Femtosecond IR-pump-IR-probe experiments with independently tunable pulses are used to monitor the ultrafast response of selected IR absorption bands to vibrational excitation of other modes of Fmoc-nitrophenylalanine. The absorptions of both NO2-bands change rapidly within <2 ps upon excitation of other vibrational modes. The results point to considerable coupling between the monitored NO2 modes and the initially excited modes or low-frequency modes. The latter are populated by a rapid energy redistribution process. The strong IR absorption of the NO2 stretching bands and the intense coupling to other modes makes the nitro group of nitrophenylalanine a sensitive monitor for vibrational energy arriving at this amino acid.