Reduced Acid Dissociation of Amino-Acids at the Surface of Water
J. Am. Chem. Soc. , Volume 139 - Issue 10 p. 3716- 3720
We use surface-specific intensity vibrational sum-frequency generation (VSFG) and attenuated total reflection spectroscopy (ATR) to probe the ionization state of the amino-acids L-alanine and L-proline at the air/water surface and in the bulk. The ionization state is determined by probing the vibrational signatures of the carboxylic acid group, representing the non-dissociated acid form, and the carboxylate anion group, representing the dissociated form, over a wide range of pH values. We find that the carboxylic acid group deprotonates at a significantly higher pH at the surface than in the bulk.