The shape of immunoglobulin G molecules in solution.

We have studied the shape of rabbit Immunoglobulin G molecules in solution by using singlet-singlet energy transfer to determine the minimum distance between the two hapten binding sites. A hybrid antibody was prepared in which one site specifically bound the energy donor, epsilon-dansyl-lysine, and the other site bound the energy acceptor, fluorescein. For this donor-acceptor pair, R(0) was calculated to be 4.8 +/- 0.2 nm (48 +/- 2 A). From a comparison of the lifetime of the donor's excited state in the presence or absence of acceptor, it was found that no energy transfer had occurred in the hybrid. Since the maximum distance over which transfer is measurable was 8.2 nm (82 A; 1.7 R(0)), and since the Fab moieties exhibit segmental flexibility, the average distance between the two hapten-binding sites was estimated to be 9.2-10 nm (92-102 A). If one assumes that the length of the Fab fragment is 7 nm (70 A), the corresponding minimum angle between Fab moieties, alpha(M), would be 80-95 degrees . The molecules in solution, thus, have an open Y- or T-shaped configuration in which the hapten binding sites are not more than 2.5 nm (25 A) from the extreme ends of the Fab fragments. The existence of conformations in which alpha(M) is less than 80 degrees , as has been observed in some antibody-antigen complexes, must therefore be the result of definite conformational changes.