**2.3 Non-resonance Raman experiments**

Powder samples of native and aggregated HEWL were prepared by drying the solutions under nitrogen at room temperature, which removed the acetic acid. Raman spectra (785-nm excitation) of HEWL powder samples and pure dipropyl di- and trisulfide liquids were recorded using a Renishaw inVia confocal Raman spectrometer equipped with a research grade Leica microscope and 50× objective (numerical aperture, 0.55). Five accumulations of 30 s each were collected for each sample in the range of 400–1800 cm<sup>−</sup><sup>1</sup> . Wire 4.0 software was used for data collection. A laser power of approximately 11.5 mW was used to avoid sample photo-degradation.

## **2.4 TCEP test for trisulfides**

A reaction with tris(2-carboxyethyl)phosphine (TCEP) reducing agent was used as a test for trisulfides [35]. Hen egg white lysozyme (HEWL) in native and aggregated form was incubated at pH 2.0 and room temperature for 90 minutes in the presence of TCEP. The reaction products were analyzed using normal Raman spectroscopy. Powder samples of HEWL-aggregates incubated at different concentrations of TCEP were prepared for non-resonance Raman spectroscopic analysis by drying the corresponding solutions under a nitrogen flow.

## **2.5 Deep UV resonance Raman spectroscopy (DUVRR)**

DUVRR spectra (199.7 nm excitation) of 25 mg/mL HEWL were collected using a home built instrument equipped with a CCD camera (Roper Scientific, Inc.) cooled in liquid nitrogen [36]. A spinning quartz NMR tube with a magnetic stirrer was used for sampling. Each spectrum recorded an average of 20 accumulations with 30 s acquisition time. GRAMS/AI 7.0 software (Thermo Galactic, Salem, NH) was used for data processing.
