Ans. aRef.: Harper's Illustrated Biochemistry, 30th edn.Option aTrueYes, due to its pyrolidone ring is stable at neutral pH (pH 7)Option bFalseNo, glycine is simplest amino acid and not affected by pHOption cFalseNo, it is steric hinderance is there due to branched chain amino acidOption dFalseNo, false, it is not affected by pH due to guanidino groupOption eFalseNo, not affected due to aromaticityAt physiological pH (around 7.4) the carboxyl group of the amino acid is unprotonated and the amino group is protonated. Thus an amino acid with no ionizable R-group would be electrically neutral at this pH. This species is termed a zwitterion.Histidine is a unique amino acid as pKa of its imidazole group permits it at pH 7 to function either as a base or as an acid. A minor change in pH changes the ionization charge on histidine to buffer pH change. Therefore it can serve as a buffer and is most stable at physiological pH.However, when histidine is incorporated into a protein, its side chain can be either positively charged or neutral, depending on the ionic environment provided by the polypeptide chains of the protein.This is an important property of histidine that contributes to the role it plays in the functioning of proteins such as hemoglobin.Extra edge: A tetrahydral carbon atom with four distinct parts is known as chiral. Only GLYCINE does not show chirality.Amino Acid ClassificationNon-polar Amino Acids.Aliphatic: glycine, alanine, valine, isoleucine, leucine.Aromatic: phenylalanine, tryptophan.Cyclic: Proline..Polar Amino Acids.Sulfur-containing: cysteine, methionine..Hydroxyl-containing: serine, threonine.Aromatic: tyrosine.Acidic Amide: asparagine, glutamine.Charged Amino Acids (at physiological pH).Acidic: aspartic acid, glutamic acid.Basic: histidine, lysine, arginine.