[RASMB] concentration range for non-ideal behavior

[John J. Correia]

John, The nonideality above 1 mg/ml rule depends upon the numerical contribution from a (1 + Ksc) term where c is on a weight scale. Ks is phenomenological and not easy to guess at. If Ks = 0.01 for example at 1 mg/ml the term starts to be significant. Thus if Ks is smaller than higher concentrations are required to nonideality effects. Since nonideality is either charge and/or excluded volume dependent, small molecules will not contribute much and uncharged molecules will not contribute. Thus working with small macromolecules at their pI will give ideal behavior over a wide range of concentrations. In addition mAb’s often self-associate albeit weakly thus potentially masking nonideality.

From: John Sumida <jpsumida at u.washington.edu<mailto:jpsumida at u.washington.edu>>
Date: Tuesday, January 20, 2015 at 8:00 PM
To: "rasmb at rasmb.org<mailto:rasmb at rasmb.org>" <rasmb at rasmb.org<mailto:rasmb at rasmb.org>>
Subject: [RASMB] concentration range for non-ideal behavior

Dear RASMB,

My recollection is that concentrations in excess of 1.0 mg/ml should be avoided due to the possibility of non-ideal interactions that would complicate the analysis of SV and SE measurements.  However in two recent cases (in one case a mAb was being studied, and in another a globular viral capsid protein), we exceeded this value by a factor of five and failed to observe any evidence of non-ideal behavior. In both cases, it was possible to fit both SV and SE data without assuming any non-ideal interactions when the experiment was performed in PBS or HEPES.

I would be curious to hear if it is the experience of the community that non-ideality is commonly to be expected at concentrations >1.0mg/ml or whether the concentration cutoff for non-ideality is less of an actual cutoff being more a function of the particular properties of the molecule being studied.

Best regards
John Sumida
University of Washington
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