An Amino Acid Is Made to Replace Another Amino Acid $\alpha$

An amino acid is made to replace another amino acid in membrane-spanning $\alpha$ -helices by site-directed mutagenesis of the gene coding for the protein. This is done in order to determine how close together two such $\alpha$ -helices are in the structure of an integral protein. The amino acid is added to facilitate the attachment of a nitroxide group which can be used to detect position The amino acid substituted is:____________________.

A) phenylalanine
B) alanine
C) cysteine
D) methionine
E) proline