The capsid of hepatitis B virus (HBV) is a significant viral antigen and important diagnostic indicator. to ~8 MDa. As such, native MS and GEMMA provide valuable alternatives to a more time-consuming cryo-electron microscopy CH5132799 analysis for preliminary characterisation of virus-antibody complexes. =4 and =3 and consisting of 240 and 180 monomeric subunits, respectively [1]. Cp dimers are the building blocks for capsid formation and are stabilised by an intermolecular four-helix bundle and a disulphide bond within the bundle (Cys61) [2C4]. Together, these two capsids are known as the hepatitis B FKBP4 core antigen (HBcAg). At a structural level, conversation of HBcAg with a number of antibodies has been investigated, mainly by cryo-electron microscopy (cryo-EM) [5C11]. =4 capsids have four quasi-equivalent variants of each epitope (designated A, B, C, D) whereas =3 capsids have only three (A, B, C), all within 60 copies per capsid. Take note, however, that likewise called sites (e.g. the B-epitope on =3 as well as the B-epitope on =4) aren’t necessarily comparable [12]. Data reveal both, that epitopes are very diverse which there may be pronounced CH5132799 variants in binding affinity between quasi-equivalent variations of confirmed epitope. Nearly all mapped epitopes can be found as conformational discontinuous epitopes, although one conformational linear epitope and one non-conformational linear epitope are also noticed [13, 14]. The places of nearly all these epitopes have already been mapped towards the higher region from the four-helix pack that constitutes the capsid spike. Nevertheless, a definite epitope continues to be determined for the monoclonal antibody 3120 [15]. Characterisation by cryo-EM of Fab 3120-binding mapped its epitope to the ground region from the capsids, and pronounced distinctions in the occupancies from the seven quasi-equivalent variations from the epitope (from 0 to 100 %) had been observed, reflecting distinctions in affinity due to nuances in framework [15]. Since Fab 3120 detects, and has defined historically, the -epitope on HBcAg [5], it really is found in diagnostic assays commonly. Furthermore, the antibody is certainly specific for constructed capsids and will not bind to dimeric capsid subunits [16]. Among the individual antibodies isolated from HBV scientific sera, some had been shown to have got an identical binding fingerprint [11], indicating that 3120-like antibodies certainly are a main element of the anti-HBcAg response to HBV infections. Following launch of electrospray ionisation [17], indigenous mass spectrometry (MS) provides emerged as a very important way of the characterisation of proteins assemblies with regards to molecular pounds (Mw), structure and stoichiometry. Application of the methodology to a number of macromolecular systems, including viral contaminants [18C21], heterogeneous proteins assemblies [22, 23] and membrane-bound proteins assemblies [24], provides demonstrated that lots of structural properties of huge non-covalent proteins complexes could be partly conserved in the gas stage. Ion flexibility mass spectrometry (IMMS) provides recently been combined to CH5132799 indigenous MS and produces additional information associated with size, charge and shape [25C29]. Up coming to indigenous MS, gas-phase electrophoretic flexibility molecular evaluation (GEMMA) has surfaced alternatively solution to characterise macromolecular contaminants such as proteins complexes, virus-antibody and infections complexes [30C38]. Both techniques commence with electrospray ionization (ESI) of proteins assemblies from a pseudophysiological buffer (e.g. aqueous ammonium acetate). In indigenous MS, the multiply billed ions that are created undergo intensive desolvation and so are eventually separated by mostly time-of-flight (ToF) mass analysers. In GEMMA, the original ESI procedure is accompanied by charge manipulation that decreases the particles to singly charged ions that are separated by their electrophoretic mobility diameter (EMD) at atmospheric pressure, which corresponds to particle size in the case of globular analytes. Finally, they are detected by vapour condensation around the separated singly charged particles with subsequent laser light scattering [31, 39]. In this study, we used a combination of ESI-based techniques, MS and GEMMA, to probe the conversation of Fab 3120 with HBcAg. We demonstrate the potential of these techniques for monitoring the concentration-dependent antibody-antigen binding process. We show that interactions of Fab 3120 with HBcAg can be maintained in the gas phase, enabling the separation and semi-quantitative monitoring of HBcAg-Fab complexes in the megadalton mass range. Most significantly, both techniques have the ability to distinguish between Fab 3120 binding to =3 and =4 capsids. Our native MS and GEMMA data were found to be in close agreement and consistent with earlier measurements made at a single Fab 3120/HBcAg dimer ratio by cryo-EM [15]. As such, they establish a proof-of-concept for analysing such virus-antibody immune complexes, qualitatively and semi-quantitatively by.