Final answer:
The sulfur atom in cysteine's side chain is sp3 hybridized and has two lone pairs of electrons. It helps stabilize the tertiary protein structure through disulfide bonding, not hydrogen bonding. The incorrect statement is that it undergoes hydrogen bonding.
Step-by-step explanation:
The student's question concerning which statement is not true about the side chain of cysteine can be addressed by examining the properties of cysteine's side chain.
A. The sulfur in cysteine's sulfhydryl group is indeed sp3 hybridized.
B. The side chain can stabilize tertiary protein structure through the formation of disulfide bonds between cysteines.
C. The sulfur atom in the cysteine side chain has two lone pairs of electrons, which is a correct statement.
D. However, cysteine does not undergo hydrogen bonding through its sulfur atom; this statement is incorrect. Cysteine participates in disulfide bonding, which helps in stabilizing protein structures, but not in hydrogen bonding.