INTRODUCTION - Nanoindentation with Biological Applications - page 10

is a chain of amino acids, linked together by peptide bonds. Peptides
bonds are inflexible and therefore chain rotation can only occur on the
bonds surrounding these peptide bonds. While the primary structure of
proteins arises from covalent peptide linkages, proteins have secondary,
tertiary and quaternary structures that arise from the folding of long
polypeptide chains (frequently more than one chain per protein) into
compact, three-dimensional strcutres due to the actions of ionic bonds
plus “weak” interactions, including hydrogen bonding, van der Waals
bonds, and hydrophobic interactions. These higher-order structures form
spontaneously, as the folded structure is the lowest energy conformation
for that particular sequence of amino acids, and arises due to interactions
between the side-chains (and based on their classification as basic,
acidic, uncharged polar, or nonpolar).

Amino
group

Carboxyl
group

i

Side Chain Peptide Bonds
Figure 1-5. An amino acid on the left and a protein chain, made by polymerization of
amino acids, showing the peptide bonds, on the right.

2.1.3. Sugars

Monosaccharides, or simple sugars, are the basic units of carbohydrates.
Glucose, fructose, galactose, xylose and ribose are all examples of
monosaccharides ( Fig. 1-6 ). Most have the chemical composition
C X (H 2 O) Y , although there are a few exceptions, including deoxyribose
from DNA. Monosaccharides are the building blocks for disaccharides,
such as sucrose.