Structural Basis of Polydopamine Film Formation: Probing 5,6

Forum Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 7670−7680

www.acsami.org

Structural Basis of Polydopamine Film Formation: Probing 5,6Dihydroxyindole-Based Eumelanin Type Units and the Porphyrin Issue Maria L. Alfieri,† Raffaella Micillo,† Lucia Panzella,† Orlando Crescenzi,† Stefano L. Oscurato,‡ Pasqualino Maddalena,‡ Alessandra Napolitano,† Vincent Ball,§,∥ and Marco d’Ischia*,† †

Department of Chemical Sciences and ‡Department of Physics “Ettore Pancini″, University of Naples Federico II, I-80126 Naples, Italy § Université de Strasbourg, Faculté de Chirurgie Dentaire, 8 rue Sainte Elisabeth, 67000 Strasbourg, France ∥ Institut National de la Santé et de la Recherche Médicale, Unité Mixte de Recherche 1121, 11 rue Humann, 67085 Strasbourg Cedex, France S Supporting Information *

ABSTRACT: The role of 5,6-dihydroxyindole (DHI)-based oligomers, including porphyrin-like tetramers, in polydopamine (PDA) film formation was addressed by a comparative structural investigation against model polymers from DHI and its 2,7′-dimer. MALDI-MS data showed that (a) PDA is structurally different from DHI melanin and does not contain species compatible with DHI-based oligomers as primary building blocks; (b) PDA films and precipitate display a single main peak at m/z 402 in common; (c) no species matching the range of m/z values expected for cyclic porphyrin-type tetramers was detected in DHI melanin produced in the presence or in the absence of folic acid (FA) as templating agent, nor by oxidation of the 2,7′-dimer of DHI as putative precursor. 15N NMR resonances and Raman spectra predicted by extensive DFT calculations on porphyrin-type structures at various oxidation levels did not match spectral data for PDA or DHI melanin. Notably, unlike PDA, which gave structurally homogeneous films on quartz on atomic force microscopy (AFM) and micro-Raman spectroscopy, DHI melanin did not form any adhesive deposit after as long as 24 h. It is concluded that PDA film deposition involves structural components unrelated to DHI-based oligomers or porphyrin-type tetramers, which, on mechanism-based analysis, may arise by quinone−amine conjugation leading to polycyclic systems with extensive chain breakdown. KEYWORDS: eumelanin, polydopamine, 5,6-dihydroxyindole, polymerization, film, porphyrin, tetramer, MALDI-MS

■

INTRODUCTION

Because of the growing importance of PDA in materials science and technology, defining structure−property−function relationships is a major goal for optimizing its performances for tailored applications. Until 2012 two different speculative structural models were commonly assumed: the “open-chain polycatechol/quinone” model, based on linear sequences of catecholamine units linked through biphenyl-type bonds, and the “eumelanin” model, which envisaged a 5,6-dihydroxyindole (DHI) polymer arising by cyclization of dopaminequinone (Scheme 1).7,8 Since 2012, several groups have begun to address in detail the chemical nature of PDA, its basic scaffolds, and functional groups. The following structural models have been proposed in order of time:

Polydopamine (PDA), a black insoluble and structurally disordered eumelanin-like material produced by the oxidative polymerization of dopamine under alkaline conditions, stands todate as the state-of-the-art for surface functionalization and coating and for various nanotechnological and biomedical applications.1 Inspired by the robust adhesion properties of catechol- and amine-rich mussel adhesive proteins such as Mytilus edulis foot protein-5 (Mefp-5), PDA provides technologically versatile coatings susceptible to functionalization and chemical manipulations. PDA thin films can be deposited at the interface of different materials, including metals, oxides, inorganic semiconductors, ceramics, and polymers, can bind cells, biomolecules, and metal ions, and can be used to control or modify the hydrophobicity of a variety of interfaces. PDA film properties, including hydrophilicity and thickness, can be finely tuned by a variety of experimental parameters including dopamine concentration,2 nature of the buffer,3 oxidant,4−6 and pH.2 © 2017 American Chemical Society

Special Issue: 10 Years of Polydopamine: Current Status and Future Directions Received: July 4, 2017 Accepted: September 22, 2017 Published: September 22, 2017 7670

DOI: 10.1021/acsami.7b09662 ACS Appl. Mater. Interfaces 2018, 10, 7670−7680

Forum Article

ACS Applied Materials & Interfaces

than the others, implying repetitive structural motifs despite inherent chemical diversity; and (iii) larger oligomers are less likely to form.18 Parallel to these structural studies, a number of papers over the past few years interpreted PDA properties in terms of a simplified structural model first proposed for eumelanins by Kaxiras et al. in 200619 and involving porphyrin-type building blocks derived from oxidative cyclization of DHI tetramers built via 2,7′bondings (Scheme 2).

Scheme 1. Simplified View of the Possible Oxidative Pathways Leading to Eumelanin Building Blocks Emphasizing the Generation of Amine-Containing Open Chain Oligomers versus Cyclized Indole-Type Speciesa

Scheme 2. Putative Pathways of Porphyrin Tetramer Formation during PDA Synthesis

a

Time-dependent density functional theory (DFT) calculations suggested that the structural model reproduces fairly well the main features of eumelanin, including broadband absorption spectrum, X-ray scattering data, the ability to capture and release metal ions, and the characteristic size of eumelanin protomolecules. Subsequent DFT calculations showed that such tetramers can not only associate with an interlayer distance of 3.0−3.3 Å but can also bind in a covalent manner through interlayer C−C bonds.20 Additional indirect support to the porphyrin model came from combined molecular dynamics simulations and high resolution transmission electron microscopy (TEM) data, suggesting that most eumelanin/PDA properties could be explained by selfassembly of porphyrin-like indole tetramers.21 Moreover, a detailed ES (+)-MS analysis of PDA reported a signal at m/z 595.1841 compatible with a cyclotetramer.14 Other studies based on MALDI-MS showed that folic acid (FA) influences the morphology and nanostructure of PDA, favoring formation of species giving a cluster of peaks at 584−589 Da that were fitted to a cyclic tetramer structure proposed as an important structural motif.22 More recently, an integrated approach based on an electrochemical fingerprinting technique suggested that natural eumelanin pigments contain porphyrinlike protomolecules at appreciable concentrations.23 Despite the evident importance of elucidating PDA structure for understanding and controlling its properties for technological applications, the picture emerging from these studies is still confusing and partly based on theoretical models that are not adequately supported by unambiguous experimental evidence. It is therefore mandatory for progress in PDA research that critical open questions about the structural components responsible for adhesion are definitively settled. A crucial issue of central

Highlighted are: (a) the oxidative coupling of 5,6-dihydroxyindole (DHI) leading to two main dimers, the 2,4′- and the 2,7′-biindolyl; (b) the possible occurrence of late coupling cyclization pathways as an alternate route to indolic oligomers from early cyclization.

