Probing the Interaction of Benzo[a]pyrene Adducts and Metabolites with Monoclonal Antibodies Using Fluorescence Line-Narrowing Spectroscopy

Molecular modeling application on hapten epitope prediction: an enantioselective immunoassay for ofloxacin optical isomers

To deepen our understanding of the physiochemical principles that govern hapten-antibody recognition, ofloxacin enantiomers were chosen as a model for epitope prediction of small molecules. In this study, two monoclonal antibodies (mAbs) mAb-WR1 and mAb-MS1 were raised against R-ofloxacin and S-ofloxacin, respectively. The enantioselective mAbs have a high sensitivity and specificity, and the enantioselectivity is not affected by heterologous coating format reactions. The epitopes of the ofloxacin isomers were predicted using the hologram quantitative structure-activity relationship (HQSAR) and comparative molecular field analysis (CoMFA) approaches. The results consistently show that the epitope of the chiral hapten should be primarily composed of the oxazine ring and the piperazinyl ring and mAbs recognize the hapten from the side of this moiety. The enantioselectivity of mAbs is most likely due to the steric hindrance caused by the stereogenic center of the epitope. Modeling of chiral hapten-protein mimics reveals that ofloxacin isomers remain upright on the surface of the carrier protein. Suggestions to improve the enantioselectivity of antibodies against ofloxacin isomers were also proposed. This study provided a simple, efficient, and general method for predicting the epitopes of small molecules via molecular modeling. The epitope predictions for small molecules may create a theoretical guide for hapten design.

Probing the physicochemical interactions of 3-hydroxy-benzo[a]pyrene with different monoclonal and recombinant antibodies by use of fluorescence line-narrowing spectroscopy

Characterization of interactions between antigens and antibodies is of utmost importance both for fundamental understanding of the binding and for development of advanced clinical diagnostics. Here, fluorescence line-narrowing (FLN) spectroscopy was used to study physicochemical interactions between 3-hydroxybenzo[a]pyrene (3OH-BaP, as antigen) and a variety of solvent matrices (as model systems) or anti-polycyclic aromatic hydrocarbon antibodies (anti-PAH). We focused the studies on the specific physicochemical interactions between 3OH-BaP and different, previously obtained, monoclonal and recombinant anti-PAH antibodies. Control experiments performed with non-binding monoclonal antibodies and bovine serum albumin (BSA) indicated that nonspecific interactions did not affect the FLN spectrum of 3OH-BaP. The spectral positions and relative intensities of the bands in the FLN spectra are highly dependent on the molecular environment of the 3OH-BaP. The FLN bands correlate with different vibrational modes of 3OH-BaP which are affected by interactions with the molecular environment (π-π interactions, H-bonding, or van-der-Waals forces). Although the analyte (3OH-BaP) was the same for all the antibodies investigated, different binding interactions could be identified from the FLN spectra on the basis of structural flexibility and conformational multiplicity of the antibodies' paratopes.

Spectral differentiation and immunoaffinity capillary electrophoresis separation of enantiomeric benzo(a)pyrene diol epoxide-derived DNA adducts

