Hexamethylenediamine-Mediated Polydopamine Film Deposition: Inhibition by Resorcinol as a Strategy for Mapping Quinone Targeting Mechanisms

Abstract

Hexamethylenediamine (HMDA) and other long chain aliphatic diamines can induce substrate-independent polymer film deposition from dopamine and several other catechols substrates at relatively low concentrations, however the mechanism of the diamine-promoted effect has remained little understood. Herein, we report data indicating that: (a) film deposition from 1 mM HMDA and dopamine is not affected by chemical oxidation with periodate but is markedly inhibited by resorcinol, which also prevents PDA film formation at 10 mM monomer concentration in the absence of HMDA; (b) N-acetylation of HMDA completely inhibits the effect on PDA film formation; (c) HMDA enables surface functionalization with 1 mM 5,6-dihydroxyindole (DHI) polymerization at pH 9.0 in a resorcinol-inhibitable manner. Structural investigation of the polymers produced from dopamine and DHI in the presence of HMDA using solid state ¹³C and ¹⁵N NMR and MALDI-MS suggested formation of covalent cross linked structures. It is concluded that HMDA enhances polydopamine adhesion by acting both on dopamine quinone and downstream, e.g., via covalent coupling with DHI. These results provide new insights into the mechanisms of PDA adhesion and disclose resorcinol as a new potent tool for targeting/mapping quinone intermediates and for controlling polymer growth.

Keywords: 5; 6-dihydroxyindole; coating; hexamethylenediamine; polydopamine; resorcinol.

Scheme 1

Synthetic pathways for polydopamine formation.

Scheme 2

Schematic outline of possible mechanisms underlying HMDA-promoted film deposition from dopamine.

Figure 1

UV-vis spectra of PDA/HMDA films with diamine added at different times after the beginning of dopamine autoxidation. Each curve refers to a separate experiment in which HMDA was added at the stated time. T0 refers to the reference experiment in which HMDA was added at the beginning of the reaction.

Figure 2

Kinetics of PDA/HMDA film formation: absorbance of the films obtained in the absence or in the presence of periodate in 0.05 M carbonate buffer pH 9.0 at a selected wavelength (400 nm). No detectable absorption was measured in the autoxidation experiments in the absence of HMDA.

Figure 3

UV-vis spectra of quartz substrates dipped for 24 h into 1 mM solution of DA with 1 mM HMDA in the absence or in the presence of 1 or 5 mM resorcinol (Res).

Figure 4

Uv-vis absorption spectra of quartz substrates subjected to dip-coating for 24 h with 10 mM dopamine in the absence and in the presence of equimolar amounts of resorcinol (Res).

Figure 5

UV–vis spectra of quartz substrate dipped into 1 mM solution of DHI in 0.05 M carbonate buffer (pH 9.0) in the presence of equimolar amount of HMDA over 24 h of DHI/HMDA coated coverslip.

Figure 6

AFM and micro-Raman analysis of the sample DHI/HMDA (A) Bright-field image of the investigated sample region collected by 20× microscope objective. (B) Raman spectrum (C) AFM image of the area indicated by the yellow square in the optical image. (D) Micro-Raman image relative to the red sample region in the optical image. FILM THICKNESS: 30 ±10 nm.

Figure 7

¹³C and ¹⁵N NMR spectra of PDA obtained in presence and in absence of the diamine (HMDA).

Figure 8

¹³C and ¹⁵N NMR spectra of the oxidation products of DHI obtained in presence and in absence of the diamine (HMDA).

Figure 9

Segmental spectra of MALDI-MS spectra of the solid separated from PDA/HMDA (A) and PDA (B) mixture after centrifugation. Asterisks indicate specific peaks of PDA/HMDA.

Figure 10

Tentative structure representative of the possible species responsible for the peak at m/z 501 in the PDA/HMDA mixture.

Figure 11

Segmental spectra of MALDI-MS of the solid separated from DHI/HMDA (A) and DHI melanin mixture after centrifugation (B). Asterisks indicate specific peaks of DHI/HMDA.

Figure 12

Tentative structure representative of the possible species responsible for the peak at m/z 425 in the PDA/HMDA mixture.

Scheme 3

Schematic outline of possible mechanisms underlying film deposition from dopamine or DHI.