In a crystal of the peptide analogue Fmoc-Leu-ψ[CH2NCS], researchers identified a short, directional contact between two isothiocyanate groups and interpreted it as being stabilized by cooperative σ-hole and π-hole contributions. Their evidence came from experimental charge-density analysis of the low-temperature structure—not simply from noticing that two groups sit close together. The result applies to this compound and crystal, not to every isothiocyanate or to a biological effect.
What are σ-holes and π-holes?
Both terms describe regions of relatively low electron density that can be relatively electropositive, but they refer to different locations in a molecule.
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σ-hole
A σ-hole is an electropositive region associated with the extension of a covalent σ bond. Its location and direction can help explain why an intermolecular contact is favored in a particular orientation.
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π-hole
A π-hole is an electropositive region above or below a molecular framework or bond region. It can provide another site for an approaching molecule to interact with.
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These descriptions are not competing explanations: a single intermolecular contact can involve contributions from both kinds of region. The interaction discussed here is directional, rather than a claim that any two isothiocyanate groups attract in any orientation. The authors’ paper and Chemistry World’s contemporary coverage provide the basis for this interpretation.
What did researchers find in the isothiocyanate peptide crystal?
Rumpa Pal, Govindappa Nagendra, M. Samarasimhareddy, Vommina V. Sureshbabu and Tayur N. Guru Row studied Fmoc-Leu-ψ[CH2NCS], an isothiocyanate-based peptide analogue. In its low-temperature crystal structure, measured at 100 K, they observed a short intermolecular N=C=S···N=C=S contact. They characterized the contact as stabilizing, with σ-hole and π-hole contributions acting cooperatively. The RSC paper abstract identifies this as an experimental charge-density result.
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That distinction matters: a short distance in a crystal structure is an observation, but by itself it does not establish what holds the molecules together or how electron density is distributed. The authors’ interpretation draws on charge-density analysis. The supporting information documents the crystallography, refinement, multipole modelling and theoretical charge-density analysis behind the structural interpretation.
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In this example, the two labels describe complementary electropositive features contributing to the same N=C=S···N=C=S intermolecular contact. The authors’ conclusion is not that one hole replaces the other, but that both contribute cooperatively to stabilization. This offers a more specific account of the contact than its length alone: geometry and the experimentally modelled electron density support the proposed interaction.
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The result is an example of how weak, directional intermolecular interactions can help organize molecules in a crystal lattice. It does not establish a universal bonding mechanism, and it does not show that the compound has a therapeutic effect.
Why did the crystal structure change when it was cooled?
For this sample, the authors reported a reversible isomorphous phase transition on cooling. The author-version manuscript describes it as a single-crystal-to-single-crystal change around 230 K. Both structures were reported in space group P4₁, while Z′—the number of crystallographically independent molecules in the asymmetric unit—changed from 1 at room temperature to 2 at low temperature. The highlighted contact is in the structure measured at 100 K, a distinct temperature from the transition point. The author-version manuscript reports these transition details.
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The reported structural change is evidence about this compound under the study’s conditions. It should not be generalized to all isothiocyanates. The paper appeared in Chemical Communications, volume 51, pages 933–936 (2015); the RSC record lists its first publication date as 24 November 2014. PubMed’s bibliographic record provides publication details.
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Weak directional contacts can matter to how molecules pack and to the properties of a solid. The researchers presented this type of structural understanding as relevant to studying relationships between molecular structure and activity, including in pharmaceutical research. That is motivation for further investigation, not evidence that this peptide analogue is an anticancer treatment or that this one crystal contact predicts biological activity. The Indian Institute of Science annual report likewise describes the cooperative holes in the context of holding molecules together in the crystal lattice.
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