Antibody cross-reactivity makes separation and differentiation of enantiomeric analytes one of the most challenging problems in immunoanalytical research, particularly for the analysis of structurally related biological molecules [such as benzo( a)pyrene (BP) metabolites and BP-derived DNA adducts]. It has recently been shown that the interaction of enantiomers of BP tetrols (BPT) with a promiscuous anti-polycyclic aromatic hydrocarbon ( anti-PAH) monoclonal antibody (mAb) allowed for separation of all four enantiomeric isomers using immunoaffinity capillary electrophoresis [ Grubor, N. M. , Armstrong, D. W. , and Jankowiak, R. ( 2006) Electrophoresis 27, 1078 ] and unambiguous spectral resolution using fluorescence line narrowing spectroscopy (FLNS) [ Grubor, N. M. , Liu, Y. , Han, X. , Armstrong, D.W. , and Jankowiak, R. ( 2006) J. Am.Chem. Soc. 128, 6409 ]. Here, we expand the use of the above two methodologies to the group of biologically important molecules that are products of BP diol epoxide (BPDE)-induced DNA damage. Four diastereomeric anti-BPDE-derived deoxyguanosine (dG) adducts, that is, (+)- and (-)- anti-trans-BPDE- N (2)-dG and (+)- and (-)- anti-cis-BPDE- N (2)-dG, were electrophoretically separated and spectroscopically differentiated using 8E11 mAb raised against BP-DNA conjugates. In fluorescence line narrowing spectroscopy (FLNS) experiments, complexes of BPDE-dG adducts with mAb revealed differences in fluorescence origin band positions, bandwidths, and vibrational patterns for all four BPDE- N (2)-dG adducts. Narrow fluorescence origin bands observed for (-)- trans-BPDE-dG (70 cm (-1)) and (+)- trans-BPDE- N (2)-dG (80 cm (-1)) suggest spatial constraint within the mAb binding pocket. Broader origin bands observed for cis type adducts ( approximately 120 cm (-1)) in 8E11 mAb suggest different binding geometries and/or conformational changes, as also indicated by changes in vibrational frequencies observed for the (+)- anti-cis and (-)- anti-cis adducts complexed with mAb. FLNS revealed that binding conformations and interactions within the mAb binding pocket are different for each adduct, enabling unambiguous positive identification. The methodologies described in this manuscript could also be used for analysis of DNA adducts following enzymatic hydrolysis of BPDE-adducted DNA to free nucleosides.

Solid-liquid extraction fluorescence line narrowing spectroscopy of fluoroquinolones in aqueous samples

We present a simple, selective, and sensitive method for the analysis of fluoroquinolones in water samples without previous separation. Octadecyl silica membranes are used with the dual purpose of sample preconcentration and solid substrate for fluorescence line narrowing spectroscopy. Measurements at liquid helium temperature (4.2 K) are easily made with the aid of a cryogenic fiber-optic probe. The entire procedure takes less than 15 minutes per sample and it consumes only 100 microL of organic solvent. Unambiguous fluoroquinolone determination is accomplished via wavelength-time matrices, which provide simultaneous information on spectral peak purity and fluorescence lifetime. We show that 10 mL of water sample are enough to detect analyte concentrations at parts-per-billion levels. The potential of this approach for the analysis of real world samples is demonstrated with the analysis of a synthetic mixture of seven fluoroquinolones spiked in a heavily contaminated water sample.

Cross-reactivity and conformational multiplicity of an anti-polycyclic aromatic hydrocarbon mAb

Cross-reactivity and multispecific functionality of antibodies play a central role in the immune system. The Ab's promiscuity is attributed to structural flexibility and conformational multiplicity of its binding sites governed by the rearrangement of hydrogen bonding centers. However, antibodies whose recognition and binding rely on less directional hydrophobic interactions might follow different interaction pathways. We investigated interaction of anti-polycyclic aromatic hydrocarbon mAb with two biologically important cross-reactants, pyrene and benzo(a)pyrene. Complex formation was characterized by means of low-temperature laser-induced fluorescence spectroscopy in both low- and high-resolution fluorescence line-narrowing (FLN) modes. It is shown that the FLN spectroscopy can identify various haptens cross-reacted with an Ab, as well as simultaneously differentiate between free and immunocomplexed haptens. In addition, our results suggest an interesting case of an Ab binding a particular cross-reactant by adopting two distinct conformations of its binding sites. The existence of the multiple conformations for anti-polycyclic aromatic hydrocarbon mAb that are trapped at low temperature can be rationalized through the existing models for Ab binding. Finally, as revealed by FLN spectra of immunocomplexed chromophores, pi-pi interactions, rather than hydrogen bonding, play the central role in complex formation.