Background:The human IgG1 antibody subclass is the most abundant one and is widely used in therapeutic applications. Results: Ultracentrifugation and x-ray/neutron scattering, together with atomistic modeling, revealed asymmetric concentration-independent IgG1 solution structures. Conclusion:The complement and receptor Fc binding sites are not hindered by the Fab regions, explaining its full activity. Significance: These solution structures clarify IgG1 activity and its therapeutic applications.

Background: The human IgG4 antibody subclass does not activate complement and forms half-antibodies.Results: Ultracentrifugation and x-ray/neutron scattering together with atomistic modeling revealed asymmetric concentration-dependent IgG4 solution structures.Conclusion: The complement and receptor Fc binding sites are hindered by the Fab regions, explaining loss of activity.Significance: These solution structures clarify IgG4 function and its therapeutic applications.

Solution structures for antibodies are critical to understand function and therapeutic applications. The stability of the solution structure of rabbit IgG in different buffers and temperatures was determined by analytical ultracentrifugation and X-ray and neutron scattering. Rabbit IgG showed a principally monomeric species, which is well resolved from small amounts of a dimeric species. The proportion of dimer increased with increased concentration, decreased temperature and heavy water from 8% to 25% in all buffers except for high salt (250 mM NaCl). The Guinier X-ray radius of gyration R(G) likewise increased with concentration in 137 mM NaCl buffer but was unchanged in 250 mM NaCl buffer. The Guinier neutron R(G) values increased as the temperature decreased. The X-ray and neutron distance distribution curves P(r) revealed two peaks, M1 and M2, whose positions did not change with concentration to indicate unchanged structures under all these conditions. The maximum dimension increased with concentration because of dimer formation. Constrained scattering modeling reproducibly revealed very similar asymmetric solution structures for monomeric rabbit IgG in different buffers, in which the Fab-Fc and Fab-Fab pairs were separated by maximally extended hinge structures. The dimer was best modeled by two pairs of Fab regions forming tip-to-tip contacts. The intact rabbit IgG structures explained the ability of its two ligands, the Fc receptor and complement C1q, to bind to the top of its Fc region that is fully accessible and unhindered by the Fab regions.