Using a combination of experimental techniques (circular dichroism, differential scanning 
calorimetry, and NMR) and molecular dynamics simulations we performed an extensive study of 
denaturation of the Trp-cage miniprotein by urea and guanidinium. The experiments, despite 
their different sensitivities to various aspects of the denaturation process, consistently 
point to simple, two-state unfolding process. Microsecond molecular dynamics simulations 
with a femtosecond time resolution allow to unravel the detailed molecular mechanism of 
Trp-cage unfolding. The process starts with a destabilizing proline shift in the hydrophobic 
core of the miniprotein, followed by a gradual destruction of the hydrophobic loop and the 
alpha-helix. Despite differences in interactions of urea vs guanidinium with various peptide 
moieties, the overall destabilizing action of these two denaturants on Trp-cage is very similar.