Biochemistry : Hydrogen Bonds

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #2 : Molecular Bonds And Forces

Which of the following most correctly describes hydrogen bonding?

Possible Answers:

A partially negative hydrogen attracts a lone electron pair from a partially positive atom

A neutral hydrogen attracts a lone electron pair from a charged atom

A partially negative hydrogen attracts a lone electron pair from a partially negative atom

A partially positive hydrogen attracts a lone electron pair from a partially negative atom

A partially positive hydrogen attracts a lone electron pair from a partially positive atom

Correct answer:

A partially positive hydrogen attracts a lone electron pair from a partially negative atom

Explanation:

Hydrogen bonding occurs when a molecule contains a hydrogen atom bonded to fluorine, oxygen, or nitrogen. This hydrogen becomes partially positive in charge, while the attached atom becomes partially negative. This is due to unequal electronegativity causing increased electron density for the more electronegative atom, and therefore a more negative charge. The partially positive charge then forms an electrostatic attraction to another partially negative atom nearby.

Example Question #3 : Molecular Bonds And Forces

In the DNA double-helix structure, what kind of bonds connect the complementary bases together?

Possible Answers:

Phosphodiester bonds

Hydrogen bonds

None of these answers

Covalent bonds

Ionic bonds

Correct answer:

Hydrogen bonds

Explanation:

In the double-helix structure of DNA, there are 2 hydrogen bonds between adenine and thymine, and 3 hydrogen bonds between cytosine and guanine. Phosphodiester bonds are covalent bonds between the 5-C sugar and phosphate group of nucleotides (and are much stronger than the hydrogen bonds between complementary bases).

Example Question #4 : Molecular Bonds And Forces

What is not true about hydrogen bonds?

Possible Answers:

Hydrogen bonds increase the boiling point of water

Hydrogen bonds are found in protein structure

Hydrogen bonds are found between water molecules

Hydrogen bonds are found in DNA structure

Hydrogen bonds can be found between metals

Correct answer:

Hydrogen bonds can be found between metals

Explanation:

Hydrogen bonds are found in primary structure of protein, as well as between the bases in DNA structure. Hydrogen bonds are only found between hydrogens attached to oxygen, nitrogen, or fluorine. They increase the attraction between water molecules, therefore are harder to break in large numbers, causing an increase in boiling point.

Example Question #5 : Molecular Bonds And Forces

In a solution of water, which of the following amino acids do you expect to be on the outside of a protein in its native state?  

Possible Answers:

Isoleucine

Valine

Phenylalanine

Glutamic acid

Correct answer:

Glutamic acid

Explanation:

In a solution of water, the outside of a folded protein is going to be in direct contact with water. Therefore, polar or ionic attractions are most favored on the outside of a protein in a solution of water since these can form attractions. The only amino acid listed that can form either polar or ionic attractions is glutamic acid.

Example Question #6 : Molecular Bonds And Forces

A researcher adds three molecules together and notices no reactions. Upon further analysis, he notices that molecule A and molecule B form strong intermolecular bonds whereas molecule C doesn’t form any bonds between A and B. Which of the following might be true regarding these three molecules?

I. Molecule A might have nitrogen

II. Molecule B might have fluorine

III. Molecule C might have nitrogen

Possible Answers:

II and III

I, II and III

I and II

I and III

Correct answer:

I and II

Explanation:

An example of strong intermolecular bond is a hydrogen bond. A hydrogen bond occurs between a hydrogen atom on a molecule and an either nitrogen, oxygen, or fluorine atom on an adjacent molecule. The question states that there are bonds between molecule A and molecule B. We can assume that these are hydrogen bonds. This means that molecule A can have hydrogen atom and/or nitrogen, oxygen, or fluorine atoms. Similarly, molecule B can also have hydrogen atom and/or nitrogen, oxygen, or fluorine atoms.

Molecule C doesn’t form hydrogen bonds with either of the other two molecules. This means it cannot have hydrogen and/or nitrogen, oxygen, or fluorine atoms. This is because if molecule C had any of these atoms then it would interact with at least one of the other molecules and form hydrogen bonds (because molecule A and molecule B have these other atoms).

