Passage
The primary structure of a protein is a series of amino acids that are connected in a long chain via C−N peptide bonds (Figure 1) .
Figure 1 Peptide bonds within a protein segment (Note: Rⁿ = amino acid side chains) Protein chain folding is guided by multiple intramolecular interactions that can occur between two peptide bonds, between a peptide bond and a side chain, or between two side chains, as shown in the protein in Figure 2. Although these interactions are intramolecular because they are within the same molecule, they act effectively like intermolecular interactions because proteins are such large molecules.
Figure 2 Intramolecular interactions between the peptide chain and amino acid side chains within a hypothetical proteinBecause intramolecular interactions play a significant role in maintaining the three-dimensional folded structure of a protein, having a greater number of interactions or stronger interactions will result in increased protein stability. The energy required to break these interactions within a protein is related to free energy, ΔG. The greater the ΔG value, the more stable the protein.To find the overall contribution of particular intramolecular interactions to protein stability, researchers made several protein variants in which one amino acid is replaced by another to change the noncovalent interactions between the side chains. The difference in free energy between the variant and the original protein, Δ(ΔG) , indicates the change in overall stability of the protein. A negative Δ(ΔG) means the variant protein is less stable than the original.Table 1 shows the results of the amino acid substitution experiments for a protein with the approximate energy difference when each amino acid is changed.Table 1 Change in Free Energy Based on Changing Intramolecular Interactions Caused by Amino Acid Substitutions
Adapted from: C. N. Pace, J. M. Scholtz, G. R. Grimsley, "Forces stabilizing proteins." FEBS Lett. ©2014 Wiley.
-In the protein shown in Figure 2, the folding of region I is predominantly stabilized by:

Question

Passage
The primary structure of a protein is a series of amino acids that are connected in a long chain via C−N peptide bonds (Figure 1) .

Figure 1 Peptide bonds within a protein segment (Note: Rⁿ = amino acid side chains) Protein chain folding is guided by multiple intramolecular interactions that can occur between two peptide bonds, between a peptide bond and a side chain, or between two side chains, as shown in the protein in Figure 2. Although these interactions are intramolecular because they are within the same molecule, they act effectively like intermolecular interactions because proteins are such large molecules.

Figure 2 Intramolecular interactions between the peptide chain and amino acid side chains within a hypothetical proteinBecause intramolecular interactions play a significant role in maintaining the three-dimensional folded structure of a protein, having a greater number of interactions or stronger interactions will result in increased protein stability. The energy required to break these interactions within a protein is related to free energy, ΔG. The greater the ΔG value, the more stable the protein.To find the overall contribution of particular intramolecular interactions to protein stability, researchers made several protein variants in which one amino acid is replaced by another to change the noncovalent interactions between the side chains. The difference in free energy between the variant and the original protein, Δ(ΔG) , indicates the change in overall stability of the protein. A negative Δ(ΔG) means the variant protein is less stable than the original.Table 1 shows the results of the amino acid substitution experiments for a protein with the approximate energy difference when each amino acid is changed.Table 1 Change in Free Energy Based on Changing Intramolecular Interactions Caused by Amino Acid Substitutions

Adapted from: C. N. Pace, J. M. Scholtz, G. R. Grimsley, "Forces stabilizing proteins." FEBS Lett. ©2014 Wiley.
-In the protein shown in Figure 2, the folding of region I is predominantly stabilized by:

Quizplus · Accepted Answer

11ecba2d_11ec_f7e8_a3bf_adb0394abfa1_MD0008_00 Hydrogen bonding is a special type of dipole-dipole interaction that occurs between molecules in which hydrogen is covalently bonded to the highly electronegative elements oxygen, nitrogen, or fluorine. In such a bond, there is a large dipole moment produced that gives the hydrogen atom a partial positive charge and the other atom a partial negative charge. Attractions between these partial charges result in hydrogen bond donor and acceptor pairs that form strong noncovalent interactions. Hydrogen bonding is one of the stronger intramolecular and intermolecular forces, and it is stronger than other types of dipole-dipole attractions. This strength is due to the large dipole moments involved as well as the small size of hydrogen. This small size allows the partially positive hydrogen to be spatially closer to a partially negative atom, which creates a stronger interaction than most other dipole-dipole interactions. In region I of the protein, the partial positive charge from the dipole of the N−H bond is attracted to the partial negative charge from the dipole of the C=O bond adjacent to the peptide bonds within the protein structure. This forms strong noncovalent hydrogen bonding interactions, which cause the β-sheet formation in the protein. (Choice A) A salt-bridge interaction is a linkage formed by the electrostatic attractions between two fully charged amino acid side chains. This is in contrast to species that are attracted because of a dipole moment involving partial charges induced by electronegativity differences. (Choice B) Covalent bonding involves breaking molecular bonds and making new bonds by sharing electrons. The interactions in region I are noncovalent; the original molecular bonds remain intact. (Choice C) Hydrophobic interactions occur between molecules that are nonpolar (lack a significant permanent dipole) and have very low water solubility. The dipole interactions of region I are not hydrophobic. Educational objective: Hydrogen bonding is one of the stronger intramolecular and intermolecular forces. It is a special kind of noncovalent dipole-dipole interaction between dipoles formed when hydrogen is covalently bound to the highly electronegative elements nitrogen, oxygen, or fluorine.

Passage the Primary Structure of a Protein Is a Series of of Amino