(a) A supramolecular aggregate consisting primarily of 5,6dihydroxyindoline and dopaminochrome held together through a combination of charge transfer, π−π stacking, and hydrogen-bonding interactions.9 (b) A physical trimer of (dopamine)2/DHI, derived from a self-assembly mechanism.10 (c) A trimer of DHI and other MALDI-MS-detectable species with m/z 401 and 448 on PDA-coated glass.11 (d) A three-component structure of polydopamine, comprising uncyclized (catecholamine) and cyclized (indole) units, as well as pyrrolecarboxylic acid moieties, with partial incorporation of tris(hydroxymethyl)aminomethane (Tris buffer).12,13 (e) Mixtures of different oligomers containing indole units with various degrees of (un)saturation and open-chain dopamine units.14 (f) Dopamine and CN-containing tautomers of quinone and indole species in growing films deposited on gold surfaces over an interval of time from 2 to 60 min.15 (g) A (DHI)2/PCA (pyrrolecarboxylic acid) trimer complex (m/z 402) as primary component to build up the supramolecular structure of PDA.16,17 Very recently, a set of computational methods applied to investigate nearly 3000 probable molecular structures of PDA and eumelanin indicated that (i) more planar oligomers are more stable; (ii) there is a group of tetramers relatively more stable 7671


Forum Article

ACS Applied Materials & Interfaces relevance to PDA film forming properties is Do f ilm formation properties of PDA depend on eumelanin-type, DHI-based components? Settling this issue is the main goal of this paper. Before discussing the results, however, a concise background of mechanistic arguments and a list of the main difficulties that need to be circumvented to achieve this goal with possible approaches is proposed to support the rationale of the study. Though PDA is often referred to as a synonymous of eumelanin because it complies to the definition24 of “nitrogenous biopolymer produced by oxidative polymerization of a tyrosinederived indole precursor” and shows similar physicochemical properties, it differs from typical natural or synthetic eumelanins because of the lack of carboxylated indole units (5,6dihydroxyindole-2-carboxylic acid, DHICA). Nonetheless, PDA may contain DHI units24 or mixtures of partially degraded indole units. Thus, answering the key question aforementioned requires an understanding of whether and to what extent proportion PDA films consist of intact or partially degraded DHI oligomers, and whether such components arise from polymerization of preformed DHI or from the late cyclization of linear dopamine oligomers. A strictly related issue is the role of porphyrin-like tetramers in PDA film formation. Thus far, formation of porphyrin structures via DHI chemistry has not been unambiguously demonstrated by adequate experimental data. Chemical evidence indicates that DHI tends to couple mainly via two different pathways, leading to the 2,4′- and the 2,7′-dimers, though other modes of coupling, e.g. via the 3-position, may become significant at the dimer− dimer coupling stage.25 Assuming that each coupling step of DHI proceeds statistically via 2,4′- and 2,7′-bondings (i.e., 50% each: an oversimplification which however roughly reflects the experimental proportion of the two coupling modes in dimer formation), and considering that porphyrin formation requires four sequential 2,7′-coupling steps, including the final cyclization step, the expected final yield of tetramer would not be higher than ca. 6% based on reacted DHI. Furthermore, nonquantitative conversion of dopamine to DHI, due to other competing pathways of dopamine quinone evolution, would further decrease the porphyrin contribution to the PDA structure. Overall, the above arguments would make a strong mechanismbased case against a significant involvement of porphyrin tetramers in the PDA and eumelanin structure. However, it is possible that specific control mechanisms, e.g. mediated by aggregation, induce porphyrin assembly from DHI. This point can be addressed by revisiting the polymerization of DHI and dopamine in the presence and in the absence of folic acid (FA) as a templating agent, as reported.22 It is also possible that, even though produced in very low yields, porphyrin components may be primary and specific components of the very minute fraction of adhesive PDA components leading to film formation, though a recent MS analysis of PDA film did not provide evidence in support of this view.15 In any case, obtaining direct experimental evidence for porphyrin tetramers in eumelanin-type materials is a difficult task. MALDI-MS data, which support most of the current suggestions, cannot be taken as unambiguous proof for bona f ide porphyrins. For example, the previous suggestion14 that the pseudomolecular ion peak in the MALDI-MS spectrum of PDA at m/z 595 could be assigned to a cyclotetrameric structure bearing indoline and aminochrome units would not withstand critical analysis since indoline type leucodopaminochrome structures would be readily reoxidized to dopaminochrome

and then to DHI, thus not being stable enough to provide a main peak under MALDI conditions. More convincing options for the species at m/z 595 could include mixed-type tetramers involving both DHI and uncyclized units (dopamine is more stable to autoxidation than the indoline and is a more likely structural moiety). Notably, moreover, even though mass data compatible with cyclic structures have been reported, alternative N-confused isomeric porphyrin scaffolds with the same molecular formula but predictably different properties should be considered as well. Even more fundamentally, pseudomolecular ion mass data are intrinsically unable to distinguish between a cyclic oligomer and the corresponding noncyclic structure at a higher oxidation level. Likewise, UV−visible and FT-IR spectroscopy can be decisive only after diagnostic bands that can be unequivocally attributed to those kinds of structures are identified. Based on the above survey, the specific aims of this paper are (a) to compare the structure and film-forming properties of PDA and DHI eumelanin in order to assess to what extent film formation depends on intact DHI-based components present in PDA; (b) to assess whether PDA films and bulk precipitate are made up of the same components or show different structural features and to provide an insight into main film components; (c) to verify whether porphyrin tetramers do form under favorable conditions, i.e. by autoxidation of DHI in the presence of FA as templating agent or by oxidation of the 2,7′-dimer of DHI, the putative precursor of porphyrin tetramers. Although porphyrins have not been directly implicated in adhesion, addressing this issue seems a relevant goal given the recurrent recourse to this structural model in the literature. To this aim, a computational survey of realistic porphyrin structures and their simulated NMR, UV−visible absorption, and Raman properties will be reported as a function of the oxidation state, to look for diagnostic signatures guiding identification in PDA films. To avoid possible interference of the amine-containing Tris buffer with polymer buildup and film deposition for the purpose of structural investigation, PDA synthesis was carried out in carbonate buffer unless otherwise stated.12,16

