A peptide experiment takes place among the ions supplied by salts, buffers and other components. Their concentrations and charges help define the ionic environment. Ionic strength is a useful summary of that environment, but it cannot replace the identities of the ions themselves.
Count the ions, including their charge numbers
On a concentration basis, IUPAC defines ionic strength as I = ½ Σ ci zi², summed over all ionic species. Here ci is the concentration of a species and zi is its charge number. Squaring the charge makes a divalent ion contribute differently from a monovalent ion at the same concentration.IUPAC Gold Book — Ionic strength (opens in a new tab)
| Salt input | Ions counted | Ionic strength |
|---|---|---|
| 1 mmol/L NaCl | 1 mmol/L Na+ and 1 mmol/L Cl− | 1 mmol/L |
| 1 mmol/L CaCl2 | 1 mmol/L Ca2+ and 2 mmol/L Cl− | 3 mmol/L |
For the second example the calculation is ½ × (1 × 4 + 2 × 1) = 3 mmol/L. These are arithmetic illustrations that omit other solution components. A real preparation may contain additional ions that must also be counted; the concentration printed for one salt is not necessarily the total ionic strength.
Screening changes the effective interaction
Mobile ions redistribute around charged groups and affect the electrostatic influence those groups exert at a distance. In a dilute-solution screening description, increasing ionic strength generally shortens the characteristic range of that influence. This is distinct from chemically removing the charged group.Jordan and colleagues — Screening and competitive binding in water (opens in a new tab)
Jordan and colleagues explicitly compared screened and unscreened models in an aqueous host–guest system and separately examined competitive binding by solution components. Their work shows why those two mechanisms should not be merged into a single unspecified salt effect.Jordan and colleagues — Screening and competitive binding in water (opens in a new tab)
If screening weakens a repulsive contribution, association may become easier. If it weakens an attractive contribution, an association may become less favourable. The final observation depends on the interactions operating in that system, not just on the word screening.
The same charge number does not make ions identical
Ghosh and colleagues simulated charged-peptide aggregation with different salts. Sodium and potassium, both monovalent, did not produce equivalent aggregate behaviour in their models. Ion identity and valency both mattered in the systems studied.Ghosh and colleagues — Salt effects on charged-peptide aggregation (opens in a new tab)
This is simulation evidence for those systems, not a universal ranking of salts for all peptides. It provides a reason to retain the actual ion names when comparing experiments, even if a calculation gives the same ionic strength.
Interpret a salt-dependent result with its controls
Suppose an illustrative study reports less association after adding a salt. That observation alone does not distinguish screened attraction from competition at a binding site, a conformational change or another effect. A proposed mechanism should be connected to the evidence used to separate those possibilities.
For comparison across papers, record the complete stated solution conditions. If only one salt concentration is reported, do not invent a complete ionic-strength value from it while ignoring unspecified buffer components.
- Which ionic species and concentrations were included?
- Was ionic strength calculated, reported or left unspecified?
- Did pH or other conditions change alongside the salt?
- Is the explanation supported by experiment, simulation or a qualitative model?
Sources and further detail
- IUPAC Gold Book — Ionic strength (opens in a new tab)
Concentration- and molality-based definitions include every ionic species, weighted by squared charge number.
- Jordan and colleagues — Screening and competitive binding in water (opens in a new tab)
JACS 143, 18605–18616 (2021). Primary cavitand host–guest study distinguishes screening from specific competition; its numerical findings are not transferred to peptides.
- Ghosh and colleagues — Salt effects on charged-peptide aggregation (opens in a new tab)
Journal of Chemical Physics 151, 074901 (2019), DOI 10.1063/1.5100890. Molecular simulations show ion-identity-dependent behaviour in the studied peptide systems; not an experimental universal salt ranking.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.