addition to a vertical positioning of the polar groups, there 

 is also a lateral positioning. It seems obvious that the H-bond net- 

 work cannot block the hole into which the ring must slide, so it 

 must be located to the side of it. This explains the well-known re- 

 quirement for auxin activity that the side-chain must be perpendicu- 

 lar to the plane of the ring. It explains why 2,6-dichlorophenylacetic 

 acid is a stronger auxin than 2,6-dichlorobenzoic acid. The phenyl- 

 acetic side-chain sticks out farther laterally than the short carboxyl 

 group of the benzoic acid. 



(4) Since the polar group has to be held in place, it requires se- 

 cure anchoring of the molecule into the cytoskeleton. This is a func- 

 tion of the ring which is held by van der Waals bonding to the cyto- 

 skeleton. Heavy atoms, such as chlorine, especially on the 3, 4, and 

 5 positions of the benzene ring, help this anchoring process ma- 

 terially as the strength of the van der Waals forces is a function of 

 the atomic weight. This explains the high activity of 2,4-D and 2,4,5-T; 

 further, why 4-chlorophenoxyacetic acid is highly active, while 4- 

 methylphenoxyacetic acid is poorly active (3). 



