Aqueous Capillary Electrophoresis of Peptides Part 1: Conformational States, Volumes and Estimates of the Extent of Hydration

Summary A series of eighteen peptides have been modelled. Of the 380 low-energy viable conformers generated, 64 were selected, with 2–5 per peptide representing the range of structures and size. Using the results of previous investigations of this peptide group, it was possible to place an upper limit to the extent of hydration for each peptide. The modelled peptides were then hydrated in stepwise fashion. From the volumetric and dimensional data derived, in conjunction with published mobilities, the relationship between electrophoretic mobility ( _ep ) and the hydrodynamic radius (r) was used to objectively test the fit between the experimental ( _ep ) data and and the modelled size parameters over the range of hydration. Where the fit was tightest it was presumed that the fraction of the maximum reasonable hydration best represented the average degree of hydration of the set. Using ellipsoidal volumes, Connolly surfaces and excluded volumes the average degree of hydration was found to be in the mid range considered and corresponded to the mean of the bimodal distribution of hydration for the individual peptides according to charge-based calculations. For a peptide with two positive charges and a partial negative charge on the carboxy residue (pH=2.5), about 15 waters of hydration are indicated. With 3, 5 and 6 positive charges, the estimated waters are 28, 48 and 85, respectively. Van der Waals volumes were not helpful as the precise summation of the volumes associated with each bonded atom is a poor reflection of the effective volume of the migrating peptide. Conformational changes are examined as a function of the progressive hydration, and, as might be expected, the greatest changes occur in the early stages of hydration.