Structural properties of Aβ(16-35) fragment are investigated as a model for the amyloid-β peptide excluding its coil-inducing terminals. Our replica-exchange molecular dynamics simulations using all-atom and explicit aqueous solvation widely reduce any structural bias. The principal folding pathway shows direct conversion of coil to β-sheet, without the long proposed helix intermediates. Our principal component analysis indicates that the fragment is also intrinsically disordered, as the full amyloid-β peptide. Thus, the observed folding mechanism lacks free-energy barriers and any peaks in the thermal capacity.