Fig. 3: pH-dependent structural transitions of Tyr.

a Real-time SERS spectra of Tyr in the condition changes from pH 1.0 (black) to pH 10.5 (green), and pH 13.0 (red). Integration time: 10 s. b Representative SERS spectra of Tyr at pH 10.5 with 1 s integration time, showing spectral fluctuations arisen from Tyr in distinct charge states. c Comparison of SERS spectra (upper panel) and simulated Raman spectra (lower panel) of Tyr at pH 1.0 (black) and pH 13.0 (red), respectively. The SERS spectra are adopted from (a). The simulated Raman spectra and electrostatic potential are generated from DFT calculations on Tyr in +1 charged state as NH₃⁺CH(CH₂C₆H₄OH)COOH (d) and -2 charged state as NH₂CH(CH₂C₆H₄O^-)COO^- (e), respectively. Based on least-squares procedures, a scaling factor of 0.9770 was applied to the wavenumbers above 1000 cm⁻¹ in simulated Raman spectra. The term (arb. units) is abbreviated for arbitrary units. d, e Electrostatic potential (ESP) values of Tyr in +1 charged state and -2 charged state mapped on 0.01 a.u. van der Waals surface, expressed in atomic units (a.u.).