Binding interactions of β-carboline drugs with B-isoform of human serum albumin: Spectroscopic and thermodynamic investigations

Abstract

In this study, we report the interaction of two \(\beta\)-carboline-structured drug molecules, Harmane (HM) and Norharmane (NHM), with the B-isoform of HSA at alkaline pH ~ 9.2. The spectroscopic results reveal that the neutral species of both the drugs were stabilized upon interaction with the B-isoform of HSA in the ground state. However, in the excited state, the prototropic equilibrium between the cation and the neutral species is modulated upon the interaction with the B-isoform of HSA, as supported by the time-resolved decay analysis. The effect of electrostatic interactions has also been monitored in the presence of a strong electrolyte, NaCl. The thermodynamics of the binding interactions of both drugs are enthalpically favourable (exothermic process, ∆H < 0). Additionally, at lower temperatures, the binding of both the drugs HM and NHM is enthalpically favourable, but T∆S predominates at higher temperatures more in the case of NHM than HM, so that it becomes positive. The association constant calculated from both the steady-state fluorescence and the Isothermal titration calorimetry (ITC) data reveals that with HSA, NHM binds strongly compared to HM. The binding interactions of both the drugs HM and NHM are associated with the positive heat capacity changes, depicting the hydrophobic hydration as the governing mechanism for the binding, which validates the negligible influence of the strong electrolyte NaCl on the steady-state fluorescence spectral profiles of both the drugs. Using molecular docking analysis, we have explored the probable binding sites of the drugs within the protein matrices. Overall, the present study reveals the binding of two structurally analogous drug molecules with the B-isoform of HSA, which may deliver a perspective of simple chemical manipulation of the drug structure in controlling the important physiological functions.

Graphical abstract

Harmane and Norharmane interact distinctly with the B-isoform of Human Serum Albumin (HSA) at alkaline pH ~ 9.2, preferring the IB and IIB subdomains of the protein, respectively. Being primarily governed by hydrophobic interactions, Norharmane outcompetes Harmane in terms of binding, and the hydrophobic hydration is accountable for these interactions.

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