The sidechain of tryptophan is an indole group - it is an aromatic compound and the lone pair of electrons on the indole nitrogen is not available for hydrogen bonding because it is conjugated with the aromatic system. However, the hydrogen attached the the indole nitrogen is still available for hydrogen bonding. Explain why threonine can serve as both a donor and acceptor. The alcohol sidechain can donate both its hydrogen and its two lone pairs of electrons. Explain how pH affects the activity of Histidine.
Histidine is a weak base.
Like tryptophan it is aromatic, and one of the nitrogen lone pairs is unavailable because it is conjugated with the aromatic ring system. The hydrogens attached to the ring nitrogens are also available for hydrogen bonding ion both the conjugate acid and conjugate base forms. In the structure of myoglobin, nearly all of the nonpolar residues are in the interior. This is due to the hydrophobic effect. The surface of the protein is where the polar sidechains occur, where they can be solvated through their interaction with water molecules.