Biology Reference
In-Depth Information
FIGURE 9.25
Julian M. Sturtevant (1908
e
2005). Courtesy of Yale University
Julian M. Sturtevant (
Figure 9.25
) was a pioneer in the field of biophysical chemistry with
a world reputation on the application of thermochemistry to biological systems. He pub-
lished into his 90s and died at the age of 97.
Since the DSC scan shape, and hence calculated cooperativity units, can be altered by
changing the scan rate (a faster rate produces a broader curve and hence apparent lower
cooperativity), the cooperativity unit calculations only have meaning compared to
another lipid run under identical conditions. DSCs have the ability to scan over a wide
range of temperature ramps. A 'normal' scan rate would be ~5
C/hr. This rate is suffi-
ciently fast to allow for a complete scan in a reasonable amount of time, but not so fast
as to obliterate curve structure. Also, lipids may become hydrolyzed or oxidized if the
sample is left in the DSC crucible for too long. However, some very slow scan rates
(~0.1
C/hr) may be employed to examine events occurring over a very narrow tempera-
ture range.
Fourier Transform Infrared (FT-IR) Spectroscopy
Although DSC is the best method to analyze lipid phase transitions, many other method-
ologies have successfully been employed. A very different method involves Fourier Trans-
form Infrared (FT-IR) spectroscopy. FT-IR is a versatile, non-perturbing, powerful
technique that can be used to obtain information on all regions of the phospholipid molecule
simultaneously (for general discussion of FT-IR see
[43,44]
). Instead of (slowly) scanning
through the entire spectrum as is done in most UV-visible applications, FT-IR irradiates
the sample with all IR wavelengths at once and the entire signal is captured and subjected
to Fourier analysis. This allows for multiple spectra to be rapidly obtained and the signals
averaged resulting in vastly improved sensitivity. Nanogram samples can be accurately
analyzed by FT-IR. Modern FT-IR spectrometers also have the advantage of being able to
eliminate the large, troublesome water absorbance, allowing for the ability to measure
membranes dispersed in aqueous solution
[45,46]
.
FT-IR spectroscopy detects vibrations during which the electrical dipole moment changes.
Of course this time is very short (~10
11
s
1
). Most of the measured absorbance bands are
