FREE-ENERGIES OF ADSORPTION OF DIPEPTIDES AND 2,5-PIPERAZINEDIONES ON SILICA FROM NEUTRAL AQUEOUS-SOLUTIONS AS ESTIMATED FROM HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHIC RETENTION DATA

The equilibrium constants (K) and free energies (-DELTAG) of adsorption of dipeptides and their cyclic derivatives, 2,5-piperazinediones, on silica from neutral aqueous solutions were calculated from their HPLC retention data on a silica gel column. For the majority of the dipeptides -DELTAG values were negative, ranging from -1 170 to 430 J mol-1, and K < 1, thus indicating a very weak adsorption. The 2,5-piperazinediones under study exhibited higher adsorbabilities (-DELTAG -1 070 + 3 030 J mol-1 K > 1) as compared to the respective dipeptides. The effect of the alpha-substituent structure on the adsorbability is analyzed. A linear dependence of -DELTAG on the number of aliphatic carbon atoms in a sorbate molecule was observed for the series of dipeptides and 2,5-piperazinediones derived from bifunctional aliphatic amino acids.