EPR investigation and spectral simulations of iron-catecholate complexes and iron-peptide models of marine adhesive cross-links.

Electron paramagnetic resonance (EPR) spectra are presented for iron complexes of catecholate, tironate, and a 3,4-dihydroxyphenylalanine (DOPA)-containing peptide of sequence Ac-Ala-DOPA-Thr-Pro-CONH2 ("AdopaTP"). This peptide was prepared to model potential metal-protein cross-links in the adhesive used by marine mussels, Mytilus edulis, for affixing themselves to surfaces. Spectra are shown for iron bound to each ligand in mono, bis, and tris coordination environments. For example, the catecholate complexes {Fe(cat)}, {Fe(cat)2}, and [Fe(cat)3]3- are provided. Detailed simulations are presented to describe the origin of spectra for the iron-catecholate and iron-peptide species, which show that the spectral features can be accounted for only with the inclusion of D- and E-strain. The spectroscopy of each compound is shown under both anaerobic and aerobic conditions. When exposed to air, the high-spin Fe3+ signal of [Fe(AdopaTP)3]3- decreases and an organic radical is formed. No other sample exhibited an appreciable radical signal. These data are discussed in light of the biomaterial synthesis carried out by marine mussels.

Structural effects of crosslinking a biopolymer hydrogel derived from marine mussel adhesive protein.

In an effort to explore new biocompatible substrates for biomedical technologies, we present a structural study on a crosslinked gelatinous protein extracted from marine mussels. Prior studies have shown the importance of iron in protein crosslinking and mussel adhesive formation. Here, the structure and properties of an extracted material were examined both before and after crosslinking with iron. The structures of these protein hydrogels were studied by SEM, SANS, and SAXS. Viscoelasticity was tested by rheological means. The starting gel was found to have a heterogeneous porous structure on a micrometer scale and, surprisingly, a regular structure on the micron to nanometer scale. However disorder, or "no periodic structure", was deduced from scattering on nanometer length scales at very high q. Crosslinking with iron condensed the structure on a micrometer level. On nanometer length scales at high q, small angle neutron scattering showed no significant differences between the samples, possibly due to strong heterogeneity. X-ray scattering also confirmed the absence of any defined periodic structure. Partial crosslinking transformed the viscoelastic starting gel into one with more rigid and elastic properties.