In the late 1950s and early 1960s, Robert Bruce Merrifield developed solid phase peptide synthesis (SPPS). Typically, during solid phase peptide synthesis, the molecule being synthesized (e.g. a growing peptide chain) is attached to an insoluble solid support that is swollen in a certain solvent, while reagents are added to the suspension in a dissolved state. For his pioneering work, the resin used and named after him is a chloromethylated copolymer of styrene and divinylbenzene. This discovery has survived until today, as most resins are still based on a polystyrene core.

In the following, we want to give a summary about the different parameters describing a resin.

Shape and bead size

In general, the smaller the beads are, the faster the reaction kinetics will be, due to the higher surface area to volume ratio. However, if the beads are too small, the filtration time can be extended. For practical purposes a resin with a 100-200 mesh offers the best balance.

Swelling

Reaction kinetics in SPPS are diffusion controlled. Consequently, a resin that swells more will have a higher diffusion rate of reagents into the core of the matrix, resulting in shorter reaction times and more complete chemical conversions.

Especially for the synthesis of large molecules a high swelling ensures sufficient space for the growing molecule as the functional groups are positioned further away from each other and thus peptide aggregation is minimized. Otherwise, deprotection or coupling reactions may be hampered or even completely inhibited, leading to a low yield of the final product. Note: The swelling is solvent dependent and related to the percentage of cross-linkage. As an example, one gram of 1% DVB cross-linked resin will swell 4-6 times its original volume in DCM. In contrast, one gram of 2% DVB cross-linked resin swells only 2-4 times its original volume in DCM. Attaching spacers to the core allows fine-tuning of the resin’s swelling properties.

Cross-linkage

Polystyrene resins are prepared by radical polymerization. The three-dimensional resin network is established by crosslinking the linear polymer chains with divinylbenzene (DVB). In general, the higher the percentage of cross-linkage, the lower the swelling of the corresponding resin in a given solvent. As an example, most polystyrene supports used in SPPS contain 1-2% divinylbenzene (DVB) as a crosslinking agent.

Resin substitution/loading

One of the main considerations when choosing a resin is high-loading vs. low-loading resin. The loading or resin substitution describes the quantity of accessible functional groups. The number of reactive sites is expressed in millimole per gram of resin.

For long or difficult sequences, it is often beneficial to choose a resin with low substitution, as possible side-reactions or interchain entanglements (peptide aggregation, secondary structure formation) are avoided through lower local peptide concentration and enhanced reaction space on the resin.

With a high-loading resin, higher quantities of the peptide can be synthesized per synthetic effort. Thus, such resins have the potential for larger scale peptide production.

➔ Stay tuned, Part 2 describing the different resins available at Iris Biotech as well as their applications will follow in our next newsletter.

➔ You need more information on resins? Download our resin guideline!

References:

Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide; R. B. Merrfield; J. Am. Chem. Soc. 1963; 85(14): 2149-2154. https://doi.org/10.1021/ja00897a025

Preparation, structure and morphology of polymer supports; D. C. Sherrington; Chem. Commun. 1998; 2275-286. https://doi.org/10.1039/a803757d

Calculating Resin Functionalization in Solid-Phase Peptide Synthesis Using a Standardized Method based on Fmoc Determination; O. Al Musaimi, A. Basso, B. G. de la Torre, F. Albericio; ACS Comb. Sci. 2019; 21(11): 717-721. https://doi.org/10.1021/acscombsci.9b00154

Guide for Resin and Linker Selection in Solid-Phase Peptide Synthesis; J. A. Moss; Curr. Protoc. Sci. 2005; 18.7.1-18.7.19. https://doi.org/10.1002/0471140864.ps1807s40

Linkers, resins, and general procedures for solid-phase peptide synthesis; P. Tofteng Shelton, K. J. Jensen; Methods Mol Biol. 2013; 1047: 23. https://doi.org/10.1007/978-1-62703-544-6_2