Within a folded polypeptide, the positively charged –NH3+ group of a lysine side chain is positioned near the partially negative carbonyl oxygen of the backbone on a different segment of the chain. Which non-covalent force most strongly stabilizes this intramolecular contact?
a) Hydrogen bonding between the –NH3+ hydrogen atoms and the carbonyl oxygen
b) Dipole–dipole interaction between the two polar groups
c) Ion–dipole attraction between the –NH3+ group and the carbonyl oxygen
d) Ion–ion (salt-bridge) attraction between the –NH3+ group and the carbonyl oxygen