Bioinspited self-assembling peptides serve as powerful building blocks in the manufacturing of nanomaterials with tailored-featifreS, Because of their ease of synthesis, biocornpatibility, and tunable activity, this emerging branch of biomolecules has become very popular. The triblock peptide architecture designed by the Hartgerink group is a versatile system that allows control over its assembly and has been shown to demonstrate tunable-bioactivity. Three main forces, Coulomb repulsion, hydrogen bonding and hydrophobicity act together to guide the triblock peptides' assembly into one-dimensional objects and hydrogels. It was shown previously that both the nanofiber morphology-(e.g., intersheet- spacing, formation of antipaiallel/parallel beta-sheets) and,hydrogel theology strictly depend on the-choice of the core residue where the triblock peptide fibers with aromatic cores in general form shotterfibers-and yield poor hydrogels with respect to the ones with aliphatic cores. However, an elaborate understanding of-the molecular reasons behind: these changes remained unclear. In this study, by using carefully designed computer based free energy calculation, we' analyzed the influence of the core residue on the formation of double-wall fibers and single-Wall 6-sheets: Our results demonstrate that the aromatic substitution impairs the fiber cores and this impairment is "mainly-associated with a reduced hydrophobic character of the aromatic side chains. Such weakening is most obvious in tryp-tophan-containing peptides where the fiber core absorbs A. significant amount of water. We also show that the ability of tyrosine to form side :chain hydrogen bonds plays an indispensable role in the fiber stability. As opposed to the impairment of the fiber cores, single-wall P-sheets, with aromatic faces become more stable compared to the ones with aliphatic faces suggesting that the choice of the core residue can also affect the underlying assembly mechanism. We also provide an in-depth comparison, of competing structures (zero dimensional aggregates, short and long fibers) in the triblock peptides' assembly and show that, by adjusting the length of the terminal blocks,, the fiber growth can be turned on or off-while keeping the nanofiber morphology intact.