Aquaporins are integral proteins of the plasma membrane whose role is to allow the passage of water molecules down its concentration gradient.
Recent observations have shown an interesting arrangement of charged amino acid residues along the inner surface of the channel resulting in the entry of water via its oxygen, then flipping to lead with its hydrogens half way through the channel.
How might this explain aquaporins specificity for water and not cations or protons?

Aquaporins are barrel shaped protein channels spanning across the cell membrane made of 6 $\alpha$ helices. These channels are permeable only to water molecules. This interaction is restricted and controlled by the aromatic/arginine group at the extracellular face of the channel and the NPA (Asperagine, Proline, Alanine) motif inside the channel. The aromatic/arginine group prevents the entry of other ions or molecules into the channel and also weakens the hydrogen bond, the water molecule moves through the channel with the oxygen atom facing the entrance. As, it reaches the NPA motif it interacts two asperagine amino acids of the NPA motifs, and the direction of the water molecule is flipped or reversed with oxygen atom facing the exit. This also ensures only one water molecule passes through the complex each time, thus regulating the flow of water molecules through the channel.