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.2021 Mar 4;26(5):1363.
doi: 10.3390/molecules26051363.

Dopamine Self-Polymerization as a Simple and Powerful Tool to Modulate the Viscoelastic Mechanical Properties of Peptide-Based Gels

Affiliations

Dopamine Self-Polymerization as a Simple and Powerful Tool to Modulate the Viscoelastic Mechanical Properties of Peptide-Based Gels

Galit Fichman et al. Molecules..

Abstract

Dopamine is a small versatile molecule used for various biotechnological and biomedical applications. This neurotransmitter, in addition to its biological role, can undergo oxidative self-polymerization to yield polydopamine, a robust universal coating material. Herein, we harness dopamine self-polymerization to modulate the viscoelastic mechanical properties of peptide-based gels, expanding their ever-growing application potential. By combining rapid peptide assembly with slower dopamine auto-polymerization, a double network gel is formed, where the fibrillar peptide gel network serves as a scaffold for polydopamine deposition, allowing polydopamine to interpenetrate the gel network as well as establishing crosslinks within the matrix. We have shown that triggering the assembly of a lysine-rich peptide gelator in the presence of dopamine can increase the mechanical rigidity of the resultant gel by a factor of 90 in some cases, while retaining the gel's shear thin-recovery behavior. We further investigate how factors such as polymerization time, dopamine concentration and peptide concentration alter the mechanical properties of the resultant gel. The hybrid peptide-dopamine gel systems were characterized using rheological measurements, circular dichroism spectroscopy and transmission electron microscopy. Overall, triggering peptide gelation in the presence of dopamine represents a simple yet powerful approach to modulate the viscoelastic mechanical properties of peptide-based gels.

Keywords: dopamine; hydrogel; peptide; self-assembly.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Conceptual scheme describing the assembly and gelation of peptide gelator in the presence of dopamine. Occurrence of rapid peptide assembly along with a slower dopamine auto-polymerization reaction results in a gel with increased mechanical rigidity.
Figure 2
Figure 2
Rheological studies of the 1 wt.% MAX1-dopamine gel system. (A) The corresponding images of 1 wt.% MAX1 gels 3 days post triggering gelation with or without 10 mM dopamine (B) Frequency and (C) Strain sweep rheological data collected at 0.2% strain and 6 rad/s frequency, respectively, for pre-formed gels. Closed and open symbols represent storage (G′) and loss (G″) modulus, respectively. (D) Recovery of gels was followed after two repetitive shear-thinning cycles. In each cycle, the G′ of the gels was initially measured at low strain for 10 min (0.2% strain), followed by 30 s measurements under high strain (1000% strain) that ruptures the gel network. Then, gel recovery is determined by measuring the G′ again at low strain (0.2% strain) for 10 min. All measurements were performed at a frequency of 6 rad/s. Measurement segments performed within the LVE regime (0.2% strain, 6 rad/s frequency) are marked as I, II, III. Grey arrows indicate the 30 s measurement segments performed under high strain (1000%). To the right are bar graphs of the G′ values collected for the gels at the end of each I, II and III segments. All rheological data represent the average G′ and G″ obtained from at least three independent measurements.
Figure 3
Figure 3
Secondary structural analysis of the MAX1-dopamine gel system using circular dichroism (CD) spectroscopy. (A) CD spectra at 37 °C were collected for 1 wt.% MAX1 in water (blue triangles) and in buffer, in the absence or presence of dopamine (black squares and red circles, respectively). Data collection was initiated after 20 min of sample equilibration at 37 °C. (B) Rate of β-sheet formation of MAX1 in the absence or presence of dopamine. The evolution of β-sheet is monitored by recording [θ]216 as a function of time for 1 wt.% peptide solution at 37 °C, starting 5 min after peptide assembly is initiated by the addition of buffer (pH 7.4). (C,D) Temperature-dependent wavelength CD spectra of 1 wt.% MAX1 in buffer, in the absence or presence of dopamine (C,D, respectively). (E) The evolution of β-sheet as a function of temperature is presented for MAX1 in water (blue triangles) and in buffer, in the absence or presence of dopamine (black squares and red circles, respectively).
Figure 4
Figure 4
Representative TEM micrographs showing fibrils isolated from 1 wt.% fibrillar gel networks 3 days after gelation is triggered in the absence or presence of 10 mM Dopamine (A andB, respectively). Scale bar = 100 nm. Widths of individual fibrils of each sample were determined using ImageJ software, by measuring width of fibrils from 3 separate micrographs, representing different location of the fibrils on the grid,n = 193 andn = 80 for the gel with without the dopamine, respectively.
Figure 5
Figure 5
Rheological studies of the 1 wt.% MAX1-dopamine gel system. (A) G′ (0.2% strain, 6 rad/s) of pre-formed 1 wt.% MAX1 peptide gel assembled in the presence of 10 mM dopamine at 1, 2, 3, 4 or 5 days post triggering assembly. Left bar (I) represents the G′ value of the gel prior to shear-thinning at high strain (1000% strain, 30 s) and right bar (II) represents the recovered G′ value obtained 10 min after the gel was shear-thinned. (B) Rheological studies on pre-formed 1 wt.% MAX1 gel 3 days post triggering gelation in the presence of different dopamine concentrations. (C) Pre-sheared G′ and corresponding G″ values of pre-formed 1 wt.% MAX1-dopamine gel system with varying concentrations of dopamine (1, 5, 10, 20 and 40 mM), plotted in relation to G′/G″ value range of other viscoelastic materials reported in the literature [10]. The G′/G″ values of MAX1-dopamine are plotted as symbols in red shades corresponding to dopamine concentration presented in panel B. Black squares represent G′ and G″ of 1 wt.% MAX1 following 3-day assembly without dopamine. The blue dotted line indicates a G″ that is 10% of the G′, for reference. Full statistical analysis for panels A and B is given in the supporting info, Table S1.
Figure 6
Figure 6
Rheological studies of the 0.5 wt.% MAX1-dopamine gel system. (A) Rheological studies on pre-formed 0.5 wt.% MAX1 gel 3 days post triggering gelation in the presence of different dopamine concentrations. Left bar (I) represents the G′ value of the gel prior to shear-thinning at high strain (1000% strain, 30 s) and right bar (II) represents the recovered G′ value obtained 10 min after the gel was shear-thinned. (B) Dynamic time sweep measurements of 0.5 wt.% MAX1 in the absence (black) and presence of 5 mM dopamine (red) monitoring the evolution of G′ and G″ (dark and light colors, respectively) as a function of time at 37 °C, pH 7.4. The insert depicts the first hour following the initiation of assembly, where the G′ and G″ values are plotted in a linear scale. (C) Pre-sheared G′ and corresponding G″ values of pre-formed 0.5 wt.% MAX1-dopamine gel system with varying concentrations of dopamine (5, 10 and 20 mM), plotted in relation to G′/G″ value range of other viscoelastic materials reported in the literature [10]. The G′/G″ values of MAX1-dopamine are plotted as symbols in red shades corresponding to dopamine concentration presented in panel A. Black squares represent G′ and G″ of 0.5 wt.% MAX1 following 3-day assembly without dopamine. The blue dotted line indicates a G″ that is 10% of the G′, for reference. Full statistical analysis for panel A is given in the supporting info, Table S1.
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