Structure and Dynamics of Spin-Labeled Insulin Entrapped in a Silica Matrix by the Sol–Gel Method

The double-histidine Cu²⁺-binding motif: a highly rigid, site-specific spin probe for electron spin resonance distance measurements

The development of ESR methods that measure long-range distance distributions has advanced biophysical research. However, the spin labels commonly employed are highly flexible, which leads to ambiguity in relating ESR measurements to protein-backbone structure. Herein we present the double-histidine (dHis) Cu(2+)-binding motif as a rigid spin probe for double electron-electron resonance (DEER) distance measurements. The spin label is assembled in situ from natural amino acid residues and a metal salt, requires no postexpression synthetic modification, and provides distance distributions that are dramatically narrower than those found with the commonly used protein spin label. Simple molecular modeling based on an X-ray crystal structure of an unlabeled protein led to a predicted most probable distance within 0.5 Å of the experimental value. Cu(2+) DEER with the dHis motif shows great promise for the resolution of precise, unambiguous distance constraints that relate directly to protein-backbone structure and flexibility.

Addressing the optimal silver content in bioactive glass systems in terms of BSA adsorption

Bioactive glasses doped with silver are aimed to minimize the risk of microbial contamination; therefore, the influence of silver on the bioactive properties is intensely investigated. However, information related to the role played by silver, when added to the bioactive glass composition, on biocompatibility properties is scarce. This aspect is essential as long as the silver content can influence blood protein adsorption onto the surface of the glass, thus affecting the material's biocompatibility. Therefore, from the perspective of the biocompatibility standpoint, the finding of an optimal silver content in a bioactive glass is an extremely important issue. In this study, silver-doped bioactive glasses were prepared by a melt-derived technique, which eliminates the pores' influence in the protein adsorption process. The obtained glasses were characterized by X-ray diffraction, UV-vis, X-ray photoelectron (XPS) and Fourier transform infrared (FT-IR) spectroscopy; afterwards, they were investigated in terms of protein adsorption. Both UV-vis and XPS spectroscopy revealed the presence of Ag⁺ ions in all silver containing samples. By increasing the silver content, metallic Ag⁰ appears, the highest amount being observed for the sample with 1 mol% AgO₂. Electron paramagnetic resonance measurements evidenced that the amount of spin-labeled serum albumin attached to the surface increases with the silver content. The results obtained by analyzing the information derived from atomic force microscopy and FT-IR measurements indicate that the occurrence of metallic Ag⁰ in the samples' structure influences the secondary structure of the adsorbed protein. Based on the results derived from the protein response upon interaction with the investigated glass calcium-phosphate based system, the optimal silver oxide concentration was determined for which the secondary structure of the adsorbed protein is similar with that of the free one. This concentration was found to be 0.5 mol%.

Assembly and function of the tRNA-modifying GTPase MnmE adsorbed to surface functionalized bioactive glass

Protein adsorption onto solid surfaces is a common phenomenon in tissue engineering related applications, and considerable progress was achieved in this field. However, there are still unanswered questions or contradictory opinions concerning details of the protein's structure, conformational changes, or aggregation once adsorbed onto solid surfaces. Electron paramagnetic resonance (EPR) spectroscopy and site-directed spin labeling (SDSL) were employed in this work to investigate the conformational changes and dynamics of the tRNA-modifying dimeric protein MnmE from E. coli, an ortholog of the human GTPBP3, upon adsorption on bioactive glass mimicking the composition of the classical 45S5 Bioglass. In addition, prior to protein attachment, the bioactive glass surface was modified with the protein coupling agent glutaraldehyde. Continuous wave EPR spectra of different spin labeled MnmE mutants were recorded to assess the dynamics of the attached spin labels before and after protein adsorption. The area of the continuous wave (cw)-EPR absorption spectrum was further used to determine the amount of the attached protein. Double electron-electron resonance (DEER) experiments were conducted to measure distances between the spin labels before and after adsorption. The results revealed that the contact regions between MnmE and the bioactive glass surface are located at the G domains and at the N-terminal domains. The low modulation depths of all DEER time traces recorded for the adsorbed single MnmE mutants, corroborated with the DEER measurements performed on MnmE double mutants, show that the adsorption process leads to dissociation of the dimer and alters the tertiary structure of MnmE, thereby abolishing its functionality. However, glutaraldehyde reduces the aggressiveness of the adsorption process and improves the stability of the protein attachment.