Circular dichroism of hapten-antibody complexes: calculation of the interaction of a trinitrophenyl hapten with tryptophan

Charge transfer between fluorescein and tryptophan as a possible interaction in the binding of fluorescein to anti-fluorescein antibody

Steady-state and time-resolved fluorescence studies of fluorescein (Fl) and 9-hydroxyphenylfluoron (HPF) bound to high-affinity rabbit anti-Fl IgG antibody (anti-Fl IgG) have been performed. The heterogeneity in the fluorescence properties observed for Fl bound to anti-Fl IgG is reduced for HPF bound to anti-Fl IgG. A charge transfer between a tryptophyl residue in the binding site and the hapten was considered as a possible binding interaction. Fl was observed to form complexes in solution with the amino acids tryptophan, tyrosine and methionine, probably due to charge transfer. Also, the fluorescence of tryptophyl residues of the protein is quenched on binding. While such charge transfer complexes may be present, there is no direct evidence that charge transfer complexes between Fl and tryptophan are necessarily present for Fl bound to all high-affinity anti-Fl IgG molecules.

Magnetic resonance studies of the binding site interactions between 19F-labeled nitrophenyl haptens and specific mouse myeloma immunoglobulin MOPC-315

The interactions between MOPC-315, a mouse myeloma protein with specificity for nitrophenyl haptens, and 19F-substituted haptens have been investigated using nuclear magnetic resonance (NMR) spectroscopy. The haptens studied are mono- or dinitrophenyl derivatives of gamma-aminobutyric acid, lysine, or glycine which have trifuoromethyl groups attached to the phenyl rings. Upon binding to immunoglobulin, the 19F nucleus experiences a downfield shift whose magnitude depends on the position of the trifluoromethyl group on the phenyl ring but is independent of other structural changes in the hapten such as the number of nitro groups attached to the phenyl ring. Further, the chemical shift of bound hapten is not influenced by the amount of the constant region attached to the binding site; we accordingly conclude that the presence of the distal, constant regions of the immunoglobulin molecule does not influence binding site interactions.

Purification and specificity of antibodies to adenosine

Antibodies to adenosine were elicited in rabbits by immunization with bovine serum albumin-adenosine conjugate. The antibodies were purified and fractionated on two affinity columns (Sepharose-oligo(A) and Sepharose-AMP). Two families of antibodies have been obtained. The antibodies purified on the Sepharose-oligo(A) column react with poly(A) while those purified on the Sepharose-AMP column do not, as shown by gel diffusion. The association constants for the binding of Fab fragments or IgG purified on the Sepharose-oligo(A) column and several haptens were deduced from dialysis equilibrium, fluorescence quenching and displacement of AMP-fluorescein conjugate. The antibodies mainly recognize adenine, and the ribose or the phosphate group of (or AMP derivatives) do not play a critical role in the interaction. Thermodynamic parameters for adenosine-Fab fragments complexes have been determined deltaH degrees = 16 kcal/mole and deltaS degrees = - 15 cal. degree-1 mole-1. Circular dichroism studies indicate that about three nucleotide residues penetrate the binding site of Fab fragments.

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

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.