Ketoprofen (Keto) is a representative of the group of non-steroidal anti-inflammatory drugs widely used in modern medical therapy. The intramolecular dynamics of Keto is of particular importance, since the mobility of its main structural fragments will be one of the key factors determining the efficiency of drug binding to the enzyme. Investigations of such processes taking into account the acid-base properties of Keto have not been carried out. Therefore, studying the intramolecular dynamics of Keto, considering its possible forms (molecular, anionic and ion-pair) is of current interest.

DFT calculations for molecular, anionic and ion-pair forms of Keto were performed at BP86/def2-TZVP level of theory using ORCA software. Experimental IR-spectra of the Keto and its sodium salt were recorded at room temperature on Nicolet iS5 FTIR spectrometer. A good agreement was obtained between the experimental and DFT-calculated vibrational frequencies of the investigated objects.

The intramolecular dynamics of the main structural fragments was investigated for the molecular, anionic and ion-pair forms of Keto. The most stable conformers were revealed for all considered forms and barriers of intramolecular rotation were estimated. It was shown that the structures of the studied forms of Keto are labile but characterized by different mobility. For all forms of Keto, the benzoyl fragment is characterized by the lowest mobility. The least labile is the anionic form of Keto. Dissociation of Keto leads to a decrease in the mobility of its structural fragments, and the formation of contact ion pairs reduces this effect.