Structure of Polydopamine: A Never-Ending Story?

Abstract

Polydopamine (PDA) formed by the oxidation of dopamine is an important polymer, in particular, for coating various surfaces. It is composed of dihydroxyindole, indoledione, and dopamine units, which are assumed to be covalently linked. Although PDA has been applied in a manifold way, its structure is still under discussion. Similarities have been observed in melanins/eumelanins as naturally occurring, deeply colored polymer pigments derived from L-DOPA. Recently, an alternative structure was proposed for PDA wherein dihydroxyindoline, indolinedione, and eventually dopamine units are not covalently linked to each other but are held together by hydrogen bonding between oxygen atoms or π stacking. In this study, we show that this structural proposal is very unlikely to occur taking into account unambiguous results obtained by different analytical methods, among them (13)C CPPI MAS NMR (cross-polarization polarization-inversion magic angle spinning NMR), (1)H MAS NMR (magic angle spinning NMR), and ES-HRMS (electrospray ionization high-resolution mass spectrometry) for the first time in addition to XPS (X-ray photoelectron spectroscopy) and FTIR spectroscopy. The results give rise to a verified structural assignment of PDA wherein dihydroxyindole and indoledione units with different degrees of (un)saturation are covalently linked by C-C bonds between their benzene rings. Furthermore, proof of open-chain (dopamine) monomer units in PDA is provided. Advanced DFT calculations imply the arrangements of several PDA chains preferably by quinone-hydroquinone-type interactions in a parallel or antiparallel manner. From all of these results, a number of hypotheses published before could be experimentally supported or were found to be contradictory, thus leading to a better understanding of the PDA structure.

Lipase immobilized on polydopamine-coated magnetite nanoparticles for biodiesel production from soybean oil

“…By comparing these images, magnetite functionalization with polydopamine represented by the presence of three sharp peaks, i.e., aromatic C-C stretching (1486 cm -1 ), NH3 in-plane bending (1422 cm -1 ), and C-O-H symmetric bending (1266 cm -1 ) could be detected. In fact, the presence of these bands represents the presence of polydopamine, however, it is worth noticing that at the present time there is no general agreement about the real structure of this particular coating material (Liebscher et al, 2013). The AFM images of Fe3O4@PD nanoparticles, in the contact mode and in the phase contrast mode, can be seen in Figures 3A and 3B, respectively, corroborating the results obtained from the DLS measurements.…”

Neurotransmitter Detection Using Corona Phase Molecular Recognition on Fluorescent Single-Walled Carbon Nanotube Sensors

“…23,25,26 The time dependence of the sensor response can be explained in terms of a polymerization product C that quenches the SWCNT fluorescence. Dopamine (A) is oxidized to dopamine quinone (B), which then polymerizes to give C:…”

“…It is well-known that dopamine is a reactive molecule and undergoes oxidation to dopamine quinone and other products readily. 23,25,26,32 We attribute the time-dependent response (Figure 3b) to these reactions. Interestingly, polymerization of dopamine might have been a primary contributor to results observed in previous work from our lab, where dopamine was shown to quench the SWCNT fluorescence of DNA-wrapped SWCNTs.…”

Simple Coatings to Render Polystyrene Protein Resistant

“…These bonding theories imply that the DOPA and polystyrene aromatic rings align parallel to each other [43][44][45][46][47]. As there is debate about how exactly the polymer is formed and adheres to surfaces and secondary coatings [48], we tested if Pluronic F127 could in fact bind to such a polydopamine coating and create a protein rejecting interface. Interestingly, a polystyrene surface coated in polydopamine film results in the largest frequency shift of all tested surfaces, possibly due to less surface induced denaturation of BSA on this hydrophilic surface compared to the hy "glue" between polystyrene and F127, the ex situ coating method first described by Messersmith et al [22] was employed.…”