OLIGOGLYCINES AND OLIGOALANINES AS TESTS FOR MODELING MOBILITY OF PEPTIDES IN CAPILLARY ELECTROPHORESIS

Peptides in homologous series of oligoglycines and oligoalanines with degree of polymerisation n = 2 to 6 are baseline separated in 20 mM ci tric acid-lithium citrate buffers in the pH range 2.51 to 3.02. Mobili ties determined as a function of pH allow calculation of the mobility mu+ for the fully protonated peptide. Values of mu+ for oligoglycines are systematically 0.6 . 10(-4) cm2 V-1 s-1 higher than those for olig oalanines, consistent with the 40% greater partial molar volume of ala nine than glycine. Since these mobilities are those of peptides with a constant charge q = 1, semi-empirical models can be systematically te sted for their predictions on how peptide mobilities vary with chain l ength or molecular mass. Within each homologous series the equation in troduced by Grossman et al. [Anal. Biochem., 179 (1989) 28] for scalin g peptide mobility with chain length, mu nz n-0.43, accounts for vari ation in mu+ with excellent correlation coefficients (r > 0.996). Scal ing mu nz M(r)-2-3 according to Offord [Nature, 211 (1966) 591] also gives excellent correlation coefficients (r > 0.996). Since a wide ran ge of exponents can be used to describe the data with good precision, no scaling relationship can be considered uniquely suitable for peptid e mobility modelling.