Final answer:
Cystine is formed through the oxidation of two cysteine molecules, resulting in a disulfide bond with no hydrogen atoms attached to sulfur that could be donated. Thus, cystine has 0 hydrogen bond donors.
Step-by-step explanation:
The student's question pertains to the amino acid cystine and its potential to act as a hydrogen bond donor. A hydrogen bond donor is a molecule that can donate a hydrogen atom to form a hydrogen bond. The amino acid cysteine (cys) has a sulfhydryl (SH) group, and when two cysteine molecules are oxidized, they form cystine through a disulfide bond. Each cysteine contributes one sulfur atom to the disulfide bond, which means there are no hydrogen atoms directly attached to the sulfur atoms that could be donated for hydrogen bonding in cystine. Consequently, cystine has 2 hydrogen bond acceptors (from the lone pairs of electrons on the sulfur atoms) and 0 hydrogen bond donors.