Chemistry Reference
In-Depth Information
Table 21.1
Mid-IR signal characteristics of carbohydrates.
Spectral Region (cm
−1
)
Functional Group Vibration
Intensity
1470-1460
C-H asymmetric bending
strong
1460-1400
C-O symmetric stretching
strong
1420-1406
C-H in-plane bending
weak
1340-1280
C-O stretching
strong
1175-1125
C-H in-plane bending
weak
1150-1070
C-O-C asymmetric
stretching
strong
1045-1035
C-O stretching
medium-strong
1005-990
C-H bending
very strong
Table 21.2
Mid-IR signal assignment of phospholipid sub-structures.
Spectral Region (cm
−1
)
Functional Group Vibration
Division
1750-1700
C
=
O stretching
interfacial
1500-1450
CH
2
bending
chain
1395-1360
CH
3
symmetric bending
chain
1230-1210
PO
2
−
asymmetric stretching
head
1105-1070
PO
2
−
symmetric stretching
head
1065-1050
C-O-P stretching
interfacial
995-970
CH
3
-N asymmetric stretching
head
Proteins conduct a wide range of essential functions in body. Tissue construction and repair, water balance
process, enzymatic and hormonal activities, immune service infrastructures, transport services (e.g., in
hemoglobin), contractile action and antibody forming are some of the most well-known functions of polymers.
The proper function of proteins is due to appropriate structural changes which would occur intramolecularly.
There are several rearrangements in proteins, for example; torsion, hydrogen bonding, reorientation,
nucleation, secondary structure folding, aggregation and side chain fluctuation all of which occur very fast
and usually within 10
−3
-10
−12
seconds. IR spectroscopy has been widely applied in the study of these structural
changes, especially for folding, secondary structure analysis and ligand interaction monitoring. The main
reported features are related to amide functional groups of proteins which are called amide A, B and I-VII.
Amides A and B's signals appear above the 3000 cm
−1
wavenumber and the others are below 1800 cm
−1
. It is
mentionable that in sight of technical limitations only amides I and II (sometimes also amide III) could be
utilized for further analytical aims as the most specific signals of protein structures.
Amide I is the most widely used band due to the protein backbone conformation and is particularly sensitive
to structural conformation. This signal is in close relation with secondary protein structures such as carbonyl
and C-N stretching; the major and minor contribution respectively. On the other hand nitrogen joint bonds are
the main signals of amide II. The main assigned ones are C-N stretching and N-H bending. Although amide
II is not as informative as amide I, it is included with amide I characteristics to improve the analysis efficiency.
Table 21.3 shows more detailed information about IR spectral signals of analytically useful amide structures.
Of course these bands are usually observed in coupling form and their separation is impossible.
DNA and RNA nucleic acids are vital structures which are very important for a bio-system. They carry the
genetic codes which define the protein structures. The main constituents of nucleic acids are the phosphate
group, saccharide sector and base structure. Base-pairing, base-stacking, base-sugar coupling and phosphate-
sugar interaction are the main reasons why the IR spectrum of nucleic acid is really complex to interpret. In
