Squaramide macrocycles that prefer sulfate in water
Meta-linked squaramide macrocycles bind SO42− so tightly in wet DMSO that NMR cannot measure Ka, and they still select sulfate in 50% water.
Source
Macrocyclic squaramides: anion receptors with high sulfate binding affinity and selectivity in aqueous media
Study at a glance
- Design
- Other — Synthesis and aqueous NMR titration of squaramide macrocycles for sulfate recognition
- N
- Anion-receptor binding study — no sample N
- Population
- Macrocyclic squaramide receptors in water/DMSO mixtures
- Outcome
- Sulfate binding affinity and selectivity versus competing anions
Structured fields used in claim comparison tables when every cited study has a complete layer.
Inspect source passages
Squaramide macrocycles that prefer sulfate in water
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
1 and 2 bind SO42− with Ka > 10^4 M−1 by NMR.
Both 1 and 2 were found to bind to SO 4 2− with affinities too high to measure accurately (>10 4 M −1 ) using NMR but significant differences in binding affinities for the other anions were observed, with [3]-MSQ 2 showing excellent selectivity for SO 4 2− , whereas 1 also bound strongly to H 2 PO 4 − and AcO − ions.
Sulfate hydration free energy is −1090 kJ mol−1.
However, binding of sulfate in aqueous solution is very challenging as a result of the high hydration energy of this anion (Δ G = −1090 kJ mol −1 ), which poses additional problems in terms of receptor selectivity in water since other anions, e.g.
What they did
Authors synthesized [2]- and [3]-squaramide macrocycles, titrated anions by 1H NMR in 0.5–50% water/DMSO, crystallized a 1:1 sulfate complex, and compared selectivity with chromate and with the sulfate-binding protein benchmark.
What they found
Meta receptors 1, 2, 5, and 6 are potent, selective sulfate ligands even in 50% water. 1 and 2 have Ka(SO42−) > 10^4 M−1; 2 is more sulfate-selective versus phosphate and acetate. The crystal shows a bowl/cone geometry with four NH···O hydrogen bonds. Receptor 6 discriminates sulfate from chromate better than the sulfate-binding protein.
The limits
What it doesn't show
Process-scale nuclear-waste extraction and clinical plasma sulfate assays are suggested applications, not demonstrated; some isomers are too insoluble for full Ka tables.
Key terms
- Squaramide
- Four-membered ring diamide that is a strong hydrogen-bond donor for anions.
- MSQ
- Macrocyclic squaramide receptor ([2] or [3] repeat).
- Sulfate hydration penalty
- ΔG ≈ −1090 kJ mol−1 makes aqueous sulfate recognition hard.
- 1:1 bowl complex
- Crystal motif of 1·SO42− resembling a calixarene cone.
- Selectivity ratio
- Ka(sulfate)/Ka(competitor); for 6, Ka(sulfate) is at least 10^4 M−1.
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The preferred anion is:
Common questions
Which anion is preferred?
Sulfate, even in highly aqueous DMSO.
How tight is binding of 1 and 2 to sulfate?
Ka > 10^4 M−1, too high for accurate NMR.
What is the crystal geometry?
1:1 bowl/cone with C2v-like symmetry.
Why is aqueous sulfate hard?
Huge hydration energy (~−1090 kJ mol−1).
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