Surface-sensitive Raman spectroscopy of collagen I fibrils

Abstract

Collagen fibrils are the main constituent of the extracellular matrix surrounding eukaryotic cells. Although the assembly and structure of collagen fibrils is well characterized, very little appears to be known about one of the key determinants of their biological function-namely, the physico-chemical properties of their surface. One way to obtain surface-sensitive structural and chemical data is to take advantage of the near-field nature of surface- and tip-enhanced Raman spectroscopy. Using Ag and Au nanoparticles bound to Collagen type-I fibrils, as well as tips coated with a thin layer of Ag, we obtained Raman spectra characteristic to the first layer of collagen molecules at the surface of the fibrils. The most frequent Raman peaks were attributed to aromatic residues such as phenylalanine and tyrosine. In several instances, we also observed Amide I bands with a full width at half-maximum of 10-30 cm(-1). The assignment of these Amide I band positions suggests the presence of 3(10)-helices as well as α- and β-sheets at the fibril's surface.

Figure 1

Far-field Raman spectrum of Collagen I fibrils. (A) Contact-mode AFM deflection image in air of a dense network of fibrils obtained with a 0.01 N/m MSNL cantilever. (B) Optical image of a sheet of Collagen I fibrils. (C) Raman difference spectra of the sheet and a dense network of fibrils. Positions of the major bands are indicated. The 2432 cm⁻¹ band is attributed to glass. The FWHM of the Amide I bands are 90 cm⁻¹.

Figure 2

Silver-surface-enhanced Raman spectrum of Collagen I fibrils. (A) Differential interference contrast image of several Collagen I fibrils coated with polydisperse Ag nanoparticles. Each bright dot corresponds to at least one nanoparticle. (B) AFM contact mode height image of 30 nm Ag nanoparticles attached to collagen fibrils on glass obtained in air with a 0.03 N/m MSNL cantilever. (C) Raman spectra single Collagen I fibrils with Ag nanoparticles attached and a spectrum of a fibril without nanoparticles (bottom spectra). The Raman scattering signal was enhanced by a factor of 10⁴. (D) Histogram of peak positions observed in 87 silver SERS spectra of Collagen I fibrils.

Figure 3

Gold-surface-enhanced Raman spectrum of Collagen I fibrils. (A) Differential interference contrast image of several Collagen I fibrils coated with 65 nm Au nanoparticles. Each bright dot corresponds to at least one nanoparticle. (B) AFM tapping mode height image in air of 65 nm Au nanoparticles attached to collagen fibrils on glass obtained with a TESP-SS cantilever. (C) Raman spectra single Collagen I fibrils with Au nanoparticles attached and a spectrum of a fibril without nanoparticles (bottom). The Raman scattering signal was enhanced by a factor of 10⁵. (D) Histogram of peak positions observed in 87 gold SERS spectra of Collagen I fibrils.

Figure 4

Tip-enhanced Raman spectrum of Collagen I fibrils. (A) Raman spectra of a single Collagen I fibril with Ag-coated tips in contact with the fibril and out of contact (bottom). The Raman scattering signal was enhanced by a factor of 8000. (B) Histogram of peak positions observed in 154 TERS spectra taken on 15 Collagen I fibrils.

Figure 5

Tip-enhanced Raman line scan of a Collagen I fibril collected in tapping mode in air at an oscillation amplitude of 10 nm. (A) AFM height image of the Collagen I fibril obtained with a TESP-SS cantilever before Raman line scanning. (White horizontal line across the fibril) The 1.14 μm path of the Raman line scan. (B) The tip-enhanced Raman spectra at different points along the 30 point path. (C) Height profile of the Collagen I fibril after Raman profile (squares) and the intensities of the 1330 (circles) and 1485 cm⁻¹ (triangles) peaks along the Raman line scan. The height profile and the intensity profile at 1330 cm⁻¹ both have a FWHM of 100 nm.

Figure 6

Spectra of Collagen I fibrils with peaks in the Amide I region generated using TERS and SERS. (A) Tip-enhanced spectra of Collagen I fibrils obtained with a silver-coated 0.01 N/m MSNL tip (1 and 3) and a CONT tip (2). (B) Surface-enhanced spectra of Collagen I fibrils obtained with polydisperse Ag nanoparticles (4), 30 nm Ag nanoparticles (5 and 7), and 65 nm Au nanoparticles (6).

Figure 7

Comparison of the diffraction-limited and enhanced Raman Amide I peaks for Collagen I fibrils. (A) Peak fitting of the Amide I band observed in a sheet of collagen presented in Fig. 1B. The positions, height, and FWHM of the Gaussians are presented in Table 1. (B) Histogram of the Amide I peak positions observed in SERS and TERS. The average FWHM of the Amide I band in 42 silver SERS, 37 gold SERS, and 29 silver TERS spectra used were 27 ± 12 cm⁻¹, 22 ± 11 cm⁻¹, and 15 ± 7 cm⁻¹, respectively.

Figure 8

Proposed orientation of the phenylalanine residue with respect to the metal surface. The coordinate system of the phenyl molecule is shown.