Three techniques, electrospray mass spectrometry, ultrafiltration, and proton relaxometry, are compared in the context of the quantitative analysis of non-covalent binding between human serum albumin (HSA) and MRI contrast agents. The study of the affinity by proton relaxometry reveals the association constant and the number of interaction sites assuming that all sites are identical and independent. Ultrafiltration was adapted for the study of paramagnetic complexes. This technique confirmed the results obtained by relaxometry. Electrospray mass spectrometry, an original method able to study non-covalent binding because of its soft ionization process that allows for the survival of weak binding, provides qualitative and quantitative results. Electrospray mass spectrometry confirmed the affinity measured by proton relaxometry and ultrafiltration. This technique requires very small amounts of products and directly gives the stoichiometry of the association, information not easily obtained by classic techniques. Nevertheless, proton relaxometry remains a useful and mandatory technique for determining the enhancement of the relaxation subsequent to the binding although it demands large amounts of compounds. It is to be pointed out that even if the three techniques lead to a similar ranking of the affinity of the contrast agents for HSA, the absolute values of the association constants disagree as a result of the difference in the experimental conditions (presence of salt, native protein or desalted one, approximations in the fitting of the data, liquid or gas phases).
Stable gadolinium(III) chelates are nowadays routinely used as contrast agents for magnetic resonance imaging (MRI). Their non-covalent binding to human serum albumin (HSA) has shown to improve their efficacy. Non-covalent interactions lead to complex formation that can be quantified by several techniques that are mostly tedious and time-consuming. In this study, electrospray ionization mass spectrometry (ESI-MS) was used to investigate the interaction between HSA and several gadolinium(III) complexes. The results were compared with those obtained in the liquid phase. Four gadolinium complexes were investigated: Gd-DTPA 1, Gd-C(4)Me-DTPA 2, Gd-EOB-DTPA 3, and MP-2269 4. Relaxometry studies show that complexes 1 and 2 have no significant affinity for HSA, while complexes 3 and 4 have increasing affinities for the protein. 1:1 and 1:2 complexes between HSA and MP-2269 were detected by ESI-MS for a twofold excess of the contrast agent, whereas a ligand/protein molar ratio of 4:1 was necessary to observe a 1:1 stoichiometry for Gd-EOB-DTPA, an observation that is in good agreement with the known weaker affinity of the contrast agent for the protein. At a fourfold molar excess, no supramolecular complex was observed for Gd-DTPA 1 and Gd-C(4)Me-DTPA 2; a tenfold molar excess was necessary to detect a 1:1 complex, confirming the very weak affinity of these contrast agents for HSA.
Upon collisional activation at high kinetic energy (8 keV), the molecular ions of pyridine N-oxides 1–5 undergo an unexpected loss of 16 mass units (oxygen) provided molecular oxygen or nitric oxide is used as the target gas instead of helium. Molecular ions of pyridines are produced in the former experiments. This peculiar behavior seems to be correlated with the high multiplicity of the target molecules (triplet ground state for O2 and doublet for NO). Ab initio calculations suggest that a lower-lying quartet state of pyridine N-oxide ions might be involved in the oxygen-loss process. In the low-translational-energy regime ( c. 20–30 eV), a loss of oxygen is also detected whatever the nature of the collision gas (argon, oxygen or nitric oxide) and the relative intensity of this reaction increases with the kinetic energy of the ions. At near thermal energies ( c. 5 eV), the molecular ions of the pyridine N-oxides react with nitric oxide apparently generating two different ion–molecule complexes or intermediates responsible for the production of [M + NO]+ and [M – O]•+ ions. The deoxygenation mechanism appears therefore highly dependent upon the experimental conditions including target gas and kinetic energy.
Four Gadolinium⋅DTPA complexes bearing long lipophilic alkyl chains were synthesized: two bis[amide] and two 4-substituted derivatives. In two of them (one bis[amide] and one 4-substituted), the alkyl chain ends with a carboxylate function. Their relaxometric properties in H₂O show the self aggregation of Gd⋅DTPA-BdodecylAmide, the better stability of the 4-substituted derivatives vs. Zn transmetallation, and the very good stability of Gd⋅(4-(carboxyundecylisothiourea-Bz)DTPA). Amongst the four compounds, only Gd⋅(4-(carboxyundecylisothiourea-Bz)DTPA) shows a strong interaction with human serum albumin (HSA) as demonstrated by proton relaxometry and ESI mass spectrometry. These data highlight the importance of the negative charge on the alkyl chain in the context of the interaction of Gd⋅(4-substituted DTPA) derivatives with HSA.
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