Physical Chemistry of Foods

(Section 7.3). It is to be expected that the charged groups can be strongly
hydrated (Section 3.2) and that the aliphatic groups and the aromatic ones
will be hydrophobic (except His owing to its charge and possibly Tyr owing
to its 22 OH).
As an estimate for the tendency to form hydrophobic bonds, one often
assigns ahydrophobicityto the side groups. One way of finding such values is
by determining the solubility of the amino acid in water and in an organic
solvent, say ethanol. The Gibbs free energy of transfer of the amino acid
from ethanol to water is then given by

DE?WG¼RTln

csat;E

csat;W



ð 7 : 1 Þ

expressed in kJ per mole. Herecsatis the solubility of the amino acid per unit
volume. Because the peptide bond is hydrophilic, the free energy of transfer
of glycine (side group 22 H) is subtracted, to obtain a value that is presumed
to be characteristic for the side group of the amino acid in a peptide chain.
In other words,

F¼DE?WGðamino acidÞ
DE?WGðglycineÞð 7 : 2 Þ

whereFis the hydrophobicity in kJ per mole of residue, mostly according to
the scale of Tanford–Bigelow, which is also used to calculate an average
hydrophobicity (over all residues) of a protein. For hydrophobic residues
F>0. There are, however, several other scales of hydrophobicity, by taking
another organic solvent (or taking the vapor phase instead), or by
determination on side group analogues (e.g., methane for alanine), all
puttingF¼0 for glycine. Unfortunately, the scales differ considerably.
Values according to two scales are given in Table 7.1. There is general
consensus on Leu, Ile, Val, Phe, and Trp being hydrophobic (and also Ala,
but with a smallF), whereas no agreement exists about Tyr, Pro, Met, or
Cys.

7.1.2 Primary Structure

The composition of proteins is variable. The percentage of a certain amino
acid may readily vary by a factor of 10 among a number of proteins. Some
amino acids do not occur in some proteins, notably Cys. The composition
largely determines the nutritional quality and to a considerable extent the
chemical reactivity of a protein. Most other properties depend on the
primary structure, i.e., the sequence of amino acid residues, because the
primary structure determines the higher structures (see below), which in turn
determine properties like conformational stability and solubility. Never-