Sedimentation velocity (SV) experiments

Sedimentation velocity experiments in the analytical ultracentrifuge were performed in a Beckman (Palo Alto, CA, USA) Optima XL-I analytical ultracentrifuge equipped with Rayleigh interference optics and an automatic on-line data capture system. Lignin solutions (~ 350 µl) in DMSO:water (9:1) or reference DMSO:water (9:1) (density = 1.09036 g mL⁻¹; viscosity = 0.029 Poise at 20.0 °C), were loaded into 12 mm pathlength double-sector epoxy cells with sapphire windows. A low loading concentration of 0.2 mg mL⁻¹ was used to minimise the effects of hydrodynamic non-ideality. Samples were run at a rotor speed of 49,000 rpm (~ 120,000 g) at 20.0 °C. Concentration profiles and movement of the sedimenting boundaries within the AUC cells were recorded using the Rayleigh interference optical system registering concentration (in fringe displacement units relative to the meniscus) versus radial position, r⁵⁹. The data were analysed by means of the SEDFIT analytical algorithm⁶⁰, which gives a distribution of sedimentation coefficient, s, and provides an assessment of the polydispersity. The c(s) vs s model was used to analyse the data, thereby generating an apparent distribution of sedimentation coefficients (s, in Svedberg units S = 10⁻¹³ s) in the form of c(s) versus s, rather than the ls-g(s) model, because of the very low s values and incomplete resolution of the sedimenting boundaries from the air-solution meniscus. Because of the very low concentrations employed, correction for non-ideality (extrapolation to infinite dilution) was not necessary, and the assumption s°_20,w ~ s_20,w is reasonable. A value for the partial specific volume, ῡ, of 0.61 mL g⁻¹ was used⁶¹.