Self-assembly and soluble aggregate behavior of computationally designed coiled-coil peptide bundles

Coiled-coil peptides have proven useful in a range of materials applications ranging from the formation of well-defined fibrils to responsive hydrogels. The ability to design from first principles their oligomerization and subsequent higher order assembly offers their expanded use in producing new materials. Toward these ends, homo-tetrameric, antiparallel, coiled-coil, peptide bundles have been designed computationally, synthesized via solid-phase methods, and their solution behavior characterized. Two different bundle-forming peptides were designed and examined. Within the targeted coiled coil structure, both bundles contained the same hydrophobic core residues. However, different exterior residues on the two different designs yielded sequences with different distributions of charged residues and two different expected isoelectric points of pI 4.4 and pI 10.5. Both coiled-coil bundles were extremely stable with respect to temperature (Tm > 80 C) and remained soluble in solution even at high (millimolar) peptide concentrations. The coiled-coil tetramer was confirmed to be the dominant species in solution by analytical sedimentation studies and by small-angle neutron scattering, where the scattering form factor is well represented by a cylinder model with the dimensions of the targeted coiled coil. At high concentrations (5-15 mM), evidence of interbundle structure was observed via neutron scattering. At these concentrations, the synthetic bundles form soluble aggregates, and interbundle distances can be determined via a structure factor fit to scattering data. The data support the successful design of robust coiled-coil bundles. Despite their different sequences, each sequence forms loosely associated but soluble aggregates of the bundles, suggesting similar dissociated states for each. The behavior of the dispersed bundles is similar to that observed for natural proteins.