Differential interactions of a biological photosensitizer with liposome membranes having varying surface charges

Abstract

The present work demonstrates the interaction of promising cancer cell photosensitizer, harmane (HM), with liposome membranes of varying surface charges, dimyristoyl-l-α-phosphatidylcholine (DMPC) and dimyristoyl-l-α-phosphatidylglycerol (DMPG). Electrostatic interaction of the cationic probe (HM) with the surface charges of the lipids is responsible for differential modulation of the spectral properties of the drug in different lipid environments. Estimation of partition coefficient (K(p) (±10%) = 5.58 × 10(4) in DMPC and 3.28 × 10(5) in DMPG) of HM between aqueous buffer and lipid phases reflect strong binding interaction of the drug with both the lipids. Evidence for greater degree of partitioning of HM into DMPG membrane compared to DMPC membrane has been deduced and further substantiated from experimental studies such as steady-state fluorescence anisotropy, micropolarity determination. The molecular modeling investigation by docking simulation coupled with fluorescence quenching experiment has been exploited to substantiate the location of drug at the lipid head-group region. Modulation of the dynamical properties of the drug within the lipid environments has also been addressed. Rotational relaxation dynamics studies unravel the impartation of a significant degree of motional restriction on the probe molecule within the lipids and reinforce the differential interactions of HM with the two lipid systems along the lines of other findings. Fluorescence kinetics studies reveal a faster association (in terms of apparent rate constants describing the process of interaction) of the drug with DMPG membrane compared to DMPC. This result is argued in connection with the electrostatic interaction between the drug and the liposome surface charges.

Unraveling the binding interaction and kinetics of a prospective anti-HIV drug with a model transport protein: results and challenges

“…In the study of interaction of small drug molecules with relevant biological and biomimicking targets the kinetics of interaction is considered to have vital diagnostic importance. 49,54,55,[72][73][74][75] The kinetics of the drug (1-PI)-protein (BSA) association reaction is studied by monitoring the quenching of protein fluorescence at l em = 346 nm upon interaction with the drug 1-PI. Fig.…”

Effect of biological confinement on the photophysics and dynamics of a proton-transfer phototautomer: an exploration of excitation and emission wavelength-dependent photophysics of the protein-bound drug

“…36 A detailed discussion on Benesi-Hildebrand equation is avoided here since it is routine and profusely available in the literature. 16,[36][37][38] We thus start with the equation:…”

“…in which I 0 , I and I N terms designate the emission intensities of the drug, respectively, in the absence of BSA, at intermediate concentration of BSA and at the level of saturation of interaction. 16,[36][37][38] Thus an analysis of the fluorescence data on eqn (5) paves way for simplistic mapping of the spectroscopic modulations through the estimation of binding constant of the drug-BSA interaction process. As seen in Fig.…”

“…Any modulation in the rigidity of the surrounding environment of the fluorophore will be reflected through anisotropy values. The so-called environment induced motional restriction on the mobility of the probe in protein medium [15][16][17][18][19][20][21][22]26,27,37,38 is manifested through anisotropy variation, thereby furnishing clues to assess the location of the probe within the complex biological environments. In the present case, the fluorescence anisotropy of the drug (Fig.…”

“…The variation of anisotropy of the fluorophore after binding to the protein is governed by the simultaneous operation of several factors like: (i) the three-dimensional shape and motions of the protein and probe-protein complex, (ii) the global Brownian tumbling of the probe-protein complex. [15][16][17][18][19][20][21][22]26,27,37,38 Naturally, the overall dimension of the probe-protein complex will be significantly larger compared to the unbound fluorophore per se, and will in turn ascertain a significant reduction in the overall tumbling motion of the former, leading to an increase in anisotropy. [15][16][17][18][19][20][21][22]26,27,37,38 Determination of the binding constant from steady-state anisotropy.…”