Keratin is a protein in the intermediate filament family and the key component 
of hair, nail, and skin. Here we report a bottom-up atomistic model of the 
keratin dimer, using the complete human keratin type k35 and k85 amino acid 
sequence. A detailed analysis of geometric and mechanical properties through 
full-atomistic simulation with validation against experimental results is 
presented. We introduce disulfide cross-links in a keratin tetramer and compare 
the mechanical behavior of the disulfide bonded systems with a system without 
disulfide bonds. Disulfide bond results in a higher strength (20% increase) and 
toughness (49% increase), but the system loses α-helical structures under 
loading, suggesting that disulfide bonds play a significant role in achieving 
the characteristic mechanical properties of trichocyte α-keratin. Our study 
provides general insight into the effect of disulfide cross-link on mechanical 
properties. Moreover, the availability of an atomistic model of this protein 
opens the possibility to study the mechanical properties of hair fibrils and 
other fibers from a bottom-up perspective.