■

EXPERIMENTAL SECTION

Synthesis of PDA and DHI melanin and coating experiments. PDA and DHI melanin were prepared respectively by autoxidation of dopamine hydrochloride (30 mg, Sigma-Aldrich) or DHI (30 mg, prepared according to Edge et al.26) dissolved in 0.1 M bicarbonate buffer (pH 9.0)3 or 0.05 M tris(hydroxymethyl)aminomethane (Tris) buffer (pH 8.5)3 (final concentration 10 mM). The reaction mixtures were left under vigorous stirring and after 24 h were acidified to pH 2 with 4 M HCl. The dark pigments were collected by centrifugation at 7000 rpm at 4 °C, washed 3 times with water, and lyophilized. The yields of PDA and DHI melanin were 28% and 79%, respectively. Quartz substrates were cleaned by soaking in piranha solution (96% H2SO4 /30% H2O2 5:1) overnight, rinsed with distilled water, dried under vacuum, and dipped in autoxidation mixtures. After 24 h, substrates were sonicated in a 1:1 v/v methanol/water solution and dried. UV−vis analysis of the coated substrates obtained was performed on a Jasco V-730 spectrophotometer. In separate experiments, dopamine or DHI (0.3 mg/mL) in the absence or in the presence of FA (0.15 mg/mL) was first dissolved in deionized water and stirred for 1 day at 60 °C in the dark; then NaOH aqueous solution (0.1 M) was used to adjust the pH value (pH ca. 8.5) and the mixture was stirred for 3 h at 60 °C in the dark.22 In the case of DHI, the phase of preincubation with FA was performed by sealing the reaction mixture under argon to prevent the indole oxidation. Then, the dark suspension was centrifuged at 10000 rpm for 30 min to collect the precipitate. The sediment was washed several times with water and then dried by lyophilization. 7672


Forum Article


Figure 1. (a) Quartz substrates immersed in the autoxidation mixture for dip-coating experiments comparing PDA (left) and DHI (right) melanin. (b) UV−visible absorption spectra of quartz substrates subjected to dip-coating with dopamine (blue curve) and DHI (red curve).

Figure 2. (a) AFM image of a representative region of the PDA film sample. (b) AFM height profile measured along the red line in panel (a). (c) MicroRaman spectrum resulting from the average of 625 spectra collected with a spatial resolution of 2 μm in the sample scanning over an area of 50 × 50 μm2. Sample preparation for MALDI-MS analysis.22 α-Cyano-4hydroxycinnamic acid (CHCA, 98% purity) was purchased from SigmaAldrich. HPLC-grade acetonitrile, trifluoroacetic acid (TFA), and bidistilled water were used. The solution of matrix CHCA (10 mg/mL) was prepared in acetonitrile/water (1:1, v/v) containing 0.1% TFA. One mL of the analyte was premixed with 1 mL of the matrix in a centrifuge tube, and then 2 μL of the resulting mixture were pipetted on the MALDI target plate and air-dried for MALDI-ToF MS analysis. MALDI spectra were recorded on a Sciex 4800 MALDI ToF/ToF instrument. The laser was operated at 3700 Hz in the positive reflectron mode. The mass spectrometer parameters were set as recommended by the manufacturer and adjusted for optimal acquisition performance. The laser spot size was set at medium focus (B50 mm laser spot diameter). The mass spectra data were acquired over a mass range of m/z 100− 4000 Da, and each mass spectrum was collected from the accumulation of 1000 laser shots. Raw data were analyzed using the computer software provided by the manufacturers and reported as monoisotopic masses. AFM and micro-Raman analysis. The combined AFM and microRaman analysis was conducted with the integrated apparatus Alpha300 RS (WITec, Germany). The system can be switched at will between AFM and confocal micro-Raman configurations, allowing a combined topographical and spectral characterization of a specified microregion of the sample. The film topography was investigated by AFM operating in AC mode. For the micro-Raman analysis, a laser beam at λ = 488 nm was used as excitation light source. The beam was focused onto the sample surface by means of a microscope objective (NA = 0.75) working in epiillumination mode. The diffraction-limited focused spot in the objective focal plane had a fwhm of approximately 320 nm. The light backscattered from the sample was collected by the same objective and sent to the spectrograph through a confocal optical collection path.

Computational methods. All DFT calculations were performed with the Gaussian package of programs,27 with the PBE0 hybrid functional.28 The 6-31+G(d,p) basis set was employed for geometry optimizations. For each species, an extensive exploration of the different tautomers/conformers, as well as different protonation states, was carried out. In those cases where conformational enantiomers exist, a single enantiomeric series has been examined. Computations were performed either in vacuo or by adoption of a polarizable continuum medium (PCM)29−32 to account for the influence of the solution environment. In view of the faster convergence, a scaled van der Waals cavity based on universal force field (UFF) radii33 was used, and polarization charges were modeled by spherical Gaussian functions;34,35 nonelectrostatic contributions to the solvation free energy were disregarded at this stage: these terms were accounted for in single-point PCM calculations (at the PCM geometries) employing radii and nonelectrostatic terms of the SMD solvation model.36 Vibrational−rotational contributions to the free energy were also computed. For the preparation of Raman scattering activity plots, harmonic frequencies were scaled by a factor of 0.954737 and a Gaussian line width of 20 cm−1 was used. UV−vis spectra of the main species were computed in vacuo or in solution using the time-dependent density functional theory (TDPBE0) approach38−42 with the 6-311++G(2d,2p) basis set. To produce graphs, transitions below 5.6 eV were selected, and an arbitrary Gaussian line width of 0.25 eV was imposed; the spectra were finally converted to a wavelength scale. NMR shielding tensors were computed within the Gauge-Including Atomic Orbitals (GIAO) ansatz43,44 with the 6-311+G(d,p) basis set. Computed 15N isotropic shieldings were converted into chemical shifts 7673


