Application Case of Differential Scanning Calorimeter for Testing Proteins

1. Preamble

When studying structural changes in microfiber-matrix complexes in keratin samples, differential scanning calorimetry (DSC) can be used to analyze changes in keratin’s thermal stability, phase transition behavior, as well as possible melting or decomposition processes before and after heat treatment.

For example, the DSC curve can reveal endothermic or exothermic peaks during the heating process, which correspond to the movement of keratin molecular chains, the melting of microfibers, or the denaturation of proteins.

By comparing the DSC curves before and after stretching and heat treatment, researchers can understand the effects of these treatments on the thermal properties and structural stability of keratin complexes.

For instance, heat treatment may cause increased intermolecular cross-linking of keratin, thereby improving the thermal stability of the material, while stretching may alter the crystallinity of keratin, influencing its thermal behavior.

2. Experimental Steps

Material preparation: Untreated brown hair, untreated hair, and bleached yellow hair.

Sample preparation: After cleansing the hair, divide it into several small portions of about 5–7 mg.

Moisture content: The hair samples were stored for 48 hours in a chamber with 50% relative humidity at 23 °C. After equilibration, the moisture content of bleached hair was 11.48%, and that of untreated brown hair was 10.15%.

Measuring instruments: Instrument name: DSC 600 Differential Scanning Calorimeter

Instrument brand: ThermCon Scientific

Experimental procedure: The samples were heated to 70 °C and maintained at this temperature for 30 minutes to remove water, then cooled to 25 °C and subsequently heated to 270 °C at a rate of 10 °C/min to record the absorption curves.

3. Results and Discussion

1. Data analysis
Figure 1 shows a typical DSC curve of an untreated hair sample. The peak temperature is defined as the denaturation temperature (Td) of the α-helix in intermediate filaments (IFs), including phase transition and protein degradation. The area of the peak represents the denaturation enthalpy △Hd, expressed as the amount of heat absorbed per gram of dry hair (J/g). △Hd is used to calculate the relative helix content (RHC): RHC = 100 × (△Hd / △Hd0) %, where △Hd0 is the average denaturation enthalpy of untreated hair.

2. Influence of environmental conditions on hair
Different sections of an 18 cm untreated brown hair sample were examined. From the tips to the roots, △Hd decreases while Td consistently increases. The RHC value of the hair tips is 85.6% compared to the roots. This clearly demonstrates that environmental exposure has a strong influence on hair, since the hair tips are in contact with the environment for a longer period.

3. Influence of bleaching and relaxation treatment on hair
Figure 2 shows the influence of bleaching time on Td and RHC of untreated hair. As bleaching time increases, RHC values decrease. △Hd of untreated hair: 3.85 J/g; △Hd of bleached hair: 1.07 J/g. RHC of bleached hair was 72.8% lower than that of untreated hair. This indicates that bleaching treatment causes severe damage to hair. SEM revealed visible holes and fractures in the hair surface. With increasing bleaching time, Td rises due to the higher cross-linking density of the matrix. In black hair (U.S.), Td also increases and △Hd decreases with extended relaxation treatment.

4. Restoration of helix peak by protein derivatives
Hair samples were treated with different protein derivatives before and after bleaching. After treatment with hydrolyzed wheat protein-cystine-polysiloxane, hydrolyzed wheat protein, and propyl polysiloxane, the RHC of hair samples increased by 77.5%, 72.0%, and 88.8%, respectively. Td was significantly reduced after protein treatment. Leave-in products had a stronger recovery effect on absorption peaks than rinse-off products. Bleached hair, with its porous surface, allows polymers to penetrate, explaining the significant increase in △Hd following protein derivative treatment. For relaxed hair, an increase in △Hd and a decrease in Td were also observed after applying protein derivatives.

4. Experimental Figure

Figure 3: Test results and analysis of human hair tissue conducted by our company.