You could try DMSO-d6. I can't guarantee, especially not for this special conjugate. However, DMSO is principially capable of dissolving chitosan derivatives.

For 1H NMR analysis of folic acid–chitosan conjugates, deuterated dimethyl sulfoxide (DMSO-d6) is the most suitable solvent, as it effectively dissolves both folic acid and chitosan, which have limited solubility in water and many organic solvents, while providing clear spectra with minimal background interference at 2.50 ppm, avoiding overlap with folic acid’s aromatic and heterocyclic proton signals (6–9 ppm) or chitosan’s sugar backbone protons (3–5 ppm). DMSO-d6 is chemically inert to the conjugate’s functional groups, ensuring stability during analysis, unlike deuterated water (D2O), which may require pH adjustments that could alter the conjugate’s structure or cause signal overlap with residual water at 4.7 ppm. Deuterated acetic acid/D2O mixtures or trifluoroacetic acid (TFA-d) may dissolve chitosan but risk protonating folic acid or complicating spectra due to acidity. To use DMSO-d6, ensure high purity to avoid impurities, and if solubility is limited, gentle heating (40–50°C) or sonication can help, avoiding prolonged heating to prevent degradation. Calibrate the NMR spectrometer to the DMSO-d6 signal for accurate chemical shift referencing, making it the optimal choice for clear, reliable spectra of folic acid–chitosan conjugates.