Investigations on UV-vis, steady-state fluorescence and synchronous fluorescence properties of human hemoglobin (HHb) reveals greater spectral shifts in peaks arising due to alterations in microenvironment of Tyr-compared to Trp-during interactions with gold nanoparticle (GNP). Besides, interactions of Au-20 nanocluster with modified Tyr/Trp (possessing methyl substituted amide bonds) by quantum chemical calculations demonstrate significant role of amide bonds and the spectral shift, binding energy and alteration in bond distances appear to be higher for modified Tyr-compared to modified Trp. Moreover, close resemblance of frequency shift of modified Tyr/Trp-in presence of Au 20 is observed with respect to the experimental FT-IR study of HHb upon interaction with GNP, suggesting participation of amide bonds in both cases. Furthermore, CD DFT calculations using optimized helical stretch of HHbin presence of Au-20 and experimental CD results of HHb in presence of GNP further indicate participation amide bonds and biocompatibility of GNP. Apparently, the theoretical and experimental interactions are going in good agreement with each other. Overall, the study of interactions of modified Tyr/Trp-as representative models of protein microenvironment and Au-20 nanocluster as prototype of GNP to develop models for exploring protein nanoparticle interactions has been highlighted. (C) 2022 Elsevier B.V. All rights reserved.

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