Synthesis, characterization, photophysical properties, and investigation of enzyme inhibition activities of quinazoline based chiral hydrazine-carbothioamides

In this study, two novel chiral quinazoline-thiosemicarbazone compounds (10a and 10b) were synthesized from (L)-valine via a multistep route involving hydroxyl protection, chlorination, hydrazination, isothiocyanate addition, and TBDMS deprotection. Structures were confirmed by ¹H-NMR, ¹³C-NMR, and IR spectroscopy. Photophysical properties were investigated in water, cationic CTAB, and anionic SDS micellar systems. Absorption band positions remained invariant across all media, indicating that micellar surface charge does not significantly perturb the ground-state electronic structure. Compound 10a showed the highest quantum yield (Φf = 0.937) and strong ICT emission with Stokes shifts of 4409–5731 cm⁻¹ in CTAB. ICT emission in SDS became prominent only at higher concentrations, suggesting that micellar surface charge governs excited-state behavior. Compound 10b exhibited quenched, single-band fluorescence with small Stokes shifts in all media, confirming that the p-nitro group suppresses ICT emission independently of micellar surface charge. Temperature-dependent studies (25–55 °C) revealed invariant absorption band positions but temperature-sensitive fluorescence intensities, the effect being more pronounced for 10a. In HSA interaction studies, simultaneous quenching of Trp214 emission and enhancement of ICT emission upon addition of 10a was consistent with FRET. Stern–Volmer analysis indicated a predominantly static quenching mechanism with a dynamic contribution. Binding constants (Ka = 6.28×10²–2.25×10⁵ M⁻¹), donor–acceptor distances (r = 13.33–14.24 Å), and Förster critical radii (R₀ = 16.23 Å for 10a, R₀ = 20.98 Å for 10b) confirmed effective energy transfer. Compound 10b also quenched HSA emission but showed considerably lower binding and quenching parameters than 10a. Positive ΔH and ΔS values together negative ΔG at all temperatures indicated hydrophobic-driven, thermodynamically favorable binding for both compounds. Enzyme inhibition assays showed that 10a and 10b exhibited activities against AChE, BChE, and α-glucosidase comparable to reference compounds. The BChE inhibitory potency of 10b (IC₅₀ = 2.38 µM) was close to that of tacrine (IC₅₀ = 2.27 µM), while 10a demonstrated superior α-glucosidase inhibition (IC₅₀ = 3.45 µM) relative to acarbose (IC₅₀ = 3.81 µM). Overall, these results demonstrate that the quinazoline-thiosemicarbazone scaffold is a promising multitarget pharmacophore for neurodegenerative and metabolic diseases, warranting further pharmacokinetic, toxicological, and structural optimization studies.