Page 361 - Carrahers_Polymer_Chemistry,_Eighth_Edition

324 Carraher’s Polymer Chemistry

right. Thus the trimer
H O O O
HN−CH −C−O−NH−CH−C−O−NH−CH−C−O−H
2
becomes (10.5)
CH 3 CH 2

OH
Gly–Ala–Ser or GlyAlaSer or EGAS, where the E signals the N-terminus.
It is important to remember that all proteins are polypeptides composed of a mixture of the
20 amino acids given in Table 10.1. The size of proteins is generally noted in either the number of
amino acids units or more likely the molecular weight in kilodaltons (kDa).
Proteins are synthesized in nature from the N-terminus to the C-terminus. While all of the amino
acids have only one variation with that variation being a lone substituent on the alpha carbon, this
variation is sufficient to produce the variety of proteins critical to life. Some of these contain an

“excess” of acid groups such as aspartic acid, while others contain one or more additional basic
groups such as arginine. Still others possess sulfur-containing moieties, namely cysteine and methi-
onine, which account for the need for sulfur to be present in our diets and eventually end up in our
fossil fuels producing sulfur oxides on burning. All possess nitrogen-containing moieties and these
also eventually end up in our fossil fuels and produce nitrogen oxides on burning. The presence of
these two elements is then connected to acid-rain production.
The amino acids may be neutral, acidic, or basic, in accordance with the relative number of amino
and carboxylic acid groups present. Cations can be formed with amino acids like tryptophane, lysine,
histidine, and arginine, which have additional amine groups, while others that contain additional
acid groups can be hydrolyzed to form anions like aspartic acid and glutamic acid. The presence of
varying amounts of these amino acid moieties within a protein are primary driving forces for the sep-
aration of proteins using electrophoresis and result in polypeptides having different isoelectric points.
If there are a number of acidic and/or basic groups on the polypeptide, the molecule is said to be a
polyampholyte or if they contain only positive or negative charges, they are called polyelectrolytes.
The behavior of these charged polypeptides is similar to the behavior of other charged polymers.
Thus, a fully hydrolyzed poly(acrylic acid) acts as a rod because the negative sites repeal one another
while a polypeptide with a large number of negative sites will also be elongated. The spacing and
number of these charged sites helps determine the tertiary structure of such polypeptides.
Even though the atoms within a peptide bond are coplanar they can exist in two possible con-
fi gurations, cis and trans.
O C C C
C − N or C − N
(10.6)
C H O H
trans cis

The trans form is usually favored whenever there is a bulky group on the adjacent alpha carbon(s)
because bulky groups will interfere more in the cis structure.
While humans synthesize about a dozen of the 20 amino acids needed for good health, the other
eight are obtained from outside our bodies, generally from eating foods that supply these essential
amino acids (Table 10.1). Different foods are good sources of different amino acids. Cereals are
generally defi cient in lysine. Thus, diets that emphasize cereals will also have other foods that can
supply lysine. In the orient, the combination of soybean and rice supply the essential amino acids
while in Central America bean and corn are used.

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