Rapid scan Fourier transform infrared (FTIR) spectroscopy and time-resolved single wavelength infrared (IR) spectroscopy have been used to follow the photochemical release of adenosine 5’-triphosphate (ATP) from P-3-(1-(2-nitrophenyl)ethyl) adenosine 5’-triphosphate (caged ATP). Vibrational difference spectra for the formation first of the aci-nitro anion intermediate and subsequently of ATP and the byproduct(s) were obtained by rapid scan FTIR spectroscopy in the millisecond-to-second time domain. Vibrational modes of the phosphate groups of ATP and caged ATP in the range 1250-900 cm(-1) could be assigned on the basis of triple and single O-18 labeling in caged ATP at the terminal phosphate group and at the bridging oxygen between the terminal phosphate and the 1-(2-nitrophenyl)ethyl group, respectively. The rapid formation and subsequent decay of the aci-nitro anion intermediate were monitored by single-wavelength time-resolved IR spectroscopy at 1251 cm(-1) (predominantly a PO2- mode of caged ATP and the aci-nitro intermediate). The appearance of the free gamma-phosphate group of ATP was monitored at 1119 cm(-1) (PO32- mode of ATP). Decay of the aci-nitro anion intermediate and formation of ATP were well fitted by single exponentials to give a mean rate constant of 218 +/- 33 s(-1) at pH 7 and 22 degrees C.