Interactions de protbines et d'ions en solution
- Influences de structure dans les associations entre prot6ines et anions organiques par EugBne FREDERICQ (Likge)
SUMMARY.
-The binding of aromatic anions by bovine serum albumin at pH 6,6 and 3,5 and by insulin at pH 5,5, in 0.1 N phcsphate buffers, has been measured by the equilibrium-dialysis method. The association constants and the affinities of the first anion have been calculated from the binding curves. Conclusions are drawn regarding the influence of the structure of anions on their affinities for proteins. The benzene derivatives present low affinities (nitro-and amino-benzoates, aminobenzenesulfonates: phthalate and salicylate). Higher affinities are obtained with naphthalene derivatives (naphthalenemono-and di-sulfonates, naphtholsulfonate, aminonaphthalenesulfonate), with indigosulfonates, with anthraquinonesulfonates and with simple azo-derivatives (monohydroxy-, dihydroxy-, amino-azobenzenesulfonates). The highest affinities are presented by : picric and flavianic acids, complex azo-derivatives (tropeolin, orange 11, xylidine red, amaranth) and sulfonphthaleins (in increasing order, phenol red, thymol blue, bromcresol purple and bromphenol blne).
It is concluded that : 1. affinity is proportional to the number of aromatic rings of the anion ; 2. the introduction of some suhstituents like OH, NO, and halogens in the aromatic rings increases affinity whereas other ones like NH, and nitrogen heterocycles depress it ; 3. the position of lateral substituents is relatively unimportant ; 4. the relative order of affinities is similar for serumalbumin and for insulin. One can also remark that in aromatic compounds, affinity is generally in qualitative relation with the resonance of the molecules.
There is no definite structural pattern of the anion confering to it a special affinity for the proteins. In the contrary, the associations studied here are quite unspecific and do not correspond t o well defined chemical combinations.
(1) The Proteins par H.