METHODS
Protein Data Bank (.pdb) format files for poly leucine α-helices
(28-mer), with either all trans or all gauche + rotamers,
were generated using the BIOVIA Discovery Studio Visualizer software
(Dassault Systèmes, San Diego). PDB files for GCN4 leucine zipper
(1ZIK), 14-3-3 protein (2BQ0), Symfoil (3O4D) and Monofoil (3OL0) are
from the RCSB database 15. All files were stripped of
HETATOMs (i.e., waters and ligands). In the case of 14-3-3 the interface
domain region relevant to the A domain was identified as residue
positions A145-A206, and those for the B domain were identified as
B3-B54/B98-B127 and these were the 3D printed regions (the B domains
being monolithic). All residues for the GCN4 leucine zipper 30-mer were
retained, as was the 42-mer region of the Monofoil polypeptide in the
trimer oligomer forming a β-trefoil fold. Domain-swapped permutants of
Monofoil 16 were generated using corresponding
internal 42-mer regions of the Symfoil (i.e., intact β-trefoil)
structure (Fig. 3).
Standard Tessellation Language (.stl) files were generated for all
structures using the Chimera software package 17.
Corresponding G-code files were generated using IdeaMaker software
(Raise3D, Irvine CA) using
standard van der Waals radii and a scale of
2.8 mm/Å. Molecules were 3D
printed using a Raise3D Pro2 printer (Raise3D, Irvine CA) and 1.75 mm
Flexmark 9 (Sigma Aldrich, St. Louis MO) thermoplastic polyurethane
(TPU) filament (Durometer hardness of 90, elongation at break of 600%,
and tensile strain of 55 MPa). Models were printed with 25% infill
(grid pattern), three surface shells, and removable supports for regions
of overhang. Printed models were stripped of supports, and weighed (with
duplicate printed models typically exhibiting a mass difference within
1-2 g). Weight was used for quality control as printing nozzle
obstruction yielded models of 5-10 g lower mass, weakened integrity, and
reduced kinetic energy upon impact. An example of a 3D printed model,
with supports trimmed, is given in Fig. 4.
Kinetic energy was imparted to models via an essentially elastic
collision by fall from defined height onto a concrete surface. Ten
repetitions were performed at each evaluated height, with essentially
random model orientation. Drop heights varied from 5.0 cm to 12.0 m. At
each height the number of repetitions resulting in model dissociation
was noted and the fraction folded calculated. Drop heights were
increased until the fraction folded yielded 0.0 (i.e., dissociation
observed in all 10 trials) for each model. Kinetic energy was calculated
as mass*g *height (g =9.8 m/s2) and the
imparted energy (J) was normalized for total model mass (J/Kg).