Forum Article


Figure 3. Segmental spectrum of MALDI-ToF (m/z: 300−600 Da) characterizations of (a) PDA film in carbonate buffer at pH = 9, (b) PDA precipitate in carbonate buffer at pH = 9, (c) PDA synthesized in the presence of FA (dopamine dissolved in water for over 24 h in the dark at 60 °C + NaOH 0.1 M to adjust the pH value to ∼8), and (d) DHI melanin in carbonate buffer at pH = 9, respectively. Asterisks indicate signals due to matrix or impurities. using as reference the values obtained at the same level for methyl 5,6dimethoxyindole-2-carboxylate (at 140.0 ppm from ammonia, cfr.).45

incubation in solutions of dopamine or DHI at alkaline pH. UV− vis analysis clearly indicated that film deposition from DHI was null or below detection. For comparison the UV−vis absorption spectra of the corresponding PDA and DHI melanin solutions are reported in the Supporting Information (Figure S1−S2). To gain additional insight into PDA films and to definitely rule out film deposition from DHI polymerization, the quartz substrates immersed into PDA and DHI oxidation mixtures were investigated by a combined atomic force microscopy (AFM) and micro-Raman analysis. Data for PDA film (Figure 2a) indicated a mean thickness of 70 nm and dispersed

■

RESULTS AND DISCUSSION Comparative structural analysis of PDA and DHI eumelanin and their film-forming properties. In an initial set of experiments, the formation on a quartz surface of thin films from DHI melanin was assessed in comparison with PDA based on the typical dip-coating methodology, to assess whether DHIrelated units play any specific role in PDA film formation. Figure 1 shows the absorption spectra of the quartz substrates after 24 h 7674


Forum Article


Table 1. Main MALDI-MS Peaks Considered for Structural Investigation of PDA and Melanins from DHI and Its 2,7′-Dimera n-mer

Dopamine oligomer calcd.b (m/z)

PDAc (m/z)

PDA filmd (m/z)

PDA + FAe (m/z)

DHI oligomer calcd.b (m/z)

DHI melaninc (m/z)

Melanin from 2,7′-dimerf (m/z)

3 4

456 (478, 494) 607 (629, 645)

402 549

402

402 549, 563, 581

444 (466, 482) 591 (613, 629)

416, 430 533, 577, 591, 621

551, 591

a

All spectra were obtained using CHCA unless otherwise indicated. Selected peaks are identified on the basis of their presence in all spectra examined (at least duplicate samples). bM + H (M + Na, M + K). cPrecipitate from the reaction in carbonate. dSolubilized in methanol/DMSO. e Precipitate obtained in the presence of FA according to Fan et al.22 f2,5-Dihydroxybenzoic acid as the matrix.

Scheme 3. Mechanism-Based Identification of Possible Structures Accounting for the Main Peak at m/z 402 in PDA Films and Bulk Polymer

sample (see Figure S4), confirming a complete overlap of the topographical and chemical features of the PDA-carbonate with the PDA-Tris films. This conclusion was in accord with a previous study based on the ESI(+) spectra of PDA samples obtained in Tris or phosphate buffer.47 The same AFM-Raman analysis revealed the absence of any material attached to the substrate in the DHI melanin reaction (see Figure S5). Overall, these data suggested that preformed DHI-based cyclized structures are unlikely to be the major determinants of PDA film-formation properties. To support this conclusion, in separate experiments different PDA films that adhered to the walls of reaction beakers and to immersed glass substrates were carefully washed with water, solubilized, and analyzed by MALDI-MS in comparison with the bulk precipitate from the same mixtures and with DHI melanin. Salient regions of representative spectra are reported in Figure 3, whereas the main peaks in common to at least two different samples, which are considered for structural analysis, are reported in Table 1.

submicron sized grains (Figure 2b). In the same sample region, the micro-Raman analysis was also performed. The Raman spectrum resulting from the average of 625 spectra collected in different positions of the sample is shown in Figure 2c, together with the positions of the observable peaks. Main bands at 1583, 1416, 1347, and 1242 cm−1 were detected, which were compatible with the presence of aromatic rings, while the broadband around 2900 cm−1 was attributable to strongly hydrogen-bonded OH and NH stretching vibrations. Notably, no intense carbonyl band in the 1650−1700 cm−1 range was detected. These results are in line with previous Raman spectroscopy studies, indicating two main bands at 1360 and 1588 cm−1 in one case,11 and at 1340 and 1600 cm−1 in the other case, due to catechol stretching vibrations.46 The combined micro-Raman imaging of the film presented in Figure S3 of the Supporting Information revealed furthermore a homogeneous chemical composition of the film presenting the typical topography shown in Figure 2a. A similar AFM-Raman analysis was conducted onto the films of PDA-Tris as a reference 7675


Forum Article


with amine-containing substrates leading to film-forming benzacridine derivatives.49,50 A critical step of this pathway would be the formation of the pyran ring by ring closure of aldehyde groups derived from a combination of carbonylforming chain fission induced by hydrogen peroxide51 (for a detailed mechanism see Scheme S2 of the Supporting Information). Path B involves sequential condensation of intact dopamine units, while path C is entirely derived from DHI coupling and oxidative quinone breakdown. Although these pathways are supported by extensive literature, some of them do not appear to be compatible with experimental evidence. In particular, path C should be involved also in DHI melanin, but no peak at m/z 402 could be detected in the relevant spectrum. As to Path B, a strength is the integrity of dopamine units, but its weakness lies in the dibenzofuran forming pathway, usually favored in acids but not under basic conditions, and in the oquinone diimine, usually not observed in the conjugation of amines with o-quinones. Conversely, Path A is the sole pathway among those considered here that leads to a flat visible light-absorbing species, purportedly ensuring adhesion. The presence of the 9,10quinone system incorporated into the acridine ring can be justified by the expected oxidizability of the aminocatechol form. Of course, definitive experimental evidence for the proposed structure is lacking and requires further work currently in progress. Attention is also called to the possibility that the main peak at m/z 402 arises from a mixture of species sharing the same molecular weight. Assuming path A as a working hypothesis, the peak at m/z 549 in PDA polymer could arise by coupling of the species at m/z 402 with a cyclized DHI-type unit (+147 mass units), while the peak at m/z 581 observed in the presence of FA could arise by muconic-type oxidative ring cleavage of an o-quinone from the species at m/z 549. The possible origin of the main peaks in DHI melanin is summarized in Scheme S3 (Supporting Information). DFT investigation of putative PDA building blocks. The results reported so far indicated that porphyrin components are unlikely to be present to a significant proportion in PDA precipitate and film and that a MALDI MS-detectable species at m/z 402 is a component of PDA films. With a view to settling these issues, a detailed DFT investigation of the postulated porphyrin compounds at the most stable oxidation level and of the postulated pyranoacridinetrione structure was carried out (Figure 4), to identify diagnostic signals in their UV−visible, Raman, and 15N NMR spectra that may be revealing of their presence in PDA films. A systematic and detailed inventory of the possible porphyrin structures at various oxidation levels was out of the scope of this study and will be reported elsewhere. Herein, we report computed UV−visible absorption and Raman spectra as well as

