Elucidation of molecular structures at buried polymer interfaces and biological interfaces using sum frequency generation vibrational spectroscopy

Abstract

Sum frequency generation (SFG) vibrational spectroscopy has been developed into an important technique to study surfaces and interfaces. It can probe buried interfaces in situ and provide molecular level structural information such as the presence of various chemical moieties, quantitative molecular functional group orientation, and time dependent kinetics or dynamics at such interfaces. This paper focuses on these three most important advantages of SFG and reviews some of the recent progress in SFG studies on interfaces related to polymer materials and biomolecules. The results discussed here demonstrate that SFG can provide important molecular structural information of buried interfaces in situ and in real time, which is difficult to obtain by other surface sensitive analytical techniques.

Figure 1

(a) SFG spectra of PMMA before, during, and after contacting water, left: ssp, right: sps. (b) SFG spectra of PBMA before, during, and after contacting water, left: ssp, right: sps. Reprinted with permission from ref. . (2001 American Chemical Society)

Figure 2

(a) Schematic for the molecular surface structure with N(R₂)=3 in water (N(R₂) is the number of carbon atoms for R₂). (b) Schematic for the molecular surface structure with N(R₂)=11 in water. Reprinted with permission from ref. . (2010 American Chemical Society)

Figure 3

Schematic demonstrations of different silane molecules at different polymer/silane interfaces. The boxes represent the bulk silane liquid. The molecular segments including headgroup, backbone, and endgroup of silane molecules are described by the figure legend. Reprinted with permission from ref. . (2003 American Chemical Society)

Figure 4

The psp spectra of hIAPP without DPPG (t=0 and 10 h) and after addition of DPPG (t=10h) at the (a) air/D₂O and (b) air/H₂O interfaces. The psp spectra of rIAPP without DPPG (t=0 and 10 h) and after addition of DPPG (t=10h) at the (a) air/D₂O and (b) air/H₂O interfaces. Reprinted with permission from ref. . (2010 American Chemical Society)

Figure 5

(a) The presence of a cationic lipid monolayer at the air-water interface aligns the first few water layers, generating strong vibrational SFG signal. (b) The binding of DNA and cationic lipids screens the electric charges, disorders the water molecules, leads to a sharp decrease of the water signal. Reprinted with permission from ref. . (2007 American Chemical Society)

Figure 6

(a) Monovalent cations do not affect the geometry of ssDNA. The underlying linker monolayer keeps its ordered conformation. (b) Divalent cations induce ssDNA deformation and the deformation perturbs the linker monolayer. (c) Hybridization in the presence of divalent cations does not introduce further disruption to the linker monolayer structure. Reprinted with permission from ref. . (2008 American Chemical Society)

Figure 7

SFG spectra of PBMA in air and water for (a) ssp, (b) sps polarization combinations. Reprinted with permission from ref. . (2002 American Chemical Society)

Figure 8

Calculated |A _yyz,as/A_yzy,as| of methyl group as a function of orientation angle θ₀ and angle distribution σ. Here the asymmetric vibrational mode of side chain methyl has the same peak width under different polarization measurements, thus $\left|{\chi }_{\mathit{yyz},as}^{(2)}/{\chi }_{\mathit{yzy},as}^{(2)}\right|=\left|A_{\mathit{yyz},as}/A_{\mathit{yzy},as}\right|$. Reprinted with permission from ref. . (2002 American Chemical Society)

Figure 9

(a) Molecular frame coordinates: the (a,b,c) axis is for methyl group fixed coordinates and the (A,B,C) axis is for isopropyl group fixed coordinates. The C axis bisects the two methyl groups (vector v), and the A axis is set in the plane of the two methyl groups. (b) The geometry of 2-propanol in laboratory frame coordinates. θ, ψ and φ are the tilt angle, the twist angle about v and the azimuthal angle about z-axis for isopropyl group. Reprinted with permission from ref. . (2006 American Chemical Society)

Figure 10

Molecular orientation of 2-propanol at the liquid/vapor interface; gray, red, and blue spheres denote carbon, oxygen, and hydrogen atoms, respectively. (a) x_iso<0.025, (b) x_iso=0.073, (c) x_iso>0.68. Reprinted with permission from ref. . (2006 American Chemical Society)

Figure 11

Schematic representation of phenol group stands up more after exposure to humid air. Reprinted with permission from ref. . (2009 American Chemical Society)

Figure 12

Schematic demonstration of the two orientations of melittin inside a lipid bilayer. Reprinted with permission from ref. . (2007 American Chemical Society)

Figure 13

(A) The definition of tilt angle (θ₁ and θ₂) and bend angle φ=(θ₁−θ₂) of alamethicin in POPC/POPC bilayer. (B) Schematic demonstration of pH dependent channel gating action of alamethicin. Reprinted with permission from ref. . (2012 American Chemical Society)

Figure 14

(Left) SFG amide I spectra of interfacial Gβγ (25 μg/mL) adsorbed onto a POPC/POPC bilayer; (Right) Gβγ orientation deduced based on SFG intensity ratio obtain from left spectra. It tilts −35° against the surface normal from the “zero” position defined earlier in ref. . Reprinted with permission from ref. . (2007 American Chemical Society)

Figure 15

SFG spectra (ssp) collected from the d-PMMA/AATM interface as a function of time. The thickness of the d-PMMA film used here is 150 nm. Reprinted with permission from ref. . (2004 American Chemical Society)

Figure 16

(Left) Time-dependent SFG signal intensities at 2840 and 2945 cm⁻¹ as AATM molecules diffuse into d-PMMA films of different thicknesses: (a) 20, (b) 70, (c) 150, (d) 210, and (e) 269 nm. (Right) Diffusion kinetics fitted by the Fickian model. Reprinted with permission from ref. . (2004 American Chemical Society)

Figure 17

Structure of a fibrinogen molecule. Reprinted with permission from ref. . (2005 American Chemical Society)

Figure 18

SFG spectra collected in the amide I range of fibrinogen adsorbed to (a) PEU, (b) SPCU, and (c) PFP in PBS buffer at different time (in min). Time dependent SFG signal of α-Helix (d) from fitting SFG spectra for fibrinogen adsorbed to PEU (closed circles), SPCU (open circules), and PFP (closed triangles). Representative error is shown for the fibrinogen/PEU sample. Reprinted with permission from ref. . (2005 American Chemical Society)

Figure 19

(a) A few possible fibrinogen configurations at the interface. The α-helix SFG signal from each set of coiled coils is shown by solid arrows, and the net α-helix SFG signal is shown by white arrows. Here αC chains are not shown. (b) Schematic of fibrinogen structural changes with time after adsorption on PEU. (c) Schematic of fibrinogen structural changes with time after adsorption on SPCU or PFP. Reprinted with permission from ref. . (2005 American Chemical Society)

Figure 20

CH₃ symmetric SFG intensity decay for DSPC/DSPC-d₈₃ bilayer at various temperatures; the blue line was recorded at 41.7 °C, green at 45.7°C, and red at 50.3°C. The dashed lines are fitted data using equation (3) in ref. . Reprinted with permission from ref. . (2004 American Chemical Society)