Minimizing the Influence of Fluorescent Tags on IgG Partition in PEG–Salt Aqueous Two‐Phase Systems for Rapid Screening Applications

Aqueous two‐phase extraction (ATPE) has been showing significant potential in the biopharmaceutical industry, allowing the selective separation of high‐value proteins directly from unclarified cell culture supernatants. In this context, effective high‐throughput screening tools are critical to perform a rapid empirical optimization of operating conditions. In particular, microfluidic ATPE screening devices, coupled with fluorescence microscopy to continuously monitor the partition of fluorophore‐labeled proteins, have been recently demonstrated to provide short diffusion distances and rapid partition, using minimal reagent volumes. Nevertheless, the currently overlooked influence of the labeling procedure on partition must be carefully evaluated to validate the extrapolation of results to the unlabeled molecule. Here, three fluorophores with different global charge and reactivity selected to label immunoglobulin G (IgG) at degrees of labeling (DoL) ranging from 0.5 to 7.6. Labeling with BODIPY FL maleimide (DoL = 0.5), combined with tris(2‐carboxyethyl) phosphine (TCEP) to generate free thiol groups, is the most promising strategy to minimize the influence of the fluorophore on partition. In particular, the partition coefficient (K_p) measured in polyethylene glycol (PEG) 3350–phosphate systems with and without the addition of NaCl using microtubes (batch) or microfluidic devices (continuous) is comparable to those quantified for the native protein.