Figure 2.

¹³C Cross-Polarization Magic Angle Spinning (CP-MAS) solid-state NMR spectra of the three pyomelanin. The peaks area allowed us to estimate gentisyl alcohol at ~ 9 to 10% and gentisaldehyde at ~ 2 to 3% in Pyo_ENZ and Pyo_BACT. The loss of carboxylic moieties observed on the Pyo_CHEM structure (ratio 1/10, black line) was explained by a partial degradation due to the alkaline conditions during the polymerization. Spectra were recorded on an Avance III spectrometer (BRUKER BIOSPIN GmbH, Germany) operated at the Larmor frequencies of 400.43 MHz and 100.70 MHz on ¹H and ¹³C nuclei, respectively, by using a 4 mm cross-polarization magic angle spinning (CP-MAS) probe head. The zirconia rotors were filled with 80 mg of fine powder polymer or standards. 2048 scans were used to acquire the ¹³C spectra, the acquisition time was 28.7 ms. The ¹H 90° pulse length and the power level were 3.8 s and 80 kHz, respectively. The spectral width was 35.7 kHz and 2048 points were acquired to describe the free induction decay. A two-pulse phase modulation (TPPM) proton decoupling (75 kHz) was used during the ¹³C acquisition. Spectra were externally referenced to the carbonyl peak of glycine at 176.03 ppm downfield of TMS. No line broadening was applied. All the spectra were acquired using the same receiver gain. sh, shoulder.