We report time-resolved photothermal infrared nanoscopy measurements across a spectral range of more than 100 cm(-1) (1565 cm(-1) to 1729 cm(-1)) at nanoscale spatial resolution. This is achieved through a custom-built system using broadly tunable external cavity quantum cascade lasers in combination with a commercially available atomic force microscope. The new system is applied to the analysis of conformational changes of a polypeptide (poly-l-lysine) film upon temperature-induced changes of the humidity in the film. Changes of the secondary structure from β-sheet to α-helix could be monitored at a time resolution of 15 s per spectrum. The time-resolved spectra are well comparable to reference measurements acquired with conventional Fourier transform infrared microscopy.