The recombinant form of goat alpha-lactalbumin has a significantly faster unfolding rate compared to the authentic form, although the two molecules differ only in an extra methionine at the N terminus of the recombinant. The mechanism of the destabilization caused by this residue was investigated through the combined use of kinetic experiments and molecular dynamics simulations. Unfolding simulations for the authentic and recombinant forms at 398 K (ten trajectories of 5 ns for each form, 100 ns total) precisely reproduced the experimentally observed differences in unfolding behavior. In addition, experiments reproduced the destabilization of a mutant protein, T38A, faithfully as predicted by the simulations. This bidirectional verification between experiments and simulations enabled the atomically detailed description of the role of the extra methionine residue in the unfolding process. (c) 2005 Elsevier Ltd. All rights reserved.