Relating electrospray ionization response to nonpolar character of small peptides

Nonpolar regions in biological molecules are investigated as a determining factor governing their electrospray ionization (ESI) mass spectrometric res ponse. Response is compared for a series of peptides whose C-terminal resid ue is varied among amino acids with increasingly nonpolar side chains. Incr eased ESI response is observed for peptides with more extensive nonpolar re gions. The basis for this increase is examined by comparing values of nonpo lar surface area and Gibbs free energy of transfer for the different amino acid residues. Comparisons of response with octadecylamine are also made, a nd this highly surface-active ion is observed to outcompete all other analy tes in ESI response. These observations are rationalized on the basis of th e equilibrium partitioning model, which is used successfully to fit experim ental data throughout the concentration range for several two-analyte syste ms. This model suggests that because excess charge exists on ESI droplet su rfaces, an analyte's relative affinity for the droplet surface determines i ts relative ESI response. Increased nonpolar character, which leads to enha nced affinity for the surface phase, results in more successful competition for excess charge and higher ESI response.