Figure 7

Binding affinity study of the 3′-sialyllactose sodium salt with polyethyleneimine-grafted-L-aspartate-L-phenylalanine dimethyl ester (PEI-g-DF). (a, b) Isothermal titration microcalorimetry data and fitting curve of 3′-sialyllactose (3 mg ml⁻¹) titrated into PEI-g-DF (12 mg ml⁻¹) in H₂O at 25 °C. (c–e) Partial hydrogen nuclear magnetic resonance (¹H NMR) spectra of PEI-g-DF (50 mg ml⁻¹; c), 3′-sialyllactose (20 mg ml⁻¹; d) and a mixture of PEI-g-DF with an equivalent amount of 3′-sialyllactose (e) in deuterated dimethylsulfoxide (d₆-DMSO) at 25 °C. The chemical shift attribution for each hydrogen proton in 3′-sialyllactose is marked by a numerical symbol in its chemical structure. The chemical shift changes of the hydrogen protons are indicated by dashed black arrows or box. (f, g) Representative bio-attenuated total reflectance Fourier transform infrared spectroscopy (Bio-ATR-FT-IR) spectra of PEI-g-DF (black), 3′-sialyllactose (red) and its mixture with an equivalent amount of PEI-g-DF (blue) in d₆-DMSO at 25 °C. Characteristic IR peaks and corresponding vibration attributions are indicated by black arrows. All the 3′-sialyllactose used in these experiments were a sodium salt to avoid the side effect of acid–alkaline interactions.