Can higher acetonitrile concentration improve peptide and peptidomimetic SPE clean-up purity and recovery?

To discover new lead compounds, researchers rely on peptide and peptoid screening, fueling the need for fast, small-scale synthesis methods to create diverse libraries. However, downstream purification often remains a bottleneck, especially when balancing speed, reproducibility, and cost. To address this challenge, Biotage® developed PeptiRen-96, a C18-based, 96-well SPE plate designed to simplify and accelerate crude peptide and peptoid clean-up.  In this blog, I explore how this SPE strategy performs using N(FkF), a short, aromatic peptoid known for its self-assembling properties and relevance in nanomaterial design (Figure 1). Its hydrophobic and aromatic character also makes it an ideal model for evaluating MeCN gradient effects on crude peptide clean-up.   

Figure 1: Structure of peptoid N(FkF).

These gradients were designed based on flash purification principles. In typical reversed-phase flash chromatography, gradient development considers both the elution strength of the solvent and where the compound of interest elutes. For instance, if a compound elutes at 35% MeOH, a standard gradient might begin at 10-20% to remove polar solvents like DMF or DMSO for 3 column volumes (CVs), followed by a ramp to 70% over 10 CVs, and ending with a final push to 100% to flush impurities. In this case, I previously determined that peptoid N(FkF) elutes around 24% MeCN.  

Building on the initial elution estimate, I designed two SPE gradients to test whether a modest increase in MeCN concentration could improve product recovery without sacrificing practical purity. This comparison focused on where the product collected, how much product remained in later fractions, and whether the workflow could be simplified for high-throughput clean-up. 

To determine how increased MeCN concentration affects recovery and purity, two gradients were tested using PeptiRen-96 on the Biotage® Extrahera™ Classic. Both methods began with conditioning in 100% MeCN containing 0.1% TFA, followed by equilibration with 100% H2O containing 0.1% TFA. Samples were loaded in methanol, then processed through wash 1 at 100% H2O containing 0.1% TFA, wash 2 at 5% MeCN containing 0.1% TFA, elution 1 at either 24% or 30% MeCN containing 0.1% TFA, and elution 2 (flush) at 70% MeCN containing 0.1% TFA. 

• Gradient A followed a stepwise gradient with wash 1 at 100% H2O, wash 2 at 5% MeCN, elution 1 at 24% MeCN, and elution 2 (flush) at 70% MeCN, all containing 0.1% TFA. 
• Gradient B mirrored Gradient A but used 30% MeCN in place of 24% for elution 1. 

A visual comparison of these gradients beginning at the equilibration step is shown in table 1 below.

Table 1.  Comparison of SPE Method used on Biotage® PeptiRen-96 

For purification:

• Gradient A used 5.2 mg of N(FkF) dissolved in 0.375 mL MeOH.
• Gradient B used 9.8 mg in the same solvent volume. 

Conditioning was performed at 100% MeCN to wet the sorbent, followed by equilibration and wash 1 with 100% H2O to promote sample retention and minimize premature elution. Wash 2, containing 5% MeCN, provided a mild organic wash before the targeted elution step. For Gradient A, elution 1 was set to 24% MeCN based on previously determined elution point of N(FkF) by analytical C18 column, with the goal of collecting the bulk of the product in this fraction. The final elution step was set at 70% MeCN by adding 50% to the 24% target elution concentration and rounding the resulting 74% to maintain approximate solvent concentrations. This stronger elution step was included to recover any product that remained retained on the sorbent, and, therefore used to evaluate whether most of the product had been successfully collected during the 24% MeCN elution. Gradient A served as the baseline for determining whether a modest increase in the primary elution concentration, from 24% to 30% in Gradient B, could improve recovery and simplify downstream handling.  

The 5%, 24% or 30%, and 70% MeCN steps were collected and then analyzed for both mass recovery and purity. 

Results

The results were as follows in table 2 below.

Table 2. Recovery and purity results for Gradient A and Gradient B

The 24% MeCN gradient split product between two fractions, while the 30% step consolidated recovery into a single, product-rich fraction. This demonstrates that increasing the mid-step MeCN concentration reduces product tailing, simplifying workflows, and minimizing the need for further fraction analysis or recombination. Although the 24% mid-step achieved 100% purity, it left a significant amount of product behind in the 70% MeCN fraction, co-eluting with impurities. In contrast, the 30% step recovered 88% purity in one fraction, offering a more practical and efficient solution particularly in high-throughput settings where time, solvent use, and sample handling are critical. 

Biotage® PeptiRen-96 provides robust reversed-phase separation for aromatic compounds like N(FkF), and when integrated with the Biotage® Extrahera™ Classic, delivers reproducible, automated workflow solutions ideal for screening, sequence refinement, and early-stage bioassay development.

Conclusion

Increasing the mid-step MeCN concentration from 24% to 30% improved recovery and workflow efficiency by collecting most of the product in the expected elution step while minimizing breakthrough into the preceding and subsequent steps. Although 24% MeCN delivered high purity, it also caused product tailing and added workflow complexity. By contrast, the 30% MeCN step achieved high recovery and strong purity in a single fraction, making it a practical option for rapid crude screening workflows. 

Accelerate peptide and peptoid purification

Looking to improve throughput while reducing manual fraction handling? Learn how PeptiRen-96 and automated SPE workflows can streamline peptide and peptoid purification.