Amino acids: zwitterions, isoelectric point

45 questions on Amino acids: zwitterions, isoelectric point from the Biomolecules chapter: 10 NEET level, 18 JEE Main, 17 JEE Advanced. Each one opens with its answer and solution.

• Which of the following natural alpha-amino acids is optically inactive? NEET level
• Which of the following is an essential amino acid (as per the NCERT list)? NEET level
• The zwitterionic (dipolar ion) form of glycine is represented by NEET level
• Which of the following amino acids is basic in nature? NEET level
• Alpha-amino acids react with ninhydrin on warming to give a coloured product. The colour is NEET level
• Naturally occurring alpha-amino acids obtained from proteins belong to which stereochemical series? NEET level
• Amino acids are crystalline solids with melting points much higher than those of haloalkanes or carboxylic acids of similar molar mass, mainly because JEE Main level
• In strongly acidic solution (pH about 1) alanine exists predominantly as JEE Main level
• An amino acid with a neutral side chain has pK_a1 = 2.20 (COOH) and pK_a2 = 9.60 (NH3+). Taking the isoelectric point as the mean of the two pKa values, its pI ... JEE Main level
• Glycine has pI = 6.0. In an electrophoresis experiment at pH 10, glycine migrates JEE Main level
• The total number of stereoisomers possible for the amino acid threonine, CH3-CH(OH)-CH(NH2)-COOH, is JEE Main level
• Assertion (A): Amino acids are amphoteric in nature. Reason (R): An amino acid contains both an acidic carboxyl group and a basic amino group. JEE Main level
• Match the amino acids with their class: P = glutamic acid, Q = lysine, R = valine; classes: 1 = acidic, 2 = basic, 3 = neutral. JEE Main level
• Glycine reacts with nitrous acid, HNO2, to give JEE Main level
• Which of the following are essential amino acids (NCERT list)? JEE Main level
• Which statements about glycine are correct? (For the pI statement, use pI = mean of pK_a1 = 2.34 and pK_a2 = 9.60.) JEE Main level
• Alanine has pK_a1 = 2.3 (COOH) and pK_a2 = 9.7 (NH3+); its pI is 6.0. Which statements are correct? JEE Main level
• Glutamic acid has three ionisable groups: alpha-COOH (pK_a = 2.2), side-chain COOH (pK_a = 4.3) and alpha-NH3+ (pK_a = 9.7). Taking pI as the mean of the two pK... JEE Advanced level
• Lysine has pK_a values of 2.2 (alpha-COOH), 9.0 (alpha-NH3+) and 10.5 (side-chain NH3+). Taking pI as the mean of the two pKa values that flank the neutral spec... JEE Advanced level
• The pI values are computed with pI = mean of the two pKa values flanking the neutral species. Data: aspartic acid pKa = 2.0 (alpha-COOH), 3.9 (side-chain COOH),... JEE Advanced level
• Alanine has pK_a1 = 2.3 and pK_a2 = 9.7. At pH 9.70 the average net charge per molecule (ignoring the cationic form, which is negligible) is JEE Advanced level
• The total number of stereocentres present in one molecule each of L-threonine, L-isoleucine, L-cysteine and glycine taken together is JEE Advanced level
• 0.15 mol of aspartic acid hydrochloride, HOOC-CH2-CH(NH3+Cl-)-COOH, is treated with just enough NaOH to convert it completely into NaOOC-CH2-CH(NH2)-COONa (plus... JEE Advanced level
• Histidine has pK_a values of 1.8 (alpha-COOH), 6.0 (imidazolium side chain, +1 -> 0 on deprotonation) and 9.2 (alpha-NH3+). The average net charge per molecule ... JEE Advanced level
• Cysteine has pK_a values of 1.96 (COOH), 8.18 (side-chain SH) and 10.28 (NH3+). Taking pI as the mean of the two pKa values flanking the neutral species, the pI... JEE Advanced level
• Aspartic acid has pK_a values 2.0 (alpha-COOH), 3.9 (side-chain COOH) and 10.0 (NH3+). Taking pI as the mean of the pKa values flanking the neutral species, whi... JEE Advanced level
• Which statements about the amino acid threonine, CH3-CH(OH)-CH(NH2)-COOH, are correct? JEE Advanced level
• Alanine has pK_a1 = 2.34. Ignoring the anionic form, at what pH is the ratio of zwitterion to cation equal to 20 : 1? Give the answer to two decimal places (use... JEE Advanced level
• Which of the following natural alpha-amino acids is optically inactive? NEET level
• The zwitterionic form of alanine is NEET level
• Which of the following is an essential amino acid? NEET level
• On heating with ninhydrin, a typical alpha-amino acid gives a NEET level
• The tetrapeptide Gly-Ala-Ser-Val (N-terminal to C-terminal) is formed from the four free amino acids. The number of peptide bonds in it and the number of water ... JEE Main level
• For glycine, pK_a1 (-COOH) = 2.34 and pK_a2 (-NH3+) = 9.60. Taking the isoelectric point as the mean of the two pKa values flanking the neutral species, pI is JEE Main level
• Aspartic acid has pK_a values of 1.88 (alpha-COOH), 3.60 (side-chain COOH) and 9.60 (alpha-NH3+). Taking pI as the mean of the two pKa values that flank the net... JEE Advanced level
• Lysine has pK_a values 2.18 (alpha-COOH), 8.95 (alpha-NH3+) and 10.53 (side-chain NH3+). Taking pI as the mean of the two pKa values flanking the net-neutral sp... JEE Advanced level
• Alanine has pK_a1 = 2.34 and pK_a2 = 9.69. In an aqueous solution at pH 12, which statements are correct? JEE Main level
• Which statements about glycine are correct? JEE Main level
• A mixture of glycine (pI = 5.97), aspartic acid (pI = 2.74) and lysine (pI = 9.74) is placed in a buffer at pH 4.0 for paper electrophoresis. Which statements a... JEE Advanced level
• Consider the tetrapeptide Ala-Thr-Ile-Gly (N-terminal to C-terminal) and suppose each stereocentre in the chain may independently have either configuration. Thr... JEE Advanced level
• Gly and Ala are the only amino acids available, both must appear in the product, and each may be used more than once. The number of different tripeptides (seque... JEE Main level
• Assertion (A): Amino acids have much higher melting points than amines or carboxylic acids of comparable molar mass. Reason (R): In the solid state amino acids ... JEE Main level
• Match the amino acid in List I with its description in List II. List I: (P) Lysine; (Q) Aspartic acid; (R) Glycine; (S) Valine. List II: (1) acidic side chain; ... JEE Advanced level
• For glycine (pK_a1 = 2.34, pK_a2 = 9.60), the average net charge of the species present at pH 2.34 is, to the nearest 0.1, JEE Main level
• Alanine has pK_a1 = 2.34 and pK_a2 = 9.69. At pH 7.0 the ratio [anion]/[cation] (i.e. H2N-CH(CH3)-COO- to H3N+-CH(CH3)-COOH) is closest to JEE Advanced level