Mixed-bed chromatography as a way to resolve peculiar protein fractionation situations

Mixed-bed chromatography is based on the use of multiple sorbents mixed together and packed in a single column. Solid-phase combinatorial libraries are a current example of mixed-bed chromatography with a large number of immobilized affinity ligands each of them attached to a different bead. They have been repeatedly reported to reduce the dynamic protein concentration range from biological extracts when used in large overloading conditions. By that way trace proteins can easily be enhanced and analyzed. When mixed libraries of ligands are used in under-saturation conditions they constitute a generic way to remove minor impurity traces still present in even highly purified biological product. Whereas ligand libraries are generally used in a neutral environment for the capturing phase, they can also be used in acidic or alkaline conditions with specific advantages due to the modulation of affinity constants. Mixed-bed cascades involving affinity libraries or ion exchangers are also approaches allowing protein fractionation. This review reports various applications of mixed-bed chromatography related to protein fractionation not only for proteomics investigations, but also for preparative purposes.