Which type of bond joins the $C_O{O}_H$ group of one molecule to the $N_H{H}_2$ group of another molecule?

• A. Ionic bond
• B. Hydrogen bond
• C. Disulfide bond
• D. Peptide bond

Answer: (D)

Verified step by step guidance

1. Identify the functional groups involved: the \( \mathrm{C O} \) (carbonyl) and \( \mathrm{O H} \) (hydroxyl) group together form a carboxyl group (\( -\mathrm{COOH} \)), and the \( \mathrm{N H H_2} \) group is an amino group (\( -\mathrm{NH_2} \)).
2. Understand that when these two groups from different molecules react, they form a covalent bond known as a peptide bond, which links amino acids in proteins.
3. Recall that a peptide bond is formed by a condensation (dehydration) reaction where the hydroxyl group (\( -\mathrm{OH} \)) from the carboxyl group and a hydrogen atom (\( \mathrm{H} \)) from the amino group are removed as water (\( \mathrm{H_2O} \)).
4. Write the general reaction for peptide bond formation: \n\n\( \mathrm{R_1{-}COOH} + \mathrm{R_2{-}NH_2} \rightarrow \mathrm{R_1{-}CO{-}NH{-}R_2} + \mathrm{H_2O} \)
5. Recognize that the bond joining the carbonyl carbon (\( \mathrm{C=O} \)) of one molecule to the nitrogen (\( \mathrm{N} \)) of another molecule is the peptide bond, a special type of amide bond critical in protein structure.