Backbone Hydrogen Bonding as a Determinant of Condensate Material States

Condensate material states ranging from liquid droplets to gels have direct functional consequences, but their molecular determinants, especially for gels, are poorly understood. Here we combined optical microscopy, NMR spectroscopy, and molecular dynamics (MD) simulations to test the hypothesis that backbone hydrogen bonding is a determinant for the material states of XYssYX tetrapeptides. IAssAI forms droplets but AIssIA forms gels; MD simulations show a higher level of hydrogen bonds in AIssIA condensates than in IAssAI condensates. Addition of a low-hydrogen-bonding peptide, AAssAA, converts AIssIA gels into droplets. The same effect is achieved by methylating the backbone amides of AIssIA to block hydrogen bonding. 1H-1H nuclear Overhauser effect spectroscopy, coupled with MD simulations, reveals strong, backbone hydrogen bonding-buttressed molecular networks in AIssIA gels. These results demonstrate that, in addition to sidechain-sidechain interaction strength, backbone hydrogen bonding, by promoting directional growth, is a determinant of condensate material states.