Protein instability during HIC: describing the effects of mobile phase conditions on instability and chromatographic retention

Retention Behavior of Polyethylene Glycol and Its Influence on Protein Elution on Hydrophobic Interaction Chromatography Media

The retention behavior of polyethylene glycol (PEG) on different types of hydrophobic interaction chromatography (HIC) resins containing butyl, octyl, and phenyl ligands was analyzed. An incomplete elution or splitting of the polymer peak into two parts was observed, where the first one was eluted at the dead time of the column, whereas the second one was strongly retained. The phenomenon was attributed to conformation changes of the polymer upon its adsorption on hydrophobic surface. The effect enhanced with increasing molecular weight of the polymer and hydrophobicity of the HIC media. Addition of PEG to the mobile phase reduced binding of proteins to HIC resins, which was demonstrated with two model systems: lysozyme (LYZ) and immunoglobulin G (IgG), and their mixtures. In case of LYZ, the presence of PEG caused reduction in the protein retention, whereas for IgG—a decrease in efficiency of the protein capture. The effect depended on the adsorption pattern of PEG; it was pronounced in the systems in which conformational changes of the polymer were suggested to occur.

Impact of organic modifier and temperature on protein denaturation in hydrophobic interaction chromatography

The goal of this study was to better understand the chromatographic conditions in which monoclonal antibodies (mAbs) of broad hydrophobicity scale and a cysteine conjugated antibody-drug conjugate (ADCs), namely brentuximab-vedotin, could denaturate. For this purpose, some experiments were carried out in HIC conditions using various organic modifier in natures and proportions, different mobile phase temperatures and also different pHs. Indeed, improper analytical conditions in hydrophobic interaction chromatography (HIC) may create reversed-phase (RP) like harsh conditions and therefore protein denaturation. In terms of organic solvents, acetonitrile (ACN) and isopropanol (IPA) were tested with proportions ranging from 0 to 40%. It appeared that IPA was a less denaturating solvent than ACN, but should be used in a reasonable range (10-15%). Temperature should also be kept reasonable (below 40°C), to limit denaturation under HIC conditions. However, the combined increase of temperature and organic content induced denaturation of protein biopharmaceuticals in all cases. Indeed, above 30-40°C and 10-15% organic modifier in mobile phase B, heavy chain (HC) and light chain (LC) fragments dissociated. Mobile phase pH was also particularly critical and denaturation was significant even under moderately acidic conditions (pH of 5.4). Today, HIC is widely used for measuring drug-to-antibody ratio (DAR) of ADCs, which is a critical quality attribute of such samples. Here, we demonstrated that the estimation of average DAR can be dependent on the amount of organic modifier in the mobile phase under HIC conditions, due to the better recovery of the most hydrophobic proteins in presence of organic solvent (IPA). So, special care should be taken when measuring the average DAR of ADCs in HIC.

Thermodynamics of coupled protein adsorption and stability using hybrid Monte Carlo simulations

A better understanding of changes in protein stability upon adsorption can improve the design of protein separation processes. In this study, we examine the coupling of the folding and the adsorption of a model protein, the B1 domain of streptococcal protein G, as a function of surface attraction using a hybrid Monte Carlo (HMC) approach with temperature replica exchange and umbrella sampling. In our HMC implementation, we are able to use a molecular dynamics (MD) time step that is an order of magnitude larger than in a traditional MD simulation protocol and observe a factor of 2 enhancement in the folding and unfolding rate. To demonstrate the convergence of our systems, we measure the travel of our order parameter the fraction of native contacts between folded and unfolded states throughout the length of our simulations. Thermodynamic quantities are extracted with minimum statistical variance using multistate reweighting between simulations at different temperatures and harmonic distance restraints from the surface. The resultant free energies, enthalpies, and entropies of the coupled unfolding and absorption processes are in qualitative agreement with previous experimental and computational observations, including entropic stabilization of the adsorbed, folded state relative to the bulk on surfaces with low attraction.

