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<div>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. </div>
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<span style="font-weight:bold">From: </span>John Sumida <<a href="mailto:jpsumida@u.washington.edu">jpsumida@u.washington.edu</a>><br>
<span style="font-weight:bold">Date: </span>Tuesday, January 20, 2015 at 8:00 PM<br>
<span style="font-weight:bold">To: </span>"<a href="mailto:rasmb@rasmb.org">rasmb@rasmb.org</a>" <<a href="mailto:rasmb@rasmb.org">rasmb@rasmb.org</a>><br>
<span style="font-weight:bold">Subject: </span>[RASMB] concentration range for non-ideal behavior<br>
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<p class="MsoNormal"><span style="font-size: 12pt; font-family: 'Courier New';">Dear RASMB,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 12pt; font-family: 'Courier New';"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size: 12pt; font-family: 'Courier New';">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 12pt; font-family: 'Courier New';"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size: 12pt; font-family: 'Courier New';">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 12pt; font-family: 'Courier New';"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size: 12pt; font-family: 'Courier New';">Best regards<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 12pt; font-family: 'Courier New';">John Sumida<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 12pt; font-family: 'Courier New';">University of Washington<o:p></o:p></span></p>
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