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 (NaIO(4)) and the transition metal ion Cu(2+) 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 NaIO(4) and Cu(2+)-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 NaIO(4) and Cu(2+), 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 NaIO(4), but remains unchanged upon addition of Cu(2+). This demonstrates that the band at 1260cm(-1) is attributed to the C O stretching vibration of the side chain hydroxyl groups in DOPA and that Cu(2+) forms complexes with DOPA rather than transform it into an o-quinone. Moreover, upon addition of NaIO(4) after cross-linking using Cu(2+), the band at 1260cm(-1) disappears, indicating that the complex formation between DOPA and Cu(2+) is reversed when DOPA is transformed into the o-quinone. These results demonstrate that NaIO(4), 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 Cu(2+), which is accumulated at high concentrations in the byssus thread of the blue mussel, is via complex formation between the metal and DOPA residues.