Observing the structural dynamics of proteins under conditions as close as possible to those in a living organism is essential for understanding the biological functions of proteins accurately. Here we demonstrate that terahertz spectroscopy is a convenient probe of conformational changes in proteins suspended in physiological buffer solution. We have observed that the partial unfolding of photoactive yellow protein leads to a clear increase in absorption at terahertz frequencies. Using normal mode and molecular dynamics simulations we show that this increase in absorption is related to an increase in the density of delocalised vibrational modes in the more flexible partially unfolded state. (c) 2008 Elsevier B. V. All rights reserved.