Question 1

Which of the following has partial double-bond character?&#10;A)hydrogen bond&#10;B)peptide bond&#10;C)phosphoanhydride bond&#10;D)van der Waals bond

Accepted Answer

Peptide bond has partial double-bond character due to resonance between the oxygen and nitrogen atoms. This resonance results in a partial double bond between the carbon and nitrogen atoms. Hydrogen bond and van der Waals bond are not covalent bonds, and phosphoanhydride bond is a covalent bond but does not have partial double-bond character.

Question 2

What is the pH of a serine solution in which the a-NH&#8323;&#8314; group,which has a pK&#8336; 9.2,is one-third dissociated?&#10;A)8.7&#10;B)8.9&#10;C)9.0&#10;D)9.5

Accepted Answer

The dissociation equation for the a-NH₃⁺ group of serine is:

NH₃⁺ ⇌ H⁺ + NH₂

The equilibrium expression is:

Ka = [H⁺][NH₂] / [NH₃⁺]

Given that one-third of NH₃⁺ is dissociated, we can find the concentration of NH₃⁺ and NH₂:

[NH₃⁺] = 2/3 [serine]
[NH₂] = 1/3 [serine]

Substitute these values into the equilibrium expression:

Ka = [H⁺] (1/3 [serine]) / (2/3 [serine])

Simplifying:

Ka = [H⁺] / 2

pKa = 9.2

pH = pKa + log([base]/[acid])
pH = 9.2 + log(1/2) = 8.9

Therefore, the pH of the solution is 8.9, which corresponds to choice B.

Question 3

Which of the following does NOT react with the -SH group on cysteine?&#10;A)N-ethylmaleimide&#10;B)benzaldehyde&#10;C)acrylonitrile&#10;D)iodoacetate

Accepted Answer

Benzaldehyde does not react with the -SH group on cysteine. The other three options, N-ethylmaleimide, acrylonitrile, and iodoacetate, all react with the -SH group through various mechanisms.

Question 4

Histidine has pK&#8336; values of 1.8,6.0,and 9.2.At which of the following pH values would histidine be found with a net negative charge?&#10;A)1.0&#10;B)4.0&#10;C)8.0&#10;D)11.0

Accepted Answer

When the pH is below the pKa of a group, that group will be protonated and positively charged. When the pH is above the pKa of a group, that group will be deprotonated and negatively charged. Therefore, histidine will have a net negative charge when the pH is greater than its highest pKa value of 9.2. Therefore, at a pH of 11.0, histidine will be negatively charged.

Question 5

Glutamic acid has pK&#8336;'s at 2.2,4.3,and 9.7.What is the isoelectric point for glutamic acid?&#10;A)3.25&#10;B)4.3&#10;C)5.4&#10;D)7.0

Accepted Answer

The isoelectric point (pI) of an amino acid with three pKa values (such as glutamic acid, which has both a carboxylic acid group and an amino group in its side chain) is calculated as the average of the pKa values that flank the zwitterionic form. For glutamic acid, the relevant pKa values are 2.2 (for the carboxyl group) and 4.3 (for the carboxyl side chain), making the pI (2.2 + 4.3) / 2 = 3.25.

Question 6

Which of the following amino acid derivatives is correctly derived from its corresponding commonly occurring amino acid?&#10;A)GABA is a neurotransmitter derived from aspartic acid.&#10;B)Histamine is a neurotransmitter derived from proline.&#10;C)Epinephrine is a hormone derived from tyrosine.&#10;D)Serotonin is a neurotransmitter derived from phenylalanine.

Accepted Answer

Epinephrine is a hormone derived from the amino acid tyrosine through a series of enzymatic reactions. This process involves the conversion of tyrosine to L-DOPA, then to dopamine, and finally to norepinephrine, which is further methylated to produce epinephrine.

Question 7

Based on their properties,which amino acids are correctly classified?&#10;A)Alanine and valine are neutral,nonpolar amino acids.&#10;B)Aspartic acid and asparagine are acidic amino acids.&#10;C)Serine and glutamine are polar,charged amino acids.&#10;D)Lysine and arginine are basic,uncharged amino acids.

Accepted Answer

Alanine and valine are both nonpolar due to their lack of charged atoms or functional groups. They are also neutral since they do not have a net positive or negative charge.

Question 8

Which of the following amino acids is classified as a basic amino acid?&#10;A)leucine&#10;B)aspartic acid&#10;C)asparagine&#10;D)lysine

Accepted Answer

Lysine is classified as a basic amino acid because it contains a positively charged amino group (-NH3+) on its side chain. Leucine is classified as a nonpolar, aliphatic amino acid. Aspartic acid and asparagine are both classified as acidic amino acids because they contain negatively charged carboxyl groups (-COO-).

