Biology Reference
In-Depth Information
associated with stretching and bending vibrations that occur at discrete positions in the mid-
IR spectral range (between 2,500 and 25,000 nm). Traditionally, IR studies do not report wave-
lengths but rather wave numbers. The spectral range for FT-IR is therefore 4,000 to 400 cm
1
.
Of particular importance to membrane studies are the absorbance bands at about 2,900 (C-H
stretching), 1,740 (C
¼
O stretching in esters and carboxylic acids), 1,400 (C-O stretching in
carboxylates). 1,235 (P
¼
O stretching in phosphate esters), and 1,000 (C-O stretching in carbo-
hydrates) cm
1
.
For phospholipids, the acyl tails (including trans/gauche ratios) are followed by C-H
vibrations (2,800
3,100 cm
1
),
the glycerol-acyl chain interface by ester stretching
e
1,240 cm
1
)
[45,46]
.
Therefore, FT-IR spectra provide a complete 'molecular snapshot' yielding information
on phospholipid conformation, and dynamics from the tip of the polar head group to
the omega end of the acyl tail.
Figure 9.26
shows the gel to liquid crystal phase transition
as determined by the absorbance maximum of the symmetric stretching band of CH
2
groups in liposomes composed of: a. 16:1,16:1 PC; b. 18:1,18:1 PC; c.14:0,14:0 PC; and
d. 16:0,16:0 PC
[46]
.T
m
s are in close agreement with values obtained by other methods
including DSC.
1,750 cm
1
) and the head group phosphate stretching (1,220
(1,700
e
e
Fluorescence Polarization
Another method that is commonly used to follow lipid phase transitions and fluidity in
membranes is fluorescence polarization. Fluorescence was first reported, and the term
coined, by George Gabriel Stokes (
Figure 9.27
) in 1852
[46,47]
. Stokes noted that the mineral
fluorite (calcium fluoride), emitted visible light when illuminated with 'invisible radiation'
(later identified as ultra-violet light). It was from this simple, but seemingly unrelated
2854
2853
2852
a
2851
b
c
d
2850
220
240
260
280
Temperature (K)
300
320
340
FIGURE 9.26
Gel to liquid crystal phase transition as determined by the absorbance maximum of the symmetric
stretching band of CH
2
groups (by FT-IR) in liposomes composed of: a. 16:1,16:1 PC; b. 18:1,18:1 PC; c.14:0,14:0 PC;
d. 16:0,16:0 PC. T
m
s are in close agreement with values obtained by other methods including DSC
[46]
.
