Answers to Concept Checks

10.1

All amino   acids,  except  glycine,    have    chiral  α-carbons.  Because the R   group   of  glycine is  a

hydrogen    atom,   it  is  not chiral  and therefore   is  not optically   active.

Amphoteric  molecules   can act as  acids   or  bases.  Carboxylic  acids   give    amino   acids   their   acidic

properties  because they    can be  deprotonated.   Amino   groups  give    amino   acids   their   basic

properties  because they    can be  protonated.

Peptide bonds   are formed  by  a   condensation    reaction,   in  which   water   is  lost,   and cleaved

hydrolytically  by  strong  acid    or  base.

The C–N bond    of  an  amide   is  planar  because it  has partial double-bond character   due to

resonance.  Double  bonds   exist   in  a   planar  conformation    and restrict    movement.

10.2
An aldehyde, ammonium chloride (NH 4 Cl), and potassium cyanide (KCN) are used to make the
aminonitrile; water is used to hydrolyze the aminonitrile to form the amino acid.
Strecker synthesis is a condensation reaction (formation of an imine from a carbonyl-
containing compound and ammonia, with loss of water), followed by nucleophilic addition
(addition of the nitrile group), followed by hydrolysis.
Gabriel synthesis begins with potassium phthalimide and diethyl bromomalonate, followed by
an alkyl halide. Water is then used to hydrolyze the resulting compound to form the amino acid.
While acids and bases are used at various times as catalysts, they are not main reactants.
Gabriel synthesis proceeds through two SN2 reactions, hydrolysis, and decarboxylation.

10.3
Inorganic phosphate contains a very negative charge. When bonded to other phosphate groups
in a nucleotide triphosphate, this creates repulsion with adjacent phosphate groups, increasing
the energy of the bond. Further, inorganic phosphate can be resonance-stabilized.
Organic phosphates are carbon-containing molecules with phosphate groups; the most
common examples are nucleotides, like those in DNA, ATP, or GTP.
The three hydrogens in phosphoric acid have very different pKa values. This allows phosphoric
acid to pick up or give off protons in a wide pH range, making it a good buffer over most of the pH