Interaction of Gold Nanoparticle with Human Serum Albumin (HSA) Protein Using Surface Energy Transfer.

Here, we study the human serum albumin (HSA) protein�Au nanoparticle interaction to identify the specific binding site of protein with nanoparticles by using the surface energy transfer (SET) method among tryptophan (Trp) of HSA, ANS-dye-labeled HSA protein, and Au nanoparticles. Here, ANS dye is used as a probe located at domain IIIA of HSA. In particular, absorbance, fluorescence quenching, decay time, circular dichroism, dynamic light scattering, and TEM measurements are performed to understand the physical properties of protein-conjugated Au nanoparticles. Using the SET method, the measured distances between the Trp residue of HSA and the binding site of HSA interacting with Au nanoparticles are 42.5, 41.9, and 48.1 Å for 1.5, 2.0, and 2.9 nm HSA-conjugated Au nanoparticles, respectively. The measured distances between the binding site of ANS dye (located at domain IIIA) in HSA to the binding site of HSA interacting with Au nanoparticles are 51, 51.5, and 54.7 Å for 1.5, 2.0, and 2.9 nm HSA-conjugated Au nanoparticles, respectively. From the protein structural data (using PyMol software), the distances from the center of domain IIIA to Cys53�Cys62 disulfide bond and Trp to Cys53�Cys62 disulfide bond are obtained to be 51.5 and 39.1 Å, respectively. Thus, the distances calculated by using SET equation (Trp to Au binding site distance and ANS to Au binding site distance) nicely match with the distances obtained from protein structural data by using PyMol software. Analysis suggests that the Au nanoparticle is attached to HSA by linkage through Cys53�Cys62 disulfide bond which is located at subdomain IA of HSA

Bulk and single-molecule fluorescence studies of the saturation of the DNA double helix using YOYO-3 intercalator dye

We report a thorough photophysical characterization of the interactions between double-stranded DNA (dsDNA) and the trimethine cyanine homodimer dye YOYO-3. The fluorescence emission of this dye is enhanced by intercalation within the DNA double helix. We have explored the saturation of the dsDNA by bound YOYO-3 at the single-molecule level by studying the single-pair Förster resonance energy transfer (FRET) from an energy donor, Alexa Fluor 488, tagged at the 5′ end of the double helix and the energy acceptor, YOYO-3, bound to the same DNA molecule. The spontaneous binding of YOYO-3 gives rise to an effective distribution of different FRET efficiencies and, therefore, donor–acceptor (D–A) distances. These distributions reveal the existence of multiple states of YOYO-3. Steady-state and time-resolved fluorescence and circular dichroism confirmed the presence of a DNA-bound aggregate of YOYO-3, conspicuous at high dye/base pair ratios. The spectral features of the aggregate suggest that it may have the structure of a parallel H-aggregate