The N-terminal amino group of the histidine brace has been postulated to play a role in intermediate formation and stabilization during LPMO catalysis. A previous neutron protein crystallography study of unreduced JdLPMO10A, a bacterial LPMO, suggested that the amino terminal is present as a mixture of the -ND -and -ND2 protonation states. The JdLPMO10A neutron structure was, however, later revisited and a joint X-ray-neutron quantum refinement concluded that the amino terminal is present solely in the -ND2 state. We therefore sought to determine the protonation state of the N-terminal amino group upon NcLPMO9D reduction by ascorbate in the presence of atmospheric O2.