Selective monitoring of 3 out of 50,000 protein vibrations

An important group in a large protein, which is the phosphate group that transiently binds to the Ca2+ transporting ATPase (Ca2+-ATPase) of the sarcoplasmic reticulum membrane, is selectively observed with IR spectroscopy. The three phosphate stretching vibrations stand out from the 50,000 protein vibrations in an isotope exchange experiment where oxygen exchange at the phosphate group is catalyzed by the Ca2+-ATPase.