Schulich Faculty of Chemistry and Solid State Institute, Technion - Israel Institute of Technology, Haifa 3200003, Israel.
Department of Materials and Interfaces, and Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
J Phys Chem B. 2021 Jan 28;125(3):895-906. doi: 10.1021/acs.jpcb.0c10089. Epub 2021 Jan 13.
A detailed understanding of the molecular structure in nanoparticle ligand capping layers is crucial for their efficient incorporation into modern scientific and technological applications. Peptide ligands render the nanoparticles as biocompatible materials. Glutathione, a γ-ECG tripeptide, self-assembles into aggregates on the surface of ligand-free silver nanoparticles through intermolecular hydrogen bonding and forms a few nanometer-thick shells. Two-dimensional nonlinear infrared (2DIR) spectroscopy suggests that aggregates adopt a conformation resembling the β-sheet secondary structure. The shell thickness was evaluated with localized surface plasmon resonance spectroscopy and X-ray photoelectron spectroscopy. The amount of glutathione on the surface was obtained with spectrophotometry of a thiol-reactive probe. Our results suggest that the shell consists of ∼15 stacked molecular layers. These values correspond to the inter-sheet distances, which are significantly shorter than those in amyloid fibrils with relatively bulky side chains, but are comparable to glycine-rich silk fibrils, where the side chains are compact. The tight packing of the glutathione layers can be facilitated by hydrogen-bonded carboxylic acid dimers of glycine and the intermolecular salt bridges between the zwitterionic γ-glutamyl groups. The structure of the glutathione aggregates was studied by 2DIR spectroscopy of the amide-I vibrational modes using C isotope labeling of the cysteine carbonyl. Isotope dilution experiments revealed the coupling of modes forming vibrational excitons along the cysteine chain. The coupling along the γ-glutamyl exciton chain was estimated from these values. The obtained coupling strengths are slightly lower than those of native β-sheets, yet they appear large enough to point onto an ordered conformation of the peptides within the aggregate. Analysis of the excitons' anharmonicities and the strength of the transition dipole moments generally is in agreement with these observations.
深入了解纳米颗粒配体覆盖层的分子结构对于将其有效地纳入现代科学和技术应用至关重要。肽配体使纳米颗粒成为具有生物相容性的材料。谷胱甘肽是一种γ-ECG 三肽，通过分子间氢键自组装在无配体的银纳米颗粒表面上形成聚集体，并形成几纳米厚的壳。二维非线性红外（2DIR）光谱表明，聚集物采用类似于β-片层二级结构的构象。通过局域表面等离激元共振光谱和 X 射线光电子能谱评估壳层厚度。通过与硫醇反应探针的分光光度法获得表面上的谷胱甘肽量。我们的结果表明，壳层由约 15 个堆叠分子层组成。这些值对应于片层之间的距离，明显短于具有相对较大侧链的淀粉样纤维，但与富含甘氨酸的丝纤维相当，其中侧链紧凑。甘氨酸的氢键二聚体和两性离子γ-谷氨酰基之间的分子间盐桥可以促进谷胱甘肽层的紧密堆积。通过使用半胱氨酸羰基的 C 同位素标记对酰胺-I 振动模式的 2DIR 光谱研究了谷胱甘肽聚集体的结构。同位素稀释实验揭示了沿着半胱氨酸链形成振动激子的模式耦合。从这些值估计γ-谷氨酰基激子链的耦合。获得的耦合强度略低于天然β-片层的耦合强度，但它们似乎足够大，可以指出聚集体中肽的有序构象。对激子的非谐性和跃迁偶极矩强度的分析通常与这些观察结果一致。
