Certainly! Let's walk through each description and match it to the **correct amino acid(s)** based on their side chain properties. --- ## **1. Positively charged side chains at pH 7.4** **Answer:** K, R, H - **K** (Lysine), **R** (Arginine), **H** (Histidine) - These amino acids have side chains that are positively charged (basic) under physiological pH. --- ## **2. Negatively charged side chains at pH 7.4** **Answer:** D, E - **D** (Aspartate), **E** (Glutamate) - These have carboxylate groups in their side chains, giving them a negative charge at pH 7.4. --- ## **3. Negatively charged side chains at pH 12** **Answer:** C, Y - **C** (Cysteine), **Y** (Tyrosine) - At very high pH, the side chains of cysteine and tyrosine can lose a proton and become negatively charged. --- ## **4. Achiral** **Answer:** G - **G** (Glycine) - Glycine’s side chain is a hydrogen atom, so it is not chiral (it does not have four different groups attached to its alpha carbon). --- ## **5. Contains two chiral carbons** **Answer:** I, T - **I** (Isoleucine), **T** (Threonine) - These amino acids have two chiral centers. --- ## **6. Side chain restricts the rotation of main chain** **Answer:** P - **P** (Proline) - Proline’s side chain forms a ring with the backbone, restricting rotation. --- ## **7. Contains carboxamide side chains** **Answer:** N, Q - **N** (Asparagine), **Q** (Glutamine) - Both have carboxamide (-CONH₂) groups in their side chains. --- ## **8. Often found in the active site of enzymes, where it can serve as either acid or base catalyst** **Answer:** H - **H** (Histidine) - Histidine's side chain can act as both a proton donor and acceptor near physiological pH. --- ## **9. Least soluble in water** **Answer:** F, W, Y - **F** (Phenylalanine), **W** (Tryptophan), **Y** (Tyrosine) - These have large aromatic side chains, making them less soluble in water. --- ## **10. Most soluble in water** **Answer:** D, E - **D** (Aspartate), **E** (Glutamate) - Their side chains are charged and hydrophilic, making them highly soluble. --- ## **11. Contains hydroxyl groups** **Answer:** S, T, Y - **S** (Serine), **T** (Threonine), **Y** (Tyrosine) - These side chains contain -OH (hydroxyl) groups. --- ## **12. N-terminal amino acid residue of proteins** **Answer:** M - **M** (Methionine) - Methionine is almost always the first amino acid in newly synthesized proteins. --- ## **13. Often found in β-turns, rarely found in α-helices** **Answer:** G, P - **G** (Glycine), **P** (Proline) - Both disrupt α-helices and are common in turns. --- ## **14. Can form disulfide crosslinks under oxidizing conditions, especially in extracellular proteins** **Answer:** C - **C** (Cysteine) - Cysteine forms disulfide bonds (S-S) important for structural stability. --- ## **15. Attachment sites of sugars in glycoproteins** **Answer:** N, S, T - **N** (Asparagine): N-linked glycosylation - **S** (Serine), **T** (Threonine): O-linked glycosylation --- ### **Summary Table** | Description | Amino Acid(s) | |-------------------------------------------------------------------------------|-----------------------| | Positively charged at pH 7.4 | K, R, H | | Negatively charged at pH 7.4 | D, E | | Negatively charged at pH 12 | C, Y | | Achiral | G | | Contains two chiral carbons | I, T | | Side chain restricts rotation of main chain | P | | Contains carboxamide side chains | N, Q | | Often found in active site (acid/base catalyst) | H | | Least soluble in water | F, W, Y | | Most soluble in water | D, E | | Contains hydroxyl groups | S, T, Y | | N-terminal amino acid residue of proteins | M | | Often in β-turns, rarely in α-helices | G, P | | Can form disulfide crosslinks | C | | Attachment sites of sugars in glycoproteins | N, S, T | Let me know if you need clarification or more details on any point!