A robust hybrid peptide crystal formed with weak hydrogen bonds

Area of Science:

• Organic Chemistry
• Crystallography
• Materials Science

Background:

• Bihelical structures are of interest in materials design.
• Diphenic anhydride and cystine di-OMe were used in an attempt to synthesize a bihelical structure.
• Initial synthesis yielded unexpected crystalline bis amide products.

Purpose of the Study:

• To characterize the unexpected crystalline bis amide product.
• To elucidate the structural features contributing to the crystal's rigidity.
• To investigate the nature of intermolecular interactions within the crystal lattice.

Main Methods:

• X-ray crystallography was employed to determine the crystal structure of the bis amide.
• An authentic sample of the bis amide was synthesized via condensation of diphenic acid dichloride with cystine di-OMe.
• Analysis of bond lengths and intermolecular contacts was performed.

Main Results:

• A stable, hard, and rigid bis amide crystal (II) was isolated and characterized.
• The crystal structure revealed a lack of normal hydrogen bonds, with only a single NH...OC linkage observed.
• Crystal rigidity is attributed to a combination of weak hydrogen bonds (CH...O, CH...pi, CH...S, NH...S) acting in concert.

Conclusions:

• The synthesized bis amide exhibits significant rigidity despite the absence of conventional hydrogen bonding.
• Weak intermolecular interactions, particularly numerous parallel CH...O, CH...pi, CH...S, and NH...S bonds, are crucial for crystal stability and rigidity.
• This finding offers insights into novel strategies for designing rigid organic materials.