Zishuai Huang, Wei Hua and Heather C. Allen Department of Chemistry and Biochemistry, The Ohio State University, 100 W. 18 th Ave., Columbus, OH, th International Symposium on Molecular Spectroscopy June 19, 2012

Background Alkali 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 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

Goals To 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 +=

Instrumental Setup

Materials Langmuir monolayer was used to investigate the binding with biological ligands at air-water interface Palmitic acid (PA, CH 3 (CH 2 ) 14 CO 2 H) 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

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

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 +

VSFG Spectra of PA in CO 2 - Region K + seems to have a greater ability of deprotonating COOH and binding to CO 2 - M n+ H2OH2O hydrated complexed 1410 cm cm -1

Summary Alkali cations in the subphase have impact on the PA monolayer More negative charge on CO 2 - is screened out with higher subphase concentration K + and Na + show a significant difference in deprotonating COOH and binding to CO 2 -