Example Question #7 : Molecular Bonds And Forces

Classically, covalent bonds are considered __________ bonds and hydrogen bonds are __________.

Possible Answers:

intramolecular . . . intramolecular

intermolecular . . . intermolecular

intermolecular . . . intramolecular

intramolecular . . . intermolecular

Correct answer:

intramolecular . . . intermolecular

Explanation:

Covalent bonds occur between atoms within (or intra) molecules. This means that covalent bonds are intramolecular bonds. Hydrogen bonds, on the other hand, occur between hydrogen atoms of one molecule and either a nitrogen, an oxygen, or a fluorine atom in another molecule. Since hydrogen bonds occur between molecules, they are classified as intermolecular bonds. There are some situations in which hydrogen bonds may be formed intramolecularly, but these are special cases. 

Example Question #8 : Molecular Bonds And Forces

A student notes that a molecule forms hydrogen bonds that involve chlorine atoms. What can you conclude about this observation?

Possible Answers:

This observation is valid

The observation is invalid because chlorine cannot form hydrogen bonds because it is a gas at room temperature

This observation is invalid because chlorine is a halogen and cannot form hydrogen bonds

This observation is invalid because chlorine cannot form hydrogen bonds due to its smaller electronegativity

Correct answer:

This observation is invalid because chlorine cannot form hydrogen bonds due to its smaller electronegativity

Explanation:

Hydrogen bonds occur between a hydrogen atom in one molecule and either nitrogen, oxygen, or fluorine atom in another molecule. It only involves nitrogen, oxygen, or fluorine atoms because these atoms have high electronegativity. Since they have very high electronegativity, these three atoms can easily attract the electron found in the hydrogen atom and form hydrogen bonds. Chlorine doesn’t have high enough electronegativity to form hydrogen bonds.

An atom's state of matter does not determine whether an atom can form hydrogen bonds. Fluorine, just like chlorine, is a halogen and it can form hydrogen bonds.

Example Question #9 : Molecular Bonds And Forces

Which of the following is not true about hydrogen bonds?

Possible Answers:

They contribute to the three dimensional structures of larger macromolecules

They can form both intermolecularly and intramolecularly

They can only form intramolecularly 

Bond strength is partially determined by the bond angle

They are stronger than van der Waals interactions

Correct answer:

They can only form intramolecularly 

Explanation:

Hydrogen bonds can occur between two molecules (intermolecular) and within parts of a single molecule (intramolecular). They are weaker than covalent bonds but are stronger than a Van der Waals interaction, and this strength is determined by a number of factors including bond angle. 

Example Question #10 : Molecular Bonds And Forces

Hydrogen bonds mostly occur between hydrogen and which other atoms?

Possible Answers:

Scandium and vanadium

Sodium, magnesium and aluminum

Silicon and Germanium

Neon, argon, and xenon

Oxygen, nitrogen, and fluorine

Correct answer:

Oxygen, nitrogen, and fluorine

Explanation:

For a hydrogen bond to occur, the atom to which the hydrogen is bonded has to have a high relative Pauling electronegativity. Only oxygen, nitrogen, and fluorine, in the upper right corner of the periodic table, have these electronegativities. Neon, argon, and xenon are, of course, inert noble gases, although xenon is sometimes assigned a high electronegativity. This is due, however, to rare bonding events -- certainly not regular hydrogen bonding. Sodium, magnesium and aluminum often form positively charged ions, so they would not tend to attract a hydrogen nucleus.

Example Question #1 : Hydrogen Bonds

What is the maximum number of hydrogen bonds that one water molecule can form?

Possible Answers:

Three

Four

Two

One

Correct answer:

Four

Explanation:

Hydrogen can form hydrogen bonds with nitrogen, oxygen, and fluorine. A water molecule consists of an oxygen atom bonded to two hydrogen atoms. Each hydrogen atom can form a hydrogen bond with a nitrogen, fluorine, or oxygen atom. Also, the oxygen, which has two lone pairs of electrons, can form two hydrogen bonds with hydrogen atoms. This sums to four hydrogen bonds per water molecule.

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