Figure 1.

Structural coarse-graining of albumin (based on PDB: 1AO6). Reverse coarsening (fine graining) to any λ level is straightforward (used in Figures 6–8). The hydrodynamic radius R_H of albumin in water has been measured at ~3.2–3.48 nm.^22,44 For the rigid model used here, n = 7 and ρ_i = R_i + d_w, where d_w is the average thickness of the hydration layer surrounding the protein. Using a single layer (d_w = 2.8 Å), eq 6 yields R_H = 3.33 nm. The calculated translational and rotational diffusion constants are D_T ~ 6.6 × 10¹¹ m² s⁻¹ and D_R ~ 4.4 × 10⁶ 1 s⁻¹, respectively, close to the experimental values.²¹ The rotational correlation times τ_D⁽¹⁾ and τ_D⁽²⁾ (Debye’s relaxation time) of the albumin macrodipole (~500 D²¹) are thus ~33 ns and ~0.1 μs, respectively. The model can be used to calculate dielectric and spectroscopic properties of albumin solutions, although the relaxation times indicate that long dynamics simulations would be needed to properly sample the conformational space, which justifies the choice of Monte Carlo sampling used in this study.