Figure 3.

We measure the absorptions of both buffer blanks (•) and BSA solutions (○), and estimate the solvent baseline arising from the bulk water remaining in solution (♦). This baseline is consistently lower than the measured solution extinction, indicating that the BSA solute does not simply displace water but also contributes to the absorption of terahertz radiation. Correction to account for hydration would further reduce the solvent baseline (data not shown).