Free-energy landscape of the main reaction pathway in PYP. The reaction coordinate has periodic boundaries (pG). Atomic structures of pG, IT, ICT, pR1, pR2 and pB are known. Early transitions from pG* require higher time resolution. The energy levels of IT and ICT are unknown (dashed lines); the free-energy levels of pR and pB are known in solution (Takeshita et al., 2002; van Brederode et al., 1995). The free-energy and entropic and enthalpic contributions to the barriers as extracted from the five-dimensional crystallographic data are shown at 300 K.