Comparative inspection of traces a and b in Figure 3 confirmed for both the film and the precipitate the intense peak at m/z 402 reported previously.16 Additional minor peaks were noted in the precipitate which were not detectable in the film. Neither the peak at m/z 402 nor the other peaks detected in PDA precipitate could be detected in the spectra of DHI melanin samples (see trace d). No significant peak attributable to cyclic porphyrin-type tetramers could be detected in any of the spectra examined. Taking these results with due caution because of the limitations inherent to the MALDI-MS methodology, it could concluded that both PDA precipitate and film consist of the species at m/z 402 as the main detectable component, though the film does not show the peaks for the minor species seen in the precipitated polymer. Interestingly, PDA synthesized in the presence of FA leads to a peak pattern similar to that observed in the carbonate reaction but with the presence of additional minor species (m/z 563, 581) the last of which could in principle be compatible with a cyclic tetramer at the 8-electron oxidation level. However, formation of this species was independent of the presence of FA (Figure S6), was not observed under the usual reaction conditions in carbonate buffer, and was not detected in DHI melanin. Moreover, FA did not affect the structure of DHI melanin (Figure S7−S8). Though not entirely conclusive, these data suggest that the species at m/z 581 cannot be unambiguously identified as a cyclic tetramer and can be accounted for by other structures (see below). It is noticed that the general spectral features in the control sample of Figure S6 are different from those of PDA film (Figure 3a) and PDA precipitate (Figure 3b). This may arguably be due to the different reaction conditions but it is difficult to provide a more detailed explanation. To explore further conditions under which the postulated porphyrin tetramer could be obtained, in subsequent experiments the 2,7′-dimer of DHI was oxidized and the resulting black eumelanin-type precipitate was subjected to MALDI-MS analysis. The spectrum showed two small peaks at m/z 551 and 591 (Figure S9), suggesting a tetramer and a degradation product thereof but no peak compatible with the postulated porphyrin structures. Toward rational models for PDA and DHI melanin building blocks. To rationalize the origin of the species responsible for the main MALDI-MS peaks of PDA film and bulk polymer, the main oxidation pathways accessible to dopamine were considered (see overall view in Supporting Information, Scheme S1). These include (a) intramolecular cyclization of dopamine quinone; (b) isomerization of dopamine quinone to quinonemethide, followed by autoxidative deamination and chain breakdown; and (c) muconic acid-type oxidative fission of o-quinones. The latter pathway was carefully tracked in the oxidative conversion of DHI to pyrrolecarboxylic acids.48 Taking Scheme S1 as a reference, the origin of the main peak of PDA at m/z 402, previously attributed to a physical trimer of two DHI units and a pyrrolecarboxylic acid, was addressed. Its odd mass (giving an even pseudomolecular ion peak) was compatible with a set of alternative structures containing either 1 or 3 nitrogen atoms, some of which are illustrated in Scheme 3 together with plausible key formation steps. In Scheme 3, path A involves a sequence of oxidative condensation and two chain breakdown and deamination processes, leading to 7-(3,4-dihydroxyphenethyl)pyrano[3,4,5kl]acridine-6,9,10(7H)-trione, which is representative of the onenitrogen option. Notably, the proposed coupling of the primary amine with the o-quinone is akin to the reaction of caffeic acid

Figure 4. Porphyrin-type DHI tetramer at the 6-electron oxidation level and truncated pyranoacridinetrione structures considered for DFT calculations. 7676


Forum Article


Figure 5. Computed UV−visible (a: in vacuo, black line, and in water, red line) and Raman (b: in vacuo) spectrum of the most significant tautomer/ conformer of the porphyrin tetramer at the 6-electron oxidation level.

Figure 6. UV−visible (a: in vacuo, black line, and in water, red line) and Raman (b: in vacuo) spectra computed of the pyranoacridinetrione model.

the 15NMR resonances for the most stable oxidation form of the porphyrin system (the 6-electron oxidation level) which was determined by assessing the relative stability of various oxidation states to disproportionation. The underlying assumption was that the porphyrin species at the oxidation level with the lowest tendency to disproportionate can be taken as a plausible reference for theoretical analysis. Figure 5 shows the simulated UV−visible and Raman spectra for the 6-electron oxidation state of the porphyrin tetramer. Interestingly, the species concentrated most of the absorption properties in the high energy region of the visible spectrum, whereas the region between 600 and 880 nm displayed very poor absorption, in contrast with the PDA spectrum (Figure 1). The computed Raman spectrum for the most stable tautomer of the 6-electron oxidation indicated an intense band in the range 1600−1700 cm−1, i.e. in a region where the Raman spectrum of PDA film is devoid of features (Figure 2, Figure 5 and Table S1). 15 N chemical shifts (ppm) computed in water for the most significant tautomers/conformers in the 6-electron oxidation state are provided in the Supporting Information (Table S2). Compared to pyrrole-type nitrogens, pyridine-type nitrogens in the porphyrin system resonate relatively downfield (ca. 275 ppm) giving average values around 217 ppm. Again, this result is in contrast with the reported 15N NMR spectrum of PDA12 in which no resonance above ca. 180 ppm was detected. Based on these results, the UV−visible, Raman, and 15N NMR spectra of the pyranoacridinetrione derivatives proposed for the main species at m/z 402 were next investigated. To simplify

calculations, a model structure truncated on the phenylethyl chain was considered. The computed absorption spectrum displayed a series of bands spanning the entire UV−visible range, thus being compatible with PDA broadband chromophore (Figure 6). Likewise, intense bands were predicted for the Raman spectrum, which matched fairly well those of the PDA film in Figure 2. Finally, a 15N NMR resonance at 137 ppm (in water) was predicted for the N-substituted acridine nitrogen, which falls within the range of 15N resonances observed in the PDA spectrum.12

