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Molarity

Amount of substance per unit volume, in moles per litre — the concentration unit that is comparable between compounds.

Molarity expresses concentration as amount of substance per unit volume — moles per litre — rather than mass per volume. Converting from one to the other requires the compound's molecular weight: molarity equals grams per litre divided by grams per mole.

It matters because mass is not a count. Research peptides span roughly 700 to 5,000 g/mol, so the same mass of a small peptide and a large one are very different numbers of molecules. Two solutions at 1 mg/mL of different compounds are not comparable; two at 1 mM are.

Because peptides are large, solutions in the low mg/mL range land in the low millimolar range, and working dilutions are usually quoted in micromolar. Converting between them requires the compound's molecular weight.

Topic: Lab Practice

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Counterion quantification by ion chromatography

The counterion can be the largest non-peptide term in a lyophilised solid, and it is not inferable from the sequence. How suppressed-conductivity ion chromatography measures trifluoroacetate, acetate, chloride and the common cations; how to dilute into the linear range; what system suitability to fix for a small anion; and how to read a molar ratio that lands far from expectation.

Zeta Potential and Electrophoretic Mobility of Peptide Assemblies

An electrophoretic light scattering instrument measures mobility; the zeta potential it prints is what a chosen model makes of that mobility. Which model applies depends on kappa-a, and the two common limits differ by a factor of 1.5 in the potential returned from the same mobility.

Weighing a Milligram: Minimum Sample Weight in Practice

Repeatability and the tolerance you commit to fix a floor below which a weighed mass carries an offset you cannot see. What static charge, buoyancy and ambient water do to a milligram of dry powder, and how that error reappears unchanged in every dilution.

The Second Virial Coefficient by Static Light Scattering

The osmotic second virial coefficient reports the net of everything a pair of molecules does to each other, read from the slope of a light-scattering dilution series. What that slope establishes, what the intercept does not, and where self-interaction chromatography takes over when the analyte is too small to scatter.

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