Dependence of the silanophilic effect on the concentration of preadsorbed salts and on the chemical structure of peptides in reversed-phase thin-layer chromatography

Abstract

The retentions of eighteen peptides in reversed-phase thin-layer chromatography were determined with methanol as the organic mobile phase. Before impregnation inorganic salts (LiCl, NaCl, KCl, CaCl 2 and MgCl 2) were preadsorbed on the silica support in the concentration range of 0.5-2 ml of 1 M salt solution per gram of silica. The majority of peptides showed a silanophilic effect: their R M values first decreased to a minimum and then increased with increasing organic phase concentration. The retention of peptides depended quadratically on the methanol concentration and linearly on the salt concentration, on the hydrated ion radii and on the charge of the cation, that is, salts could not mask the silanophilic effect of free silanol groups even at higher concentrations. Principal component analysis (PCA) proved that the linear and quadratic forms of methanol concentrations have a similar effect on the retention of peptides; however, the salt concentration, the hydrated ion radii and charge of cations modify the retention differently. The PC variables did not correlate with the lipophilicity of peptides, proving the lipophilicity modifying effect of salts. No correlation was found between the parameters of quadratic functions fitted for salt-free and salt-containing systems, proving again the lipophilicity-modifying effect of salts. On the two-dimensional non-linear map of PC variables the peptides did not form cluster according to their structural characteristics, proving the participation of each peptide substructure in the peptide-salt interaction. However, the overall polarity of peptides influenced their salt sensitivity.