Both the ascorbate-reduced and low pH neutron crystals of NcLPMO9D belonged to the monoclinic P21 space group. The crystal structures include two protein molecules per asymmetric unit (Molecule A and Molecule B) related by non-crystallographic symmetry (NCS). Analysis of the crystallographic contacts shows that the two NCS related NcLPMO9D molecules pack with their planar polysaccharide binding interfaces against each other in a binding-face to binding-face configuration also seen in PDB structures 4EIR, 5TKG, 5TKH and 5TKI (Fig. ). While present in a dimeric conformation in the crystal structure, LPMOs including NcLPMO9D function as single-domain monomeric proteins in solution, where they associate with planar crystalline cellulose as confirmed by early LPMO structural studies, and single-molecule observations of LPMO using atomic force microscopy (AFM). Within this binding-face to binding-face packing architecture, the two coordinated coppers are ~12 Å apart, precluding direct interaction. Binding interface residues Tyr25, Tyr206 and Trp207 adopt distinct conformations in Molecule A and B, contributing to different microenvironments near the Cu 2+ active site (Fig. ). Structural alignment of our NcLPMO9D Molecule A or Molecule B with the oligosaccharidebound form of LsLPMO9A (PDB 5ACI), indicates that the substrate pyranose rings of the LsLPMO9A structure superimpose with the proline rings Pro43 and Pro163 for both Molecule A or Molecule B of NcLPMO9D (Fig. ). The alignment further indicates that Tyr206 of Molecule A is positioned to interact with substrate via its phenol ring. This suggests that the crystallographic environment of the active site in Molecule A more closely mimics surface residue interaction with the pyranose ring of the glucose units and creates an occluded active site, possibly analogous to the bulk solvent-protected environment created when the planar active site of LPMO binds to its crystalline linear polysaccharide substrate. Such crystal packing in which active site residues mimic carbohydrate substrate pyranose rings has also been observed in the crystal packing of NcLPMO-3 (PDB 4EIS). Representation of the protein surface additionally illustrates the active site occlusion due to crystal packing (Fig. ).