Covalent Organic Framework Membranes Enable Precise Molecular Sieving of Lower-Molecular-Weight Peptides

<title>Abstract</title> <p>Ultrafiltration membrane is routinely used to isolate bioactive peptides from diverse high molecular weight protein hydrolysates, but its broad pore-size distribution makes efficient separation of highly active low-molecular-weight peptides (LMWPs, Mw = 180-1200 Da) difficult, typically requiring energy-intensive reversed-phase chromatography. Nanofiltration membrane technology emerges as a promising and eco-friendly alternative for LMWPs separation. However, most commercial amorphous polymer nanofiltration membranes possess ill-defined pore architectures, rendering the precise selective separation of LMWPs and elucidation of their transport mechanisms a formidable challenge. Here, we present a side-chain engineering strategy that involves grafting side chains with varying degrees of freedom onto the channels of a regular covalent organic framework (COF) membrane to regulate the spatial conformation of the membrane channels. It is found that rigid alkyl side chains (C) serve as a molecular barrier, effectively decreasing pore space and preserving the stable structure of COF pore channels compared to flexible alkyl ether side chains (PEG), resulting in an impressive separation selectivity of 426 for LMWPs with a molecular weight difference of only 950 Da, which is 30 times higher than existing commercial nanofiltration membranes. Notably, the permeation flux, approximately 7 g·h-1, is comparable to that of current commercial preparative-scale liquid chromatography columns (50 × 150 mm, C18). Interestingly, it disclosed that the membrane's separation of LMWPs is not only related to their molecular weight but also to the conformational flexibility of the peptides. This work offers valuable insights for the design of membrane materials aimed at separating LMWPs with flexible conformations and deepens the understanding of the separation mechanisms involved.</p>