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In a 100 K crystal structure of the peptide analogue Fmoc-Leu-ψ[CH2NCS], researchers identified a short contact between two isothiocyanate groups and interpreted it as stabilized by cooperative σ-hole and π-hole contributions. Their experimental charge-density analysis makes this more than a claim based on a short distance alone—but the result applies to this compound’s crystal, not to biological activity or every isothiocyanate.
What are a σ-hole and a π-hole?
Both terms describe regions of relatively positive electrostatic potential associated with a molecule. A σ-hole is found along the extension of a covalent σ bond, while a π-hole lies above or below a molecular framework or bond region. These regions can help explain why some intermolecular contacts are directional: the way molecules approach matters, not just how close they come. Royal Society of Chemistry coverage discusses σ-holes as relatively positive, low-electron-density regions.
The labels describe different locations and geometries, not mutually exclusive types of contact. A single intermolecular arrangement can involve contributions from both.
How can σ-hole and π-hole interactions work together?
In the reported example, both contributions were assigned to the same contact: N=C=S···N=C=S between neighboring isothiocyanate groups. The authors interpreted their experimental charge-density analysis as showing that σ-hole and π-hole effects act cooperatively to stabilize this contact. In other words, the proposed explanation is not that one hole replaces the other, but that both contribute within one directional intermolecular interaction. The paper’s abstract states this compound-specific finding.
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That interpretation rests on charge-density work, rather than on the observation of a short distance by itself. The paper’s supporting information documents crystallography, refinement, multipole modelling and theoretical charge-density analysis. Those methods provide a basis for analyzing the electron distribution and proposed stabilizing contributions in this crystal.
What did researchers find in the isothiocyanate peptide crystal?
Pal, Nagendra, Samarasimhareddy, Sureshbabu and Guru Row studied Fmoc-Leu-ψ[CH2NCS], an isothiocyanate-based peptide analogue. Their work focused on how its molecules pack in a crystal. The low-temperature structure measured at 100 K contains the highlighted short N=C=S···N=C=S contact. The study was published in Chemical Communications, volume 51, pages 933–936 (2015); the journal record lists its first publication date as 24 November 2014. PubMed’s bibliographic record identifies the article and authors.
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Why did the crystal structure change when it was cooled?
For this sample, the authors reported a reversible isomorphous phase transition on cooling. The RSC author-version manuscript describes it as a single-crystal-to-single-crystal change around 230 K. Both structures are reported in space group P4₁, while the number of independent molecules in the asymmetric unit, Z′, changes from 1 at room temperature to 2 at low temperature. The distinct temperature values refer to different observations: the transition occurs around 230 K, while the contact was analyzed in the structure measured at 100 K. The author-version manuscript reports the transition and crystallographic details.
These measurements describe the behavior of this crystal in this experiment. They do not establish that all isothiocyanate compounds undergo the same transition, nor do they by themselves explain every microscopic factor behind the structural change.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What does the finding establish—and what does it not?
The study provides a specific crystallographic example of weak, directional intermolecular forces contributing to crystal organization, with experimental charge-density analysis supporting a cooperative σ-hole/π-hole interpretation. Such structural work can motivate research into relationships between molecular structure and properties, including structure–activity questions. The Indian Institute of Science annual report also records the cooperative holes as contributing to the molecules’ crystal lattice.
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- Established in the reported study: a short N=C=S···N=C=S contact in the 100 K crystal of Fmoc-Leu-ψ[CH2NCS], interpreted through charge-density analysis as stabilized by cooperative σ-hole and π-hole contributions.
- Not established by this result: that the interaction causes a therapeutic effect, predicts clinical efficacy, or represents a universal bonding mechanism across isothiocyanates.
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