zishuai huang, wei hua and heather c. allen [email protected] department of chemistry and...
TRANSCRIPT
BINDING OF Na+ AND K+ TO THE HEADGROUP OF PALMITIC ACID MONOLAYERS
STUDIED BY VIBRATIONAL SUM FREQUENCY GENERATION SPECTROSCOPY
Zishuai Huang, Wei Hua and Heather C. [email protected]
Department of Chemistry and Biochemistry, The Ohio State University, 100 W. 18th Ave., Columbus, OH, 43210
67th International Symposium on Molecular Spectroscopy
June 19, 2012
BackgroundAlkali cations are critical in biological systems due to their
electrical interaction with biological ligands in cell membranes and proteins.
Na+ and K+ are important in neuron function and regulation between cells and extracellular fluid
http://highered.mcgraw-hill.com
While Na+ and K+ are in the same chemical group, they can exhibit differences when interacting with biological relevant molecules, such as phospholipids and fatty acids.
Campbell, Neil (1987). Biology. Benjamin/Cummings
GoalsTo investigate the interaction between
biologically relevant cations Na+ and K+ and carboxylic group (COOH) in fatty acid at air-aqueous interfaces
How the interaction is related to the subphase concentration and cation identity
Vibrational Sum Frequency Spectroscopy (VSFG) Principle
VSFG selection rules:
Lack of inversion is required
Vibrational mode must be both IR and Raman active
SFG signal is enhanced when incident IR frequency is resonant with a vibrational mode of a molecule
visIR SFG
+ =
(2) 2 (2) (2) 2| | | |SFG vis IR NR v vis IRv
I I I I I (2) vv
IR v v
A
i
Instrumental Setup
MaterialsLangmuir monolayer was used to investigate the
binding with biological ligands at air-water interface
Palmitic acid (PA, CH3(CH2)14CO2H) was used as a model surfactant in this study.
PA was dissolved in chloroform at 1.5 mM
NaCl and KCl solutions were filtered to remove organics before use
http://en.wikipedia.org/wiki/File:Palmitic-acid-3D-balls.png
Experiment
Salt solutions were added to Petri dishes
PA was spread to the surface of salt solutions by micro syringe
10 min was allowed for complete solvent evaporation before taking spectra
VSFG Spectra of PA in OH Region
The free OH signal (3700 cm-1) disappears in the presence of PA
Two broad continuums and one small shoulder are observed in PA on water
3000 3200 3400 3600 3800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
PA on water
VS
FG
inte
nsity
(a.
u.)
wavenumber (cm-1)
VSFG Spectra of PA in OH Region
Loss of OH oscillator signal with increasing concentration results from the screening-out effect of the cations
K+ shows greater screening-out effect to COO- than Na+
3000 3200 3400 3600 3800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.2M NaCl 0.6M NaCl
VS
FG
inte
nsity
(a.
u.)
wavenumber (cm-1)
3000 3200 3400 3600 3800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.2M KCl 0.6M KCl
VS
FG
inte
nsi
ty (a.u
.)
wavenumber (cm-1)
VSFG Spectra of PA in CO2- Region
K+ seems to have a greater ability of deprotonating COOH and binding to CO2
-
Mn+
H2O hydrated
complexed
1410 cm-1
1470 cm-1
1350 1400 1450 1500 1550
0.0
0.2
0.4
0.6
0.6M NaCl 0.6M KCl
VS
FG
inte
nsity
(a.
u.)
wavenumber (cm-1)
SummaryAlkali cations in the subphase have impact on
the PA monolayer
More negative charge on CO2- is screened out
with higher subphase concentration
K+ and Na+ show a significant difference in deprotonating COOH and binding to CO2
-
Thank you