Water-Mediated Folding and HAR of l-Asp-l-Asp-l-Asp Trihydrate: Polymorph Analysis

What is it about?

This research investigates the application of restraints for atomic displacement parameters (ADPs) of hydrogen atoms in Hirshfeld atom refinement (HAR) using two polymorphs of the water-rich l-Asp-l-Asp-l-Asp (DDD) tripeptide crystal structure. The study involved crystallizing DDD as a trihydrate and determining its structure at both 8 K and 100 K using synchrotron X-ray diffraction, followed by HAR to accurately localize hydrogen atoms, particularly those involved in hydrogen bonding. The structural analysis revealed that DDD adopts a folded, horseshoe-like conformation stabilized by three water molecules forming strong hydrogen-bonded bridges between peptide termini and carboxyl groups. Comparison of the two polymorphs showed differences in water molecule disorder and hydrogen-bonding patterns, with distinct water networks supporting the same overall peptide fold. The article further tested and optimized various hydrogen-atom ADP restraints (ISOR, DELU, RIGU, SIMU) within HAR, determining that physically reasonable ADPs should be preferred over isotropic or non-physical descriptions, despite some uncertainty in absolute accuracy. The findings highlight the essential structural role of water and hydrogen atoms in peptide conformations and emphasize the necessity of precise hydrogen modeling in X-ray crystallography. Recommendations for hydrogen-atom ADP restraint values in HAR are proposed, while acknowledging the need for broader validation across more chemical systems.

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Why is it important?

This research investigates the accurate modeling of hydrogen atoms and water networks in the crystal structure of the tripeptide l-Asp-l-Asp-l-Asp (DDD), addressing challenges in describing water and hydrogen in peptide structures using advanced quantum-crystallographic techniques. The study is significant because water molecules and hydrogen atoms play a crucial role in protein folding, proton-transfer mechanisms, and stabilization of peptide conformations, yet are often poorly resolved in crystallographic studies. By exploring both structural chemistry and methodological advances in X-ray refinement, the work contributes to a deeper understanding of peptide–water interactions and provides recommendations for improved crystallographic modeling. Key Takeaways: 1. The study determines that DDD crystallizes as a zwitterion with a highly folded, horseshoe-shaped conformation, which is stabilized by three co-crystallized water molecules that form strong hydrogen-bonded bridges between the N- and C-termini of the peptide. 2. Comparative analysis of two DDD trihydrate polymorphs at 8 K and 100 K reveals the existence of five distinct water networks that all support the same overall peptide folding, but with differences in water molecule positions and key hydrogen bonds, demonstrating the structural versatility conferred by water in peptide crystals. 3. The research implements and evaluates hydrogen-atom restraints for atomic displacement parameters (ADPs) in Hirshfeld atom refinement (HAR), finding that physically reasonable ADPs are preferable and providing specific restraint values for hydrogen atoms, thereby enhancing the accuracy and reliability of hydrogen and water modeling in X-ray crystal structure refinements.