Changes in solvent exposure reveal the kinetics and equilibria of adsorbed protein unfolding in hydrophobic interaction chromatography

Hydrogen exchange has been a useful technique for studying the conformational state of proteins, both in bulk solution and at interfaces, for several decades. Here, we propose a physically based model of simultaneous protein adsorption, unfolding and hydrogen exchange in HIC. An accompanying experimental protocol, utilizing mass spectrometry to quantify deuterium labeling, enables the determination of both the equilibrium partitioning between conformational states and pseudo-first order rate constants for folding and unfolding of adsorbed protein. Unlike chromatographic techniques, which rely on the interpretation of bulk phase behavior, this methodology utilizes the measurement of a molecular property (solvent exposure) and provides insight into the nature of the unfolded conformation in the adsorbed phase. Three model proteins of varying conformational stability, alpha-chymotrypsinogen A, beta-lactoglobulin B, and holo alpha-lactalbumin, are studied on Sepharose HIC resins possessing assorted ligand chemistries and densities. alpha-Chymotrypsinogen, conformationally the most stable protein in the set, exhibits no change in solvent exposure at all the conditions studied, even when isocratic pulse-response chromatography suggests nearly irreversible adsorption. Apparent unfolding energies of adsorbed beta-lactoglobulin B and holo alpha-lactalbumin range from -4 to 3 kJ/mol and are dependent on resin properties and salt concentration. Characteristic pseudo-first order rate constants for surface-induced unfolding are 0.2-0.9 min(-1). While poor protein recovery in HIC is often associated with irreversible unfolding, this study documents that non-eluting behavior can occur when surface unfolding is reversible or does not occur at all. Further, this hydrogen exchange technique can be used to assess the conformation of adsorbed protein under conditions where the protein is non-eluting and chromatographic methods are not applicable.

A new thermodynamic model describes the effects of ligand density and type, salt concentration and protein species in hydrophobic interaction chromatography

A new thermodynamic model is derived that describes both loading and pulse-response behavior of proteins in hydrophobic interaction chromatography (HIC). The model describes adsorption in terms of protein and solvent activities, and water displacement from hydrophobic interfaces, and distinguishes contributions from ligand density, ligand type and protein species. Experimental isocratic response and loading data for a set of globular proteins on Sepharose resins of various ligand types and densities are described by the model with a limited number of parameters. The model is explicit in ligand density and may provide insight into the sensitivity of protein retention to ligand density in HIC as well as the limited reproducibility of HIC data.

QCM-D sensitivity to protein adsorption reversibility

Using a quartz crystal microbalance with dissipative monitoring (QCM-D) we have determined the adsorption reversibility and viscoelastic properties of ribonuclease A adsorbed to hydrophobic self-assembled monolayers. Consistent with previous work with proteins unfolding on hydrophobic surfaces, high protein solution concentrations, reduced adsorption times, and low ammonium sulfate concentrations lead to increased adsorption reversibility. Measured rigidity of the protein layers normalized for adsorbed protein amounts, a quantity we term specific dissipation, correlated with adsorption reversibility of ribonuclease A. These results suggest that specific dissipation may be correlated with changes in structure of adsorbed proteins.

Generalizing a two-conformation model for describing salt and temperature effects on protein retention and stability in hydrophobic interaction chromatography

A two-conformation adsorption model that includes the effects of salt concentration and temperature on both stability and adsorption has been developed to describe the effects of secondary protein unfolding on hydrophobic interaction chromatography (HIC). The model has been applied to a biotech protein and to beta-lactoglobulin on Phenyl Sepharose 6FF low sub HIC media. Thermodynamic property models for adsorption and protein stability with parameters obtained from experimental chromatographic data successfully describe observed chromatographic behavior over ranges of temperature and salt concentration, provide predictions of distribution among different conformers, and give a basis for calculating trends in retention strength and stability with changing conditions, that might prove useful in HIC process development.

Protein instability during HIC: Hydrogen exchange labeling analysis and a framework for describing mobile and stationary phase effects

Unfolding of marginally stable proteins is a significant factor in commercial application of hydrophobic interaction chromatography (HIC). In this work, hydrogen-deuterium isotope exchange labeling has been used to monitor protein unfolding on HIC media for different stationary phase hydrophobicities and as a function of ammonium sulfate concentration. Circular dichroism and Raman spectroscopy were also used to characterize the structural perturbations experienced by solution phase protein that had been exposed to media and by protein adsorbed on media. As expected, greater instability is seen on chromatographic media with greater apparent hydrophobicity. However, increased salt concentrations also led to more unfolding, despite the well-known stabilizing effect of ammonium sulfate in solution. A thermodynamic framework is proposed to account for the effects of salt on both adsorption and stability during hydrophobic chromatography. Using appropriate estimates of input quantities, analysis with the framework can explain how salt effects on stability in chromatographic systems may contrast with solution stability.