Photochemical Reaction of 2-(3-Benzoylphenyl)propionic Acid (Ketoprofen) with Basic Amino Acids and Dipeptides

Abstract

Photoreaction of 2-(3-benzoylphenyl)propionic acid (ketoprofen, KP) with basic amino acids (histidine, lysine, and arginine) and dipeptides (carnosine and anserine) including a histidine moiety in phosphate buffer solution (pH 7.4) has been investigated with transient absorption spectroscopy. With UV irradiation KP(-) gave rise to a carbanion through a decarboxylation reaction, and the carbanion easily abstracted a proton from the surrounding molecule to yield a 3-ethylbenzophenone ketyl biradical (EBPH). The dipeptides as well as the basic amino acids were found to accelerate the proton transfer reaction whereas alanine and glycine had no effect on the reaction, revealing that these amino acids having a protonated side chain act as a proton donor. The formation quantum yield of EBPH was estimated to be fairly large by means of an actinometrical method with benzophenone, and the bimolecular reaction rate constant for the proton transfer between the carbanion and the protonated basic amino acids or the protonated dipeptides was successfully determined. It has become apparent that the bimolecular reaction rate constant for the proton transfer depended on the acid dissociation constant for the side chain of the amino acids for the first time. This reaction mechanism was interpreted by difference of the heat of reaction for each basic amino acid based on the thermodynamical consideration. These results strongly suggest that the side chain of the basic amino acid residue in protein should play an important role for photochemistry of KP in vivo.

Photoreaction of Ketoprofen with Tryptophan and Tyrosine in Phosphate Buffer Solution

“…In the phosphate buffer solution of pH 7.4, almost all KP exists in the deprotonated form of KP (KP − ) because of its acidity (p K a = 4.7). According to the previous report , an intense band around 600 nm appearing immediately after the laser irradiation was due to KP carbanion, and the band around 525 nm at 1.7 μs after the laser was assigned to EBPH. The same spectra were obtained with Tyr.…”

“…As the formation rate constant of EBPH, which equals to the disappearance rate constant of carbanion , is much larger than the disappearance rate constant of EBPH, the EBPH formation quantum yield can be expressed as follows.where [Carbanion] and [EBPH] are yields of carbanion and EBPH with an incident laser power, A total and A slow are the total and the slow component absorption intensities at t = 0 monitored at 525 nm, ε Carbanion and ε EBPH are molar absorption coefficients at 525 nm of carbanion and EBPH, respectively. In the previous work , the ε Carbanion value was estimated to be (3100 ± 100) M −1 cm −1 by means of the actinometry method with BP. Assuming that the ε EBPH value equals to that of benzophenone ketyl radical (BPK) at 545 nm (4600 M −1 cm −1 ) because the spectral feature of BPK and EBPH is quite similar, the value of Φ EBPH can be obtained.…”

“… *Reference ; †For amino acids and dipeptides, correction of the concentration should be required because actual quencher for carbanion was the protonated form to the side chain of basic amino acids and dipeptides ; ‡Reference ; §Reference ; ∥Reference ; ¶This work. …”

“…Several proton‐donating sites for these compounds are considered. As for basic amino acids and dipeptides including a histidine moiety in the phosphate buffer solution at pH 7.4, the protonated side chain is the donating site not the protonated N ‐terminal because alanine and glycine have no effect on the lifetime of the carbanion . Therefore, on phenol compounds such as Tyr, DOPA, 4‐MP, the phenolic OH group is concluded to be the proton‐donating site.…”

“…An experimental setup of the transient absorption spectrum measurement has been reported elsewhere . Briefly, a XeCl excimer laser (Lamda Physik COMPex 102; 308 nm, 200 mJ, 20 ns pulse duration) was used as an excitation light and a Xe flash lamp (Ushio UXL‒300DO; 300 W) was used as a monitoring light source.…”