■

CONCLUSIONS The combined experimental and theoretical approach reported herein has addressed critical issues concerning the structural basis of PDA film forming properties. The results can be summarized as follows: (a) PDA and DHI melanin show different peak patterns in the MALDI-MS spectra, suggesting that PDA does not contain the main structural components related to indole-based oligomers. (b) PDA films and bulk precipitate samples show a single main peak in common at m/z 402 on MALDI-MS analysis. (c) Mechanism-based analysis of plausible structures for the main peak in PDA film at m/z 402, based also on the lack of this peak in DHI melanin, would support a covalent species: at the present stage, a pyranoacridinetrione 7677


Forum Article

ACS Applied Materials & Interfaces structure seems the most likely candidate based on simulated UV−visible, Raman, and 15N NMR spectra. (d) Under the same dip-coating conditions used for PDA, DHI polymerization does not produce AFM detectable films on a quartz surface. (e) No MALDI MS-detectable species matching the m/z values expected for cyclic porphyrin-type tetramers was found in DHI melanin even in the presence of FA as templating agent under reported conditions; likewise, no evidence for a significant cyclodimerization was obtained by oxidation of the 2,7′-dimer of DHI, a putative porphyrin precursor. (f) Spectral features predicted by extensive DFT calculations for the most stable porphyrin-like structures at the 6electron oxidation state did not match experimental data for PDA or DHI melanin. Before drawing conclusions about the nature of the species accounting for PDA adhesion properties, a note of caution is in order concerning the results discussed herein. In addition to those reported in previous studies, the main information on PDA films obtained in the present study that can be confidently used for structural characterization derives from the Raman and the MALDI-MS spectra. In particular, interpretation of mass spectrometric data is subject to caveats relating to the different ionization properties of the various structural components. Notwithstanding this limitation, it can be confidently stated that PDA contains specific molecular constituents that are involved in film formation and that do not appear to relate to DHI-based units derived from cyclization of dopamine or fragmentation products thereof. Thus, it is concluded that PDA is different from DHI melanin and that it is the dif ference, rather than the analogy that determines adhesion and film-forming properties. Several arguments illustrated in this study, moreover, would not support the recent claim that the primary building block of PDA at m/z 402 is a trimer complex consisting of two 5,6-dihydroxyindole (DHI) units and one pyrrolecarboxylic acid (PCA) moiety. Considering accumulating evidence that amine containing aliphatic chains are a crucial structural element favoring formation of adhesive films from catecholic substrates,49,52 it can be speculated that PDA film formation requires aminecontaining structural components, for which various hypotheses have been proposed. Based on these results and a critical analysis of relevant literature data, it is also concluded that any interpretation of PDA or eumelanin properties based on porphyrin-type structures is at present not supported by solid experimental evidence and needs to be revisited.

■

Alessandra Napolitano: 0000-0003-0507-5370 Vincent Ball: 0000-0002-7432-4222 Marco d’Ischia: 0000-0002-7184-0029 Funding

This work was carried out in part with financial support from Kao Corporation. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Computational resources were provided by the SCoPE Data Center of the University Federico II of Naples.

■

ABBREVIATIONS DHI, 5,6-dihydroxyindole; PDA, polydopamine; MALDI-MS, matrix-assisted laser desorption/ionization−mass spectrometry; AFM, atomic force microscopy; Mefp-5, Mytilus edulis foot protein-5; HPLC, high performance liquid chromatography; Tris, tris(hydroxymethyl)aminomethane; CPPI-MAS NMR, cross-polarization polarization inversion magic angle spinning nuclear magnetic resonance; ES-MS, electrospray mass spectrometry; HR-MS, high resolution mass spectroscopy; XPS, Xray photoelectron spectroscopy; ToF-SIMS, time-of-flight secondary ion mass spectrometry; PCA, pyrrolecarboxylic acid; DFT, density functional theory; TEM, transmission electron microscopy; FA, folic acid; EPR, electron paramagnetic resonance; UV, ultraviolet; FT-IR, Fourier transform infrared spectroscopy; CHCA, α-cyano-4-hydroxy cinnamic acid

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09662. AFM, Raman, and MALDI-MS analysis of PDA and DHI melanin, computed Raman bands, and 15N chemical shifts and reaction mechanism (PDF)

■

REFERENCES

(1) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. MusselInspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318, 426−430. (2) Ball, V.; Frari, D. D.; Toniazzo, V.; Ruch, D. Kinetics of Polydopamine Film Deposition as a Function of pH and Dopamine Concentration: Insights in the Polydopamine Deposition Mechanism. J. Colloid Interface Sci. 2012, 386, 366−372. (3) Bernsmann, F.; Ball, V.; Addiego, F.; Ponche, A.; Michel, M.; Gracio, J. J. D. A.; Toniazzo, V.; Ruch, D. Dopamine-Melanin Film Deposition Depends on the Used Oxidant and Buffer Solution. Langmuir 2011, 27, 2819−2825. (4) Wei, Q.; Zhang, F.; Li, J.; Li, B.; Zhao, C. Oxidant-Induced Dopamine Polymerization for Multifunctional Coatings. Polym. Chem. 2010, 1, 1430−1433. (5) Du, X.; Li, L.; Li, J.; Yang, C.; Frenkel, N.; Welle, A.; Heissler, S.; Nefedov, A.; Grunze, M.; Levkin, P. A. UV-Triggered Dopamine Polymerization: Control of Polymerization, Surface Coating, and Photopatterning. Adv. Mater. 2014, 26, 8029−8033. (6) Ponzio, F.; Barthès, J.; Bour, J.; Michel, M.; Bertani, P.; Hemmerlé, J.; d’Ischia, M.; Ball, V. Oxidant Control of Polydopamine Surface Chemistry in Acids: A Mechanism-Based Entry to SuperhydrophilicSuperoleophobic Coatings. Chem. Mater. 2016, 28, 4697−4705. (7) Crescenzi, O.; Kroesche, C.; Hoffbauer, W.; Jansen, M.; Napolitano, A.; Prota, G.; Peter, M. G. Synthesis of Dopamines Labelled with 13C in the α- or β-Side Chain Position and Their Application to Structural Studies on Melanins by Solid-State NMR Spectroscopy 1994, 1994, 563−567. (8) Jiang, J. H.; Zhu, L. P.; Li, X. L.; Xu, Y. Y.; Zhu, B. K. Surface Modification of PE Porous Membranes Based on the Strong Adhesion of Polydopamine and Covalent Immobilization of Heparin. J. Membr. Sci. 2010, 364, 194−202. (9) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Elucidating the Structure of Poly(dopamine). Langmuir 2012, 28, 6428−6435.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +39 081-674132. ORCID

