Orientational Behavior and Vibrational Response of Glycine at Aqueous Interfaces

Abstract

Aqueous glycine plays many different roles in living systems, from being a building block for proteins to being a neurotransmitter. To better understand its fundamental behavior, we study glycine's orientational behavior near model aqueous interfaces, in the absence and presence of electric fields and biorelevant ions. To this purpose, we use a surface-specific technique called heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG). Using HD-VSFG, we directly probe the symmetric and antisymmetric stretching vibrations of the carboxylate group of zwitterionic glycine. From their relative amplitudes, we infer the zwitterion's orientation near surfactant-covered interfaces and find that it is governed by both electrostatic and surfactant-specific interactions. By introducing additional ions, we observe that the net orientation is altered by the enhanced ionic strength, indicating a change in the balance of the electrostatic and surfactant-specific interactions.

Figure 1

Comparison of chemical structures and vibrational features of different glycine species. (a) Left to right: chemical structures of cationic, zwitterionic, and anionic glycine. (b) Steady-state infrared absorption spectra (A) of glycine, recorded at acidic/neutral/basic conditions in heavy water (D₂O), plotted in function of the spatial frequency of the exciting infrared light . In the presented spectra, we subtracted the infrared absorption of the solvent and normalized the signals to the sample thickness, see Supporting Information. The main vibrational features are assigned in Table 1. (c) HD-VSFG spectra (Im(χ⁽²⁾), SSP polarization) of 1 M glycine solutions at neutral pD, at the neat D₂O/air interface and in the presence of monolayers of charged surfactants. The above spectra are presented after subtracting the corresponding HD-VSFG spectra of neat and surfactant-covered D₂O/air interfaces, see Figure S5.

Figure 2

Semiempirical framework connecting the zwitterion’s orientation with the HD-VSFG signals of its COO^– group. (a) Definition of the angle θ, as the angle between the surface normal and the symmetry axis of the zwitterion’s COO^– group. (b) Relative Im(χ⁽²⁾) contribution of the two main carboxylate modes of zwitterionic glycine, derived using the theoretical and experimental results of earlier works.⁻ These works use the assumption that the COO^– group can freely rotate around the C–C bond. We show ratios in Table 2. Below: Illustration of zwitterionic glycine molecules, oriented due to (c) the electric field induced by negative surface charges, (d) the interaction of the amine group and the surfactant monolayer, (e) the electric field induced by positive surface charges, and (f) the interaction of the carboxylate group and the surfactant monolayer.

Figure 3

Effect of the addition of salts on the HD-VSFG signals of glycine. (a) HD-VSFG spectra of 2 M glycine in DS^–-covered H₂O, with an increasing concentration of added NaCl. The negative feature at 1620 cm^–1 contains the broad HD-VSFG response of of zwitterionic glycine as well as that of the bending mode of H₂O (ν̃_IR = 1643 cm^–1). (b) Effect of exchanging salt ions (c^salt = 100 mM). For both sets of spectra, we subtracted the quadrupolar SFG contribution of neat H₂O, which does not change with added salt concentration.

Figure 4

Comparison of HD-VSFG spectra of 1 M glycine + 1 M NaCl solutions, at the neat D₂O/air interface and in the presence of monolayers of charged surfactants. The addition of Na⁺ and Cl^– ions does not change the zwitterion signals at the neat water/air interface, see Figure S12. For display purposes, we subtracted SFG contributions of the corresponding neat/surfactant-covered D₂O/air interfaces.