Investigation of Adsorption and Cross-Linking of a Mussel Adhesive Protein Using Attenuated Total Internal Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR)

Abstract

Mytilus edulis foot protein 1 (Mefp-1) contains the redox-functional amino acid 3,4-dihydroxyphenylalanine (DOPA), which is a typical feature of most mefp proteins. We have previously shown, using combined optic (ellipsometry) and acoustic (QCM-D) measurements, that the oxidizing agent sodium periodate (NaIO4) and the transition metal ion Cu2+ promote cross-linking of Mefp-1. However, different chemical reaction mechanisms can not be distinguished using these methods. In the present study, we have complemented our previous investigations using Attenuated Total Internal Reflection Fourier Transform Infrared spectroscopy (ATR-FTIR), allowing a spectroscopic analysis of NaIO4 and Cu2+-induced cross-linking of Mefp-1 adsorbed on a ZnSe surface. In aqueous solution, adsorbed Mefp-1 displays absorption bands at 1570, 1472, 1260, and 973 cm−1. Upon addition of NaIO4 and Cu2+, the absorptions at 1570, 1472, and 973 cm−1 increase by approximately a factor of two. In contrast, the band at 1260 cm−1 disappears upon cross-linking using NaIO4, but remains unchanged upon addition of Cu2+. This demonstrates that the band at 1260 cm−1 is attributed to the C‒O stretching vibration of the side chain hydroxyl groups in DOPA and that Cu2+ forms complexes with DOPA rather than transform it into an o-quinone. Moreover, upon addition of NaIO4 after cross-linking using Cu2+, the band at 1260 cm−1 disappears, indicating that the complex formation between DOPA and Cu2+ is reversed when DOPA is transformed into the o-quinone. These results demonstrate that NaIO4, which initiates a similar reaction to the naturally occurring enzyme catechol oxidase, contributes to the formation of di-DOPA cross-links. In contrast, the dominating contribution to the cross-linking from Cu2+, which is accumulated at high concentrations in the byssus thread of the blue mussel, is via complex formation between the metal and DOPA residues.