Food Biochemistry and Food Processing

5 Water Chemistry and Biochemistry 119

attached to the asymmetric alpha carbon. They are
the building blocks of proteins, polymers of amino
acids. At a pH called the isoelectric point, which
depends on the amino acid in question, the dominant
species is a zwitterion, RHC(NH 3
)(COO), which
has a positive and a negative site, but no net charge.
For example, the isoelectric point for glycine is pH
6.00, and its dominant species is H 2 C(NH 3
)
COO. An amino acid exists in at least three forms
due to the following ionization or equilibria:

RHC(NH 3
)(COOH) RHC(NH 3
)(COO)

H
, Ka1

RHC(NH 3
)(COO) RHC(NH 2 )(COO)
H
,
Ka2.

Most amino acids behave like a diprotic acid with
two dissociation constants, Ka1 and Ka2. A few
amino acids have a third ionizable group in their
side chains.
Among the 20 common amino acids, the side
chains of eight are nonpolar, and those of seven are
polar, containing –OH, CuO, or –SH groups.
Aspartic and glutamic acid contain acidic –COOH
groups in their side chains, whereas arginine, histi-
dine, and lysine contain basic –NH or –NH 2 groups.
These have four forms due to adding or losing pro-
tons at different pH values of the solution, and they
behave as triprotic acids. For example, aspartic acid
[Asp (COOH)CH 2 C(NH 3
)(COO)] has these
forms:

AspH
Asp 
H

Asp Asp
H

AspAsp2-
H

Proteins, amino acid polymers, can accept or pro-

vide several protons as the pH changes. At its iso-

electric point(a specific pH), the protein has no net

charge and is least soluble because electrostatic

repulsion between its molecules is lowest, and the

molecules coalesce or precipitate, forming a solid or

gel.

Solutions of Salts

Salts consist of positive and negative ions, and these

ions are hydrated in their solutions. Positive, hydrat-

ed ions such as Na(H 2 O) 6 
, Ca(H 2 O) 82 
, and Al

(H 2 O) 63 
have six to eight water molecules around

them. Figure 5.12 is a sketch of the interactions of

water molecules with ions. The water molecules

point the negative ends of their dipoles towards pos-

itive ions, and their positive ends towards negative

ions. Molecules in the hydration sphere constantly

and dynamically exchange with those around them.

The number and lifetimes of hydrated water mole-

cules have been studied by various methods. These

studies reveal that the hydration sphere is one layer

deep, and the lifetimes of these hydrated water mol-

ecules are in the order of picoseconds (10^12 sec-

onds). The larger negative ions also interact with the

polar water molecules, but not as strongly as do

Figure 5.12.The first hydration sphere of most cations M(H 2 O) 6
, and anions X(H 2 O) 6 ^1. Small water molecules are
below the plane containing the ions, and large water molecules are above the plane.