Circular Dichroism of Hapten—Antibody Complexes: Characterization of the Combining Sites of Native and Reformed MOPC‐315 Protein, Its Isolated Subunits, and Its Fv Fragment

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Abstract
Extrinsic Cotton effects generated by binding haptens to native and reformed MOPC-315 protein, its subunits, and its Fv fragment have been examined. The identity of the combining sites of native and reassociated proteins and Fv-315 was demonstrated by the identity of their circular dichroism (CD) difference spectra. The spectrum of TNP-aminocaproate complexed with L chains differed in maxima and minima and cross-over points and lacked the 495-nm CD peak of TNP-aminocaproate-MOPC-315 protein and Nalpha-TNP-tryptophan spectra. A negative 293-nm tryptophanyl CD band, present in spectra of MOPC-315 protins and Fv-315 but absent from spectra of L and H chains, was blue-shifted by haptens and may represent electronic interactions occurring within the MOPC-315 combining site between tryptophanyl and chromophoric residues of different subunits. This conclusion is supported by molecular models of the MOPC-315 combining site.