A recent experimental study reported that termini-uncapped A beta(16-22) (with sequence KLVFFAE) peptides self-assembled into nanofibrils at pH 2.0. The oligomerization of this uncapped peptide at atomic level in acidic pH condition remains to be determined, as computational studies mainly focus on the self-assembly of capped A beta(16-22) peptides at neutral pH condition. In this study, using replica exchange molecular dynamics (REMD) simulations with explicit solvent, we investigated the octameric structures of the uncapped A beta(16-22) and its F19W variant at acidic pH condition. Our simulations reveal that the A beta(16-22) octamers adopt various conformations, including closed beta-barrels, bilayer beta-sheets, and disordered aggregates. The closed beta-barrel conformation is particularly interesting, as the cylindrical beta-barrel has been reported recently as a cytotoxic species. Interpeptide contact probability analyses between all pairs of residues reveal that the hydrophobic and aromatic stacking interactions between F19 residues play an essential role in the formation of beta-barrels and bilayer beta-sheets. The importance of F19 and the steric effect on the structures of A beta(16-22) octamers are further examined by REMD simulation of F19W mutant. This REMD run shows that substitution of F19 by W with a more bulky aromatic side chain significantly reduces the beta-sheet content and in turn enhances the population of disordered aggregates, indicating that the steric effect significantly affect the self-assembly of low molecular weight A beta(16-22) oligomers.