Structure of collagen adsorbed on a model implant surface resolved by polarization modulation infrared reflection–absorption spectroscopy

• Amide I modes are assigned to structural units of the collagen triple helix.
• The triple helix of collagen adsorbed on the titania surface is distorted.
• Collagen interacts with water molecules adsorbed on the titania surface.
• Intramolecular hydrogen bonds at imino acid protein fragments are destabilized.
• Native collagen structure is restored upon addition of water to the protein film.

The polarization modulation infrared reflection–absorption spectra of collagen adsorbed on a titania surface and quantum chemical calculations are used to describe components of the amide I mode to the protein structure at a sub-molecular level. In this study, imino acid rich and poor fragments, representing the entire collagen molecule, are taken into account. The amide I mode of the collagen triple helix is composed of three absorption bands which involve: (i) (∼1690 cm−1) the CO stretching modes at unhydrated groups, (ii) (1655–1673 cm−1) the CO stretching at carbonyl groups at imino acids and glycine forming intramolecular hydrogen bonds with H atoms at both NH2 and, unusual for proteins, CH2 groups at glycine at a neighbouring chain and (iii) (∼1640 cm−1) the CO stretching at carbonyl groups forming hydrogen bonds between two, often charged, amino acids as well as hydrogen bonds to water along the entire helix.The IR spectrum of films prepared from diluted solutions (c < 50 μg ml−1) corresponds to solution spectra indicating that native collagen molecules interact with water adsorbed on the titania surface. In films prepared from solutions (c ⩾ 50 μg ml−1) collagen multilayers are formed. The amide I mode is blue-shifted by 18 cm−1, indicating that intramolecular hydrogen bonds at imino acid rich fragments are weakened. Simultaneous red-shift of the amide A mode implies that the strength of hydrogen bonds at the imino acid poor fragments increases. Theoretically predicted distortion of the collagen structure upon adsorption on the titania surface is experimentally confirmed.

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