The conformational flexibility of antibodies in solution directly affects their immune function. Namely, the flexible hinge regions of immunoglobulin G (IgG) antibodies are essential in epitope-specific antigen recognition and biological effector function. The antibody structure, which is strongly related to its functions, has been partially revealed by electron microscopy and X-ray crystallography, but only under non-physiological conditions. Here we observed monoclonal IgG antibodies in aqueous solution by high-resolution frequency modulation atomic force microscopy (FM-AFM). We found that monoclonal antibodies self-assemble into hexamers, which form two-dimensional crystals in aqueous solution. Furthermore, by directly observing antibody-antigen interactions using FM-AFM, we revealed that IgG molecules in the crystal retain immunoactivity. As the self-assembled monolayer crystal of antibodies retains immunoactivity at a neutral pH and is functionally stable at a wide range of pH and temperature, the antibody crystal is applicable to new biotechnological platforms for biosensors or bioassays.

Immunoglobulin G (IgG), an antibody, plays a significant role in the immune system, and the functions of IgG molecules have been studied in many research fields such as medicine and engineering. Recently, we found the self-assembly of monoclonal mouse IgG molecules on a mica substrate using atomic force microscopy (AFM); the IgG molecules self-assemble into hexamers and the hexamers form a two-dimensional (2D) crystal. The self-assembly of the IgG molecules is of great interest in terms of the enhancement of the immunoactivity of the antibodies. In this study, we investigated the self-assembly of various IgG molecules on a mica substrate to discuss if the hexamerization of the IgG molecules is a general phenomenon. We also investigated the antigen binding site in the IgG antibody hexamers, and estimated the association rate constant of the self-assembled IgG molecules based on the AFM measurements.The estimated value was lower than that reported in a previous study probably because of the limited mobility of the antigen-binding fragments on the substrate.

We investigated aqueous solutions containing nitrite ions and DMPO (5,5-dimethyl-1-pyrroline-N-oxide) by electron spin resonance (ESR) in the pH range from 1 to 6. A DMPO-OH signal was observed below pH 3.0 in the presence of nitrite ions, whereas in the absence of nitrite ion, an extremely weak signal was observed below pH 1.5. Addition of methanol, a hydroxyl radical scavenger, to this system did not lead to the appearance of a detectable DMPO-CHOH signal. The possibility of this DMPO-OH signal being due to a genuine spin trapping process with hydroxyl radical was, therefore, ruled out. The reactivities of reactive nitrogen species (RNS) in this system with DMPO have also been investigated by density functional theory (DFT) at the IEFPCM (water)/B3LYP/6-311 + G ** level of theory. On the basis of the pH dependence of the signal intensity and the redox potential E° (versus SHE) calculated by DFT theory, we propose that the origin of this signal is "inverted spin trapping" via one-electron oxidation of DMPO by HONO, followed by the nucleophilic addition of water. Prevention of these false-positive results when detecting hydroxyl radical using ESR spin trapping requires an awareness of both the presence of nitrite ions in the solution and the solution pH.