Lucia Panzella: 0000-0002-2662-8205 Stefano L. Oscurato: 0000-0002-1814-8033 7678


Forum Article

ACS Applied Materials & Interfaces (10) Hong, S.; Na, Y. S.; Choi, S.; Song, I. T.; Kim, W. Y.; Lee, H. NonCovalent Self-Assembly and Covalent Polymerization Co-Contribute to Polydopamine Formation. Adv. Funct. Mater. 2012, 22, 4711−4717. (11) Shalev, T.; Gopin, A.; Bauer, M.; Stark, R. W.; Rahimipour, S. Non-Leaching Antimicrobial Surfaces through Polydopamine BioInspired Coating of Quaternary Ammonium Salts or an Ultrashort Antimicrobial Lipopeptide. J. Mater. Chem. 2012, 22, 2026−2032. (12) Della Vecchia, N. F.; Avolio, R.; Alfè, M.; Errico, M. E.; Napolitano, A.; d’Ischia, M. Building-Block Diversity in Polydopamine Underpins a Multifunctional Eumelanin-Type Platform Tunable Through a Quinone Control Point. Adv. Funct. Mater. 2013, 23, 1331−1340. (13) Della Vecchia, N. F.; Luchini, A.; Napolitano, A.; D’Errico, G.; Vitiello, G.; Szekely, N.; d’Ischia, M.; Paduano, L. Tris Buffer Modulates Polydopamine Growth, Aggregation, and Paramagnetic Properties. Langmuir 2014, 30, 9811−9818. (14) Liebscher, J.; Mrówczyński, R.; Scheidt, H. A.; Filip, C.; Hădade, N. D.; Turcu, R.; Bende, A.; Beck, S. Structure of Polydopamine: A Never-Ending Story? Langmuir 2013, 29, 10539−10548. (15) Zangmeister, R. A.; Morris, T. A.; Tarlov, M. J. Characterization of Polydopamine Thin Films Deposited at Short Times by Autoxidation of Dopamine. Langmuir 2013, 29, 8619−8628. (16) Ding, Y.; Weng, L.-T.; Yang, M.; Yang, Z.; Lu, X.; Huang, N.; Leng, Y. Insights into the Aggregation/Deposition and Structure of a Polydopamine Film. Langmuir 2014, 30, 12258−12269. (17) Schaubroeck, D.; Vercammen, Y.; Vaeck, L. V.; Vanderleyden, E.; Dubruel, P.; Vanfleteren, J. Surface Characterization and Stability of an Epoxy Resin Surface Modified with Polyamines Grafted on Polydopamine. Appl. Surf. Sci. 2014, 303, 465−472. (18) Chen, C.-T.; Martin-Martinez, F. J.; Jung, G. S.; Buehler, M. J. Polydopamine and Eumelanin Molecular Structures Investigated with Ab Initio Calculations. Chem. Sci. 2017, 8, 1631−1641. (19) Kaxiras, E.; Tsolakidis, A.; Zonios, G.; Meng, S. Structural Model of Eumelanin. Phys. Rev. Lett. 2006, 97, 218102. (20) Meng, S.; Kaxiras, E. Theoretical Models of Eumelanin Protomolecules and Their Optical Properties. Biophys. J. 2008, 94, 2095−2105. (21) Chen, C.-T.; Ball, V.; de Almeida Gracio, J. J.; Singh, M. K.; Toniazzo, V.; Ruch, D.; Buehler, M. J. Self-Assembly of Tetramers of 5,6-Dihydroxyindole Explains the Primary Physical Properties of Eumelanin: Experiment, Simulation, and Design. ACS Nano 2013, 7, 1524−1532. (22) Fan, H.; Yu, X.; Liu, Y.; Shi, Z.; Liu, H.; Nie, Z.; Wu, D.; Jin, Z. Folic Acid−Polydopamine Nanofibers Show Enhanced OrderedStacking via π−π Interactions. Soft Matter 2015, 11, 4621−4629. (23) Kim, Y. J.; Khetan, A.; Wu, W.; Chun, S.-E.; Viswanathan, V.; Whitacre, J. F.; Bettinger, C. J. Evidence of Porphyrin-Like Structures in Natural Melanin Pigments Using Electrochemical Fingerprinting. Adv. Mater. 2016, 28, 3173−3180. (24) d’Ischia, M.; Wakamatsu, K.; Napolitano, A.; Briganti, S.; GarciaBorron, J.-C.; Kovacs, D.; Meredith, P.; Pezzella, A.; Picardo, M.; Sarna, T.; Simon, J. D.; Ito, S. Melanins and Melanogenesis: Methods, Standards, Protocols. Pigm. Cell Melanoma Res. 2013, 26, 616−633. (25) Pezzella, A.; Napolitano, A.; d’Ischia, M.; Prota, G. Oxidative Polymerisation of 5,6-Dihydroxyindole-2-Carboxylic Acid to Melanin: A New Insight. Tetrahedron 1996, 52, 7913−7920. (26) Edge, R.; d’Ischia, M.; Land, E. J.; Napolitano, A.; Navaratnam, S.; Panzella, L.; Pezzella, A.; Ramsden, C. A.; Riley, P. A. Dopaquinone Redox Exchange with Dihydroxyindole and Dihydroxyindole Carboxylic Acid. Pigm. Cell Res. 2006, 19, 443−450. (27) Frisch, M. J.; Trucks, W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.;

Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc.,: Wallingford, CT, 2013. (28) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0Model. J. Chem. Phys. 1999, 110, 6158−6169. (29) Miertuš, S.; Scrocco, E.; Tomasi, J. Electrostatic Interaction of a Solute with a Continuum. A Direct Utilization of Ab Initio Molecular Potentials for the Prevision of Solvent Effects. Chem. Phys. 1981, 55, 117−129. (30) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. New Developments in the Polarizable Continuum Model for Quantum Mechanical and Classical Calculations on Molecules in Solution. J. Chem. Phys. 2002, 117, 43−54. (31) Scalmani, G.; Barone, V.; Kudin, K. N.; Pomelli, C. S.; Scuseria, G. E.; Frisch, M. J. Achieving Linear-Scaling Computational Cost for the Polarizable Continuum Model of Solvatation. Theor. Chem. Acc. 2004, 111, 90−100. (32) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999−3094. (33) Rappé, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III; Skiff, W. M. UFF, a Full Periodic Table Force Field for Molecular Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc. 1992, 114, 10024−10035. (34) York, D. M.; Karplus, M. A Smooth Solvation Potential Based on the Conductor-Like Screening Model. J. Phys. Chem. A 1999, 103, 11060−11079. (35) Scalmani, G.; Frisch, M. J. Continuous Surface Charge Polarizable Continuum Models of Solvation. I. General Formalism. J. Chem. Phys. 2010, 132, 114110. (36) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378−6396. (37) Merrick, J. P.; Moran, D.; Radom, L. An Evaluation of Harmonic Vibrational Frequency Scale Factors. J. Phys. Chem. A 2007, 111, 11683−11700. (38) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. An Efficient Implementation of Time-Dependent Density-Functional Theory for the Calculation of Excitation Energies of Large Molecules. J. Chem. Phys. 1998, 109, 8218−8224. (39) Bauernschmitt, R.; Ahlrichs, R. Treatment of Electronic Excitations within the Adiabatic Approximation of Time Dependent Density Functional Theory. Chem. Phys. Lett. 1996, 256, 454−464. (40) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. Molecular Excitation Energies to High-Lying Bound States from TimeDependent Density-Functional Response Theory: Characterization and Correction of the Time-Dependent Local Density Approximation Ionization Threshold. J. Chem. Phys. 1998, 108, 4439−4449. (41) Adamo, C.; Scuseria, G. E.; Barone, V. Accurate Excitation Energies from Time-Dependent Density Functional Theory: Assessing the PBE0Model. J. Chem. Phys. 1999, 111, 2889−2899. (42) Scalmani, G.; Frisch, M. J.; Mennucci, B.; Tomasi, J.; Cammi, R.; Barone, V. Geometries and Properties of Excited States in the Gas Phase and in Solution: Theory and Application of a Time-Dependent Density Functional Theory Polarizable Continuum Model. J. Chem. Phys. 2006, 124, 94107. (43) Ditchfield, R. Self-Consistent Perturbation Theory of Diamagnetism. Mol. Phys. 1974, 27, 789−807. (44) Wolinski, K.; Hinton, J. F.; Pulay, P. Efficient Implementation of the Gauge-Independent Atomic Orbital Method for NMR Chemical Shift Calculations. J. Am. Chem. Soc. 1990, 112, 8251−8260. (45) Adhyaru, B. B.; Akhmedov, N. G.; Katritzky, A. R.; Bowers, C. R. Solid-State Cross-Polarization Magic Angle Spinning 13C and 15N NMR Characterization of Sepia Melanin, Sepia Melanin Free Acid and Human 7679


Forum Article

ACS Applied Materials & Interfaces Hair Melanin in Comparison with Several Model Compounds. Magn. Reson. Chem. 2003, 41, 466−474. (46) Ku, S. H.; Lee, J. S.; Park, C. B. Spatial Control of Cell Adhesion and Patterning through Mussel-Inspired Surface Modification by Polydopamine. Langmuir 2010, 26, 15104−15108. (47) Yang, H.; Lan, Y.; Zhu, W.; Li, W.; Xu, D.; Cui, J.; Shen, D.; Li, G. Polydopamine-Coated Nanofibrous Mats as a Versatile Platform for Producing Porous Functional Membranes. J. Mater. Chem. 2012, 22, 16994−17001. (48) Napolitano, A.; Pezzella, A.; d’Ischia, M.; Prota, G. New Pyrrole Acids by Oxidative Degradation of Eumelanins with Hydrogen Peroxide. Further Hints to the Mechanism of Pigment Breakdown. Tetrahedron 1996, 52, 8775−8780. (49) Iacomino, M.; Paez, J. I.; Avolio, R.; Carpentieri, A.; Panzella, L.; Falco, G.; Pizzo, E.; Errico, M. E.; Napolitano, A.; del Campo, A.; d’Ischia, M. Multifunctional Thin Films and Coatings from Caffeic Acid and a Cross-Linking Diamine. Langmuir 2017, 33, 2096−2102. (50) Iacomino, M.; Weber, F.; Gleichenhagen, M.; Pistorio, V.; Panzella, L.; Pizzo, E.; Schieber, A.; d’Ischia, M.; Napolitano, A. Stable Benzacridine Pigments by Oxidative Coupling of Chlorogenic Acid with Amino Acids and Proteins: Toward Natural Product-Based Green Food Coloring. J. Agric. Food Chem. 2017, 65, 6519−6528. (51) Napolitano, A.; Pezzella, A.; Prota, G. 6,7-Dihydroxy-1,2,3,4Tetrahydroisoquinoline Formation by Iron Mediated Dopamine Oxidation: A Novel Route to Endogenous Neurotoxins under Oxidative Stress Conditions. Tetrahedron Lett. 1999, 40, 2833−2836. (52) Chen, S.; Zhang, J.; Chen, Y.; Zhao, S.; Chen, M.; Li, X.; Maitz, M. F.; Wang, J.; Huang, N. Application Of Phenol/Amine Copolymerized Film Modified Magnesium Alloys: Anticorrosion And Surface Biofunctionalization. ACS Appl. Mater. Interfaces 2015, 7, 24510−24522.

7680


Structural Basis of Polydopamine Film Formation: Probing 5,6

Recommend Documents