34 ACTIONS OF DRUGS ON SURFACES 



thesimple esters, the rate of hydrolysis may vary, accord- 

 ing to the configuration of the ester at the interface, so 

 as to have a velocity as high as 0.18 min"^, or as low as 

 0.005 min~i under a given set of conditions. Similar 

 results have been attained for the rates of oxidation 

 in interfaces of substances containing double bonds, by 

 permanganate. 



We see, from consideration of these results, that the 

 interaction of an enzyme with its substrate may be affect- 

 ed by the orientation and other physical conditions of 

 the substrate and that these in turn may be affected by 

 drugs. 



Specificity in Surface Reactions 



Many drugs, such as adrenaline, veratrin, acetylcholine 

 and cocaine act in such very low concentration that it 

 is not at once apparent how a sufficient concentration of 

 these molecules can arise at their site of action to produce 

 a significant physico-chemical effect. However, if the 

 action of these drugs is at surfaces it is possible to see 

 both how an adequate concentration may arise, and why 

 there is a high degree of specificity in their action. Each 

 drug contains ionising groups, polar groups and practi- 

 cally non-polar hydrocarbon groups. Hence a drug has 

 an intrinsic capability of being adsorbed at a surface, 

 and when adsorbed, of altering the properties of the sur- 

 face. E.g. let us consider the concentration of adrenaline, 



