Parallel electromembrane extraction of peptides with monoterpene and medium-length fatty acid deep eutectic solvents

Electromembrane extraction (EME) involves the process of mass transfer of charged analytes from an aqueous sample through an organic liquid membrane into an aqueous acceptor medium under the influence of an electrical field. Successful solvation of the analyte within the liquid membrane is of paramount importance and involves molecular interactions with the liquid membrane. In this comprehensive investigation, parallel EME was examined using a training set of 13 model peptides employing deep eutectic solvents as the liquid membrane. These deep eutectic solvents were formulated by mixing specific monoterpenes (thymol, menthol, camphor) with medium-chain fatty acids (1-octanoic acid and 1-decanoic acid).

From an array of different liquid membrane compositions explored, it was revealed that the combination of camphor and 1-decanoic acid (in a 1:1 w/w ratio) with 2% di (2-ethylhexyl) phosphate (DEHP) delivered the most efficient extraction system. The solvation of the model peptides within this liquid membrane predominantly relied on ionic interactions between protonated basic functionalities and DEHP, along with hydrogen bond interactions between the deprotonated acid functionalities (hydrogen bond acceptor) and 1-decanoic acid (hydrogen bond donor). Selectivity was modulated by the pH of the sample and acceptor solutions, with a direct correlation to the polarity and net charge of the model peptides. The ionization of 1-decanoic acid in the interfacial region between the sample and liquid membrane emerged as an important factor influencing the selectivity.

Although parallel EME of peptides has been reported previously, the current liquid membrane provides an extraction system with sufficient stability for the first time. Selective extraction of peptides through EME holds substantial promise within the realm of next-generation environmentally-friendly sample preparation methodologies. The findings presented in this paper contribute significantly to our fundamental understanding of these processes, and may serve as an important reference for the development of future methods in this field.