Question 9

Which of the following results from resonance in peptide bonds? A)a C?-carbon that is out of the coplanar group of atoms B)unrestricted rotation in the peptide backbone at the N-C_? bond and C_?-Co bond C)approximately 40% double-bond character D)Co-N bond distance that is longer than normal,but shorter than C=N bonds

Accepted Answer

Resonance in peptide bonds results in approximately 40% double-bond character, which contributes to the stability of the peptide bond.

Question 10

What effect does the presence of the a-COO- group of an amino acid have on the pK&#8336; value of its a-NH&#8323;&#8314; group?&#10;A)The pKa value increases by >2 pH units.&#10;B)The pKa value decrease by > 2 pH units.&#10;C)The pKa value increases by ~1.5 pH units.&#10;D)The pKa value decrease by ~ 1.5 pH units.

Accepted Answer

The presence of the α-COO- group of an amino acid has an acidic effect on the α-NH₃⁺ group, decreasing its pKₐ value by approximately 1.5 pH units. This is because the negatively charged α-COO- group can form a salt bridge with the positively charged α-NH₃⁺ group, stabilizing the protonated form and making it more difficult to lose a proton.

Question 11

At what pH is the cysteine side-chain sulfhydryl group 11% dissociated if its pK&#8336; value is 8.32?&#10;A)3.2&#10;B)7.4&#10;C)8.5&#10;D)10.2

Accepted Answer

The answer of At what pH is the cysteine side-chain...

Question 12

Which of the following is a property of ninhydrin?&#10;A)It produces a yellow product upon reaction with histidine.&#10;B)It can be used to detect the location of amino acids following SDS-PAGE.&#10;C)It produces a green product upon reaction with alanine.&#10;D)It reacts with an amino acid by oxidatively deaminating its amino group.

Accepted Answer

The answer of Which of the following is a property...

Question 13

Which of the following amino acids has a side-chain pK&#8336; near neutrality and therefore plays an important role as proton donor and acceptor in many enzyme-catalyzed reactions?&#10;A)histidine&#10;B)cysteine&#10;C)aspartic acid&#10;D)arginine

Accepted Answer

The answer of Which of the following amino acids has...

Question 14

Which of the following amino acids has more than 1 chiral carbon?&#10;A)serine&#10;B)lysine&#10;C)threonine&#10;D)cysteine

Accepted Answer

The answer of Which of the following amino acids has...

Question 15

The amino and carboxyl groups of amino acids react in a head-to-tail fashion and forming a covalent linkage.What is this linkage typically referred to as?&#10;A)an aromatic bond&#10;B)a phosphoanhydride bond&#10;C)a peptide bond&#10;D)a hydrogen bond

Accepted Answer

The answer of The amino and carboxyl groups of amino...

Question 16

Which of the following amino acids most likely to be present if the pI of a peptide is 4.6?&#10;A)glutamic acid&#10;B)histidine&#10;C)arginine&#10;D)cysteine

Accepted Answer

The answer of Which of the following amino acids most...

Question 17

Which of the following tri-peptides would absorb light at 280 nm?&#10;A)Ala-Lys-His&#10;B)Ser-Gly-Asn&#10;C)Ala-Ala-Trp&#10;D)Val-Pro-Leu

Accepted Answer

The answer of Which of the following tri-peptides would absorb...

Question 18

Which of the following is an intrinsic property of the commonly occurring 20 amino acids?&#10;A)asymmetrical a-carbon&#10;B)the inability to polymerize&#10;C)polarity&#10;D)non-chirality

Accepted Answer

The answer of Which of the following is an intrinsic...

Question 19

What type of thermodynamic reaction must be coupled to peptide bond formation in biological systems?&#10;A)adverse&#10;B)compatible&#10;C)reverse&#10;D)favourable

Accepted Answer

The answer of What type of thermodynamic reaction must be...

Question 20

Which of the following amino acids has a free a-carboxyl group but no free a-amino group?&#10;A)aspartic acid&#10;B)proline&#10;C)asparagine&#10;D)lysine

Accepted Answer

The answer of Which of the following amino acids has...

Question 21

Of the following four amino acids which one absorbs light most strongly between 250 and 300 nm?&#10;A)histidine&#10;B)phenylalanine&#10;C)tryptophan&#10;D)tyrosine

Accepted Answer

The answer of Of the following four amino acids which...

Question 22

Which of the following is a characteristic of stereoisomers?&#10;A)A diastereomer is a non-superimposable mirror image.&#10;B)An enantiomer is a superimposable mirror image.&#10;C)Diastereomers rotate plane-polarized light in equal magnitude but opposite directions.&#10;D)Diastereomers have different melting points

Accepted Answer

The answer of Which of the following is a characteristic...

Question 23

Which of the following is a feature of L-isoleucine?&#10;A)It has four stereoisomers.&#10;B)L-allo-leucine would be its diastereomer.&#10;C)It contains a total of 2 symmetric carbons.&#10;D)Using the (R,S)system,it can also be named as (3S,2S)-isoleucine.

Accepted Answer

The answer of Which of the following is a feature...

Deck 4: Amino Acids