Counterion

The oppositely charged ion that accompanies a charged peptide in its solid form — part of the powder's weight, but not part of the peptide.

Peptides with basic residues carry positive charges, and electrical neutrality requires negative ions alongside them in the solid. Those are the counterions — most commonly trifluoroacetate carried over from purification, or acetate after a deliberate exchange, with chloride appearing in some pharmaceutical contexts. Together with residual water, the counterion is why a vial's powder weighs more than the peptide it contains.

The counterion fraction is not trivial. Depending on how many basic residues a sequence has, counterions can account for a tenth or more of the powder's mass — mass that a gross-weight label counts and an HPLC purity figure never sees, since the measurement is blind to species that do not absorb at the detection wavelength.

Which counterion is present is a property of the batch, not the compound, and it is established by analysis rather than assumption. Certificates that report counterion content make the powder's composition explicit; most research-grade certificates do not, leaving the salt form as a stated or assumed property of the product rather than a measured one.

For research purposes only. Not for human consumption, diagnosis, treatment, or prevention of any condition. Nothing on this page is medical advice.