Properties of swollen polymer networks. Solvation and swelling of peptide-containing resins in solid-phase peptide synthesis

Abstract

Peptide-resins containing linear peptides up to 6000 molecular weight-were prepared by stepwise solid-phase synthesis of the repeating model sequence Leu-Ala-Gly-Val-oxymethylphenylacetic acid on copoly(styrene-l% divinylbenzene) resin beads. Initial substitutions of 0.22 and 0.95 mmol/g of polystyrene were used and the resulting peptide-resins contained 11-81% peptide. Representative samples were taken after the addition of each model sequence unit and the diameters of the dry beads were measured by direct microscopic examination. The volume of 1 g of the unsubstituted dry beads was 1.0 mL and was found to increase by more than fivefold as the peptide content approached 80%. Similar measurements of the samples in CH2Cl2 or DMF showed that the swollen volumes of 1 g of the unsubstituted beads were 6.2 and 3.3 mL, respectively. The volumes of the swollen peptide-resins showed a dramatic increase in the course of the synthesis, and at 80% peptide the volumes/g of polystyrene were 12 mL in CH2Cl2 and 28 mL in DMF. There was no indication that the upper limits of swelling of the peptide-containing resin had been reached. The solvation properties of the cross-linked polymer network and the pendant peptide chains mutually affect one another and at high loadings of peptide in a solvent such as DMF the peptide component has a dominating influence on the swelling of the peptide-resin beads. The swelling of the unsubstituted resin is due to a decrease in free energy from solvation of the polystyrene and, at equilibrium, is balanced by the elastic restraining force resulting from deformation of the loosely cross-linked polymer network. The increased swelling of the peptide-resin can then be attributed to the additional net decrease in free energy from solvation of the linear peptide chains, which is counteracted by an increase in the elastic restraining force arising from further deformation of the loosely cross-linked network structure of the polymer support. No such additional counterforce is expected to arise from deformation of the linear peptide chain imposed by the expansion of the polymeric support. In either CH2Cl2 or DMF, the space available for peptide chain growth within the swollen resin beads is not a limiting factor in solid-phase peptide synthesis. After the synthesis of the 60-residue model there was actually more space within the bead for chain growth than at the beginning of the synthesis. The results of this study allow a rational choice of the level of loading of peptide on the resin and of an appropriate protocol for the synthesis of a particular peptide.