Complementary fluorescence and phosphorescence study of the interaction of brompheniramine with human serum albumin.

Binding of the antihistamine drug brompheniramine (BPA) to human serum albumin (HSA) is studied by measuring quenching of the fluorescence and room temperature phosphorescence (RTP) of tryptophan. The modified Stern-Volmer equation was used to derive association constants and accessible fractions from the steady-state fluorescence data. Decay associated spectra (DAS) revealed three tryptophan fluorescence lifetimes, indicating the presence of three HSA conformations. BPA causes mainly static quenching of the long-living, solvent-exposed conformer. RTP spectra and lifetimes, recorded under deoxygenated conditions in the presence of 0.2 M KI, provided additional kinetic information about the HSA-BPA interactions. Fluorescence DAS that were also recorded in the presence of 0.2 M KI revealed that the solvent-exposed conformer is the major contributor to the RTP signal. The phosphorescence quenching is mostly dynamic at pH 7 and mostly static at pH 9, presumably related to the protonation state of the alkylamino chain of BPA. This provides direct insight into the binding mode of the antihistamine drug, as well as kinetic information at both the nanosecond and the millisecond time scales.

pH-dependent complexation of histamine H1 receptor antagonists and human serum albumin studied by UV resonance Raman spectroscopy.

UV resonance Raman spectroscopy was used to characterize the binding of three first-generation histamine H(1) receptor antagonists-tripelennamine (TRP), mepyramine (MEP), and brompheniramine (BPA)-to human serum albumin (HSA) at pH 7.2 and pH 9.0. Binding constants differ at these pH values, which can be ascribed to the different extent of protonation of the ethylamino side chain of the ligands. We have recently shown [Tardioli et al. J. Raman Spectrosc. 2011, 42, 1016-1024] that for the solution conformation of TRP and MEP the side chain plays an important role by allowing an internal hydrogen bond with the aminopyridine nitrogen in TRP and MEP. Results presented in this paper suggest that the existence of such molecular structures has serious biological significance on the binding affinity of those ligands to HSA. At pH 7.2, only the stretched conformers of protonated TRP and MEP bind in HSA binding site I. Using UV absorption data, we derived binding constants for the neutral and protonated forms of TRP to HSA. The neutral species seems to be conjugated to a positive group of the protein, affecting both the tryptophan W214 and some of the tyrosine (Y) vibrations. BPA, for which the structure with an intramolecular hydrogen bonded side chain is not possible, is H bound to the indole ring nitrogen of W214, of which the side chain rotates over a certain angle to accommodate the drug in site I. We propose that the protonated BPA is also bound in site I, where the Y150 residue stabilizes the presence of this compound in the binding pocket. No spectroscopic evidence was found for conformational changes of the protein affecting the spectroscopic properties of W and Y in this pH range.

Interaction of bovine serum albumin with gemini surfactants.

The interactions between bovine serum albumin and cationic gemini surfactants were investigated as a function of concentration, under different pH conditions. The investigation deals with dielectric relaxation, dynamic light scattering, zeta-potential, circular dichroism, and UV spectroscopy. The interactive behavior of the anionic form is quite different from the cationic species. It indicates that protein-surfactant interactions are mostly electrostatic in nature and depend on the state of charge of bovine serum albumin. The results indicate the presence of both hydrophobic and electrostatic contributions in the interactions of gemini with bovine serum albumin. Comparison of dynamic light scattering, dielectric relaxation, electrophoretic mobility, and optical circular dichroism allows drawing some preliminary hypotheses on the different contributions to surfactant binding and supports former studies on the formation of complexes between the bovine serum albumin and the above species.

Anomalous photophysics of H1 antihistamines in aqueous solution.

Electronic absorption, emission, and excitation spectra, and fluorescence lifetimes of two H1 antihistamines--tripelennamine and mepyramine--are investigated in detail to ascertain their usefulness as fluorescent probes for ligand binding to G-protein coupled receptors. The photophysical behavior of these compounds in aqueous solution is complex due to the presence of three protonable nitrogens, intramolecular hydrogen bonding, quenching due to the formation of a charge transfer state, and intramolecular fluorescence resonance energy transfer. At physiological pH values, anomalous photophysical behavior is observed: the compounds are found to be in a ground-state equilibrium mixture of two species, one with the alkylamine tail involved in an intramolecular hydrogen bond and a second without such a bond. This internal hydrogen-bonded tail has a profound effect on the ground and excited-state properties of both tripelennamine and mepyramine, which is further elucidated by comparing them to the reference compounds 2-aminopyridine and 2-(N,N-dimethylamino)pyridine.