Switching of Trp-214 intrinsic rotamer population in human serum albumin: An insight into the aftermath of embracing therapeutic bioorganic luminophore azapodophyllotoxin into sudlow site I

Human serum albumin is perceived to be the most abundant protein in human blood plasma and functions as a major carrier of different enzymes and drugs inside human body. The present article puts in an effort to demonstrate the attitude adopted by human serum albumin towards a potential therapeutic luminophore 4-(2-Hydroxyethyl)-10-phenyl-3,4,6,7,8,10-hexahydro-1H-cyclopenta[g]furo[3,4-b]quinoline-1-one (HPFQ). HPFQ is a prodigy from azapodophyllotoxin class of compounds, which have been synthesized from the perspective of improved bioactivity than its prologue podophyllotoxins. While, HPFQ has proved to be highly bioactive against most cancer cell lines with best GI₅₀ values of <0.1 µM for a major number of cell lines; it also showed terrific fluorescent properties throughout the polarity scale, worthy of a promising imaging agent. The binding mechanism of HPFQ with HSA has been established by combining in vitro spectroscopic techniques, in silico molecular docking and induced fit docking (IFD). The competitive site-binding studies demonstrated that the otherwise anion-receptor sudlow site I of HSA nurtures neutral HPFQ with prudent affinity (Binding constant, K_b = 0.74 × 10⁵ M^-1). The time-resolve fluorescence studies reveal an appreciable reduction in HSA average radiative lifetime against an increase in HPFQ concentration and provided evidence for Forster's resonance energy transfer (FRET) being responsible for the dominant quenching mechanism, escorted by minor structural deformations in the backbone of protein structure. HPFQ institutes itself near Trp-214 within protein matrix, and subsequently the "hydrophobic amino acids" dominated cybotactic environment of Trp-214 experiences a reduction in the micropolarity. The allosteric modulation triggered by the stronger association of HPFQ with HSA leads towards minor deformation in secondary structure of protein. Sudlow site I of HSA proficiently embraces a favourable conformation like malleable dough to furnish space for arriving bioactive HPFQ molecule. HPFQ is also believed to administer the conformational regulation in HSA domain by affecting inter-conversion of HSA rotamers, which may prove to be an enlightening area to decode the preferable interaction between them. The juxtaposed spectroscopic research described herein is expected to embolden design of azapodophyllotoxin based anti-proliferative clinical agents for efficient in vivo bio-distribution employing HSA-centred drug delivery and administration systems.