Molecular dynamics simulation (300, 320, 340 K) performed on the Fab (Kol) fragment of immunoglobulin G revealed that the structural changes associated with relaxation of peptides after their release from the stabilized by the tertiary interaction native conformation may be considered characteristic of the transition from native to molten state. The configuration of peptide chains at temperatures close to melting, liberated from the constraints associated with tertiary packing, was found to deviate toward helical rather than extended forms. The direction of the shift is diagonal on the phi-psi map. The torsional angles tend to concentrate in the Cea7 region, and some leak to the alphaR area. The geometrical parameters designed to describe the configuration of the peptide chain in Fab fragment also confirmed that during melting the peptides generally moved toward helical form.