How do you remove aggregates with a HIC monolith?

A HIC monolith is a hydrophobic interaction medium whose ligands sit on the walls of open, through-going channels instead of inside porous beads. MonoCore™ HIC is a modified cellulose monolith with Butyl or Phenyl ligands, 15 µm channels and 1.8 mL or 5.4 mL bed volume, supplied as a capsule with UNF 10/32 connections for standard FPLC systems. Because the sample is carried past the ligands by flow rather than reaching them by diffusion, a HIC polishing step can be run at residence times of seconds rather than minutes.

What the HIC step has to do

Hydrophobic interaction chromatography binds proteins at high salt and releases them as the salt falls. Aggregates expose more hydrophobic surface than the monomer, so they bind more strongly — which is why HIC is a standard tool for aggregate removal in antibody and protein polishing.

Two modes are in use, and the choice decides almost everything about the step:

In bind-and-elute mode the product binds and is eluted in a descending salt gradient, with aggregates trailing at lower salt. This gives the most control and the clearest separation, at the cost of a gradient and a larger buffer volume.

In flow-through mode the salt is set so that only the more hydrophobic species bind, and the monomer passes through. This is faster and simpler, uses less buffer, and is the more common choice when aggregate levels are low and the target is well behaved.

Why the format matters in HIC

Aggregates are large. In a porous bead, they reach the binding sites by diffusing into the pore system, and the larger the species, the slower that is — which is the reason packed beds are operated at residence times of minutes. In a monolith the liquid flows through channels and passes the ligands on the way, so mass transport does not set the pace. The practical consequence is that the same separation can be run at much shorter residence times. Use the residence time calculator to see what a given flow rate means for your bed volume; the mechanism is described in convective vs. diffusive mass transport.

HIC has a second, less obvious property: it runs at high salt. Salt raises viscosity, viscosity raises pressure, and pressure is what limits how fast a packed bed can be run. An open-channel format keeps the pressure drop low, so the salt does not cost you the same amount of speed.

Butyl or Phenyl — how to choose

Butyl is an aliphatic ligand: the interaction is purely hydrophobic and generally milder, which usually means less salt is needed for binding and elution conditions are gentler. Phenyl is aromatic and adds π-interactions on top of the hydrophobic contact, which often changes selectivity for aromatic residues rather than simply binding more strongly.

Which of the two separates your aggregates better is not predictable from the sequence. Screen both on a small capsule with the same salt series, and pick by resolution, not by binding strength. MonoCore™ HIC is available with either ligand in the same format, so the screening runs on identical hardware.

Where a HIC monolith does not help

A HIC step needs salt — typically ammonium sulfate, sodium citrate or sodium chloride at a concentration where the product is still stable. If your molecule aggregates or precipitates under those conditions, no format fixes that, and an ion exchange polishing step is the better route. See chromatography media for AAV polishing for the ion exchange side of the same decision.

Binding capacity is the second thing to settle before a format decision, and it is not a number you should take from an article. It depends on the molecule, the salt condition and the residence time you intend to run. The MonoCore™ HIC datasheet states the conditions we measure under; to put a number on your own feed, record a breakthrough curve at the residence time you intend to run — what a capacity figure does and does not say.

And a HIC monolith does not make a bad buffer system good. If the salt gradient is not resolved on your current medium, the same gradient on a different format will not resolve it either.

How to evaluate one on your own feed

Run the smallest format on the FPLC system you already use. A 1.8 mL capsule with UNF 10/32 fittings needs no re-plumbing. Screen Butyl and Phenyl at two or three salt concentrations, record a breakthrough curve for the mode you intend to run, and compare resolution at the residence time you can afford in the process — not at the residence time that makes the chromatogram look best. Scale-up to 5.4 mL keeps the pressure and flow profile, so a method needs less re-optimisation when volumes grow.

Cation exchange or HIC for bispecific aggregates?

Both remove aggregate, and they fail differently. Cation exchange separates by charge, so it works when the aggregate carries a different net charge than the monomer — common for bispecifics, where mispaired species often differ in pI. Hydrophobic interaction separates by exposed hydrophobic surface, which is what aggregates have more of almost by definition, and it is the more reliable route when the charge difference is small.

In practice the decision is made on the feed, not in advance: screen both, at the same residence time, and compare resolution and yield rather than binding strength. Both chemistries exist on the same modified cellulose matrix — MonoCore™ S for cation exchange, MonoCore™ HIC with Butyl or Phenyl — in the same 1.8 mL and 5.4 mL formats with UNF 10/32 connections, so the screening runs on identical hardware and the comparison is about chemistry alone.

One practical note for bispecifics and other sensitive formats: HIC needs salt and cation exchange needs a pH window, and both can destabilise a molecule that already survived a low-pH capture step. Where the acid step itself is the problem, see why low-pH elution fails on sensitive antibody modalities.

Frequently asked questions

What is a HIC monolith?

A single piece of porous material with through-going channels, carrying hydrophobic ligands on the channel walls. Unlike a packed bed of beads, it has no interstitial volume between particles and no diffusion path into a bead interior. MonoCore™ HIC uses a modified cellulose monolith with 15 µm channels, with Butyl or Phenyl ligands.

Butyl or Phenyl for aggregate removal?

Butyl is aliphatic and usually the milder ligand; Phenyl is aromatic and shifts selectivity through additional π-interactions. Neither is generally better for aggregates — screen both under the same salt conditions and choose by resolution.

Can HIC be run in flow-through mode?

Yes, and for polishing it is often the preferred mode: the salt is set so that the monomer does not bind while the more hydrophobic aggregates are retained. It needs less buffer than a gradient elution and is easier to run at scale, but it leaves less room to correct a poorly chosen salt concentration.

How much salt does a HIC step need?

Enough for the product to bind, and no more than the product tolerates. The working point depends on the molecule and the salt: kosmotropic salts such as ammonium sulfate promote binding at lower concentrations than sodium chloride. Determine it by screening, and check aggregation and solubility of the product at the loading condition before you fix the method.

How do I measure binding capacity on a HIC monolith?

Load continuously at a fixed flow rate, record UV against volume, and read the load at 10 % breakthrough — corrected for the hold-up volume of the system. The arithmetic is the loaded mass divided by the bed volume; the hold-up correction is the part that is easy to get wrong, and how to read a capacity figure covers it. Report the value together with the residence time, because without it the number cannot be compared.