Abstract
The red-emitting fluorescent properties of bovine serum albumin (BSA)–gold conjugates are commonly attributed to gold nanoclusters formed by metallic and ionized gold atoms, stabilized by the protein. Others argue that red fluorescence originates from gold cation–protein complexes instead, not gold nanoclusters. Our fluorescence and infrared spectroscopy, neutron, and X-ray small-angle scattering measurements show that the fluorescence and structural behavior of BSA–Au conjugates are different in normal and heavy water, strengthening the argument for the existence of loose ionic gold–protein complexes. The quantum yield for red-emitting luminescence is higher in heavy water (3.5%) than normal water (2.4%), emphasizing the impact of hydration effects. Changes in red luminescence are associated with the perturbations of BSA conformations and alterations to interatomic gold–sulfur and gold–oxygen interactions. The relative alignment of domains I and II, II and III, III and IV of BSA, determined from small-angle scattering measurements, indicate a loose (“expanded-like”) structure at pH 12 (pD ~12); by contrast, at pH 7 (pD ~7), a more regular formation appears with an increased distance between the I and II domains, suggesting the localization of gold atoms in these regions.
Highlights
Introduction published maps and institutional affilBiocompatible red fluorescent bovine serum albumin (BSA)–gold conjugates offer favorable biological applications because their red fluorescence significantly differs from tissue materials’ blue or green auto-fluorescence [1]
An elegant and simple one-pot aqueous synthesis of fluorescent bovine serum albumin–gold (BSA–Au) complexes was first described by Xie et al, a pioneering work in the field [2]
We show that heavy water, compared to normal water, induces more decided effects in both global and fine structures and that these changes bring a significantly increased red fluorescence than that observed in normal water
Summary
Sigma-Aldrich (Sigma-Aldrich, St. Louis, MO, USA) and used as received. The pH of the solutions was adjusted with HCl (Sigma-Aldrich, St. Louis, MO, USA) and NaOH (Sigma-Aldrich, St. Louis, MO, USA). All solutions were prepared in ultra-pure Milli-Q water (total organic content ≤ 4 ppb; resistivity ≥ 18 MΩcm) and heavy water (deuterium oxid for NMR, 99.8% D) purchased from Acros Organics (Morris Plains, NJ, USA)
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