Insights into the morphology of human serum albumin and sodium dodecyl sulfate complex: A spectroscopic and microscopic approach.

Exploring and understanding the fundamental interaction between protein and surfactant is utmost important for various pharmaceutical and biomedical applications. However, very less is known about the arrangement of individual negatively charged sodium dodecyl sulfate (SDS) molecules on the human serum albumin (HSA). Here, we have investigated the morphology and mechanistic insights of complexation between HSA and SDS by means of photoluminescence (PL) spectroscopy, circular dichroism (CD) and PL microscopy using amine-functionalized silicon quantum dot (Si QD) as an external luminescent marker. The present study is based on a unique and dynamic SDS-Si QD system. The synthesized allylamine-functionalized Si QDs show a distinct PL band centered at 455nm upon excitation at 375nm. At neutral pH, these Si QDs form ordered aggregates in the presence of 1mM SDS due to the hydrogen bonding interaction with the sulfate head groups of surfactants, which is manifested in the appearance of a large Stokes shifted luminescence band centered at 610nm. It has been observed that the PL intensity of SDS-Si QD aggregates at 610nm decreases gradually with concomitant increase in the PL intensity of monomeric Si QDs at 455nm upon increasing the concentration of HSA from 1 to 10μM. These observations combined with PL lifetime, PL microscopy and CD results reveal that SDS forms micelle-like aggregates on the partially unfolded HSA mainly via electrostatic interaction between negatively charged sulfate head groups and positively charged residues of partially unfolded HSA. For the present HSA-SDS system, our results fit a model with type I "necklace and bead"-like structures, where micelle-like SDS aggregates wrap around by the partially unfolded HSA backbone.