There exists increasing evidence that second-shell ligands might affect the electronic structure of metalloproteins. We address this issue in the context of plastocyanins, where an H-bond between the copper-bound Cys Sgamma and a vicinal Asn NH is always observed. By performing multiple sequence alignments, we show the full conservancy of this vicinal Asn residue. By performing density functional theory (DFT) calculations on model systems, we show that inclusion of the Asn backbone affects the spin density distribution around the copper center in fair agreement with experimental data. Our calculations provide further support to the proposal that fully conserved outer shell ligands should be taken into account for a proper description of metal active sites in proteins.