Examples of hydrogen bond interactions from conserved, buried polar residues that involve N-terminal regions of α-helices. Representative structures were chosen for each family based on resolution; residues are coloured by atom type with buried, conserved polar residues shown in magenta. Hydrogen bonds are shown in black. Two examples of aspartates that hydrogen bond forward to N-termini residues in A) the glyceraldehyde 3-phosphate dehydrogenase family [PDB: 1gd1] and B) the beta-lactamase family [PDB: 1btl]. The aspartate in B) also forms distant interactions to another helix N-terminus. Two examples of aspartates hydrogen bonding back to N-terminal residues as well as to coils in the C) alcohol dehydrogenases [PDB: 2ohx] and D) matrix metalloproteinases [PDB: 1hfc]. The latter panel also displays an example of a cysteine hydrogen-bonding forward to an N-terminal residue. Two examples of aspartates forming distant hydrogen bonds to N-terminal residues in E) bacterial serine proteinases [PDB: 2sga] and F) chalcone and stilbene synthases [PDB: 1hnj]. G) Cysteine residue that hydrogen bonds forward to an N-terminal region in cytochrome P450 s [PDB: 1jfb]. H) Cysteine in zinc binding domain in Lin-11, Isl-1 and Mec-3 [PDB: 1ctl] that forms distant hydrogen bonds to an N-terminal residue. I) Cysteine hydrogen bonding back to N-terminal residues in the alcohol dehydrogenase family [PDB: 2ohx]. Pictures of all polar sidechain examples were produced using Pymol and clipping was used for improving figure clarity .