Free energies of adsorption of amino acids, short linear peptides and 2,5-piperazinediones on silica from water as estimated from high-performance liquid-chromatographic retention data

Equilibrium constants (K) and free energies (-Delta G) of adsorption of amino acids, short linear peptides and cyclic dipeptides (2,5-piperazinediones) on silica from neutral aqueous solutions were calculated from the retention values measured by means of high-performance liquid chromatography on a silica gel column. For most amino acids and linear peptides -Delta G values were negative and K < 1, thus showing very weak adsorption. 2,5-Piperazinediones exhibited higher adsorbability (for most of them, -Delta G > 0 and K > 1) as compared to related dipeptides. Influence of the structure of cr-substituent on the adsorbability is analyzed. A linear dependence of -Delta G on the number of aliphatic carbon atoms in a sorbate molecule was found for the series of aliphatic bifunctional amino acids, related dipeptides and 2,5-piperazinediones, as well as for the group from glycine to triglycyl glycine.