Problem: The X-ray crystal structure of the ACE2 receptor protein bound to the SARS CoV-2 receptor binding domain suggests how specific amino acids might interact to connect the two proteins. D30 on the folded ACE2 receptor protein on the host cell bonds with K417 on the folded SARS CoV-2 receptor binding domain. From the picture above, which one of the chemical groups labeled 1, 2, 3 or 4 (in red) on aspartic acid most likely makes this interaction at pH7?