THE PHOTOLYSIS OF TRYPTOPHAN WITH 337.1 nm LASER RADIATION

Abstract

Abstract— Aqueous solutions of L‐tryptophan were photolyzed by exposure to 337.1 nm radiation from a pulsed nitrogen laser. These data were compared with results for the 290 nm conventional‐source photolysis of tryptophan. The progress of photolysis was monitored by fluorescence analysis of tryptophan. UV absorption spectroscopy, HPLC, TLC, and proton NMR spectroscopy. The loss of Trp was observed to be first order for 290 nm photolysis but of mixed order for 337.1 nm photolysis. Five photolysis products were detected by TLC analysis, including: N‐formylkynurenine. kynurenine, tryptamine (detected after 290 nm photolysis but not 337.1 nm photolysis) and two unknown products. The tryptophan‐containing peptides N‐acetyl‐tryptophanamide (NATA) and tryptophylglycine (Trp‐Gly) were also observed to photolyze upon 337.1 nm laser radiation demonstrating that this phenomenon is not restricted to free tryptophan monomer. Since Trp is not ordinarily thought to absorb U V radiation at wavelengths as long as 337.1 nm. a number of experiments were performed in an effort to determine the mechanism of photolysis at this wavelength. Evidence is presented which indicates that the 337.1 nm laser photolysis of Trp does not result from two photon absorption, dielectric breakdown, or other laser‐specific processes. Instead. it is concluded that this photolysis results either from a very weak absorption tail extending to 337.1 nm in tryptophan itself or from a reaction involving an impurity sensitizer which absorbs the 337.1 nm radiation. The sensitizing impurity. if present, could not. however, be removed by preparative HPLC and could not be detected by TLC or fluorescence analysis.

The non-fluorescence of 4-fluorotryptophan

“…264 photolysed at 295 nm, to facilitate comparison of the present 285 nm data with previous tryptophan photolysis studies (Borkman et al, 1986;Tallmadge et al, 1989).…”

“…Tryptophan was also photolysed at 25°C and 295 nm to compare with previous work (Borkman et al, 1986;Tallmadge et al, 1989). No significant differences in the reverse-phase h.p.l.c.…”

“…Loss of starting material as well as photoproduct formation are both in evidence. Photoproducts of tryptophan that have been identified previously include Nformylkynurenine, kynurenine and tryptamine (Borkman et al, 1986). Figs.…”

A structural study of bovine lens α‐crystallin and its subunits by absorption and linear dichroism spectroscopy

“…Since it was not found in studies on other systems [15, 161 , we conclude that it is not an artifact. Although we have no explanation for the phenomenon yet, it might very well be related to the anomalous absorption at 337 nm indicated by Borkman et al both for eye lens crystallins [27] and Trp in aqueous solution [28]. Based on photolysis induced by radiation at this wavelength, they conclude it highly probable that Trp residues do absorb in the near-ultraviolet region, although to such a low extent that usually it is not detected.…”

“…If the transition dipole moment of a Trp responsible for this absorption is oriented along the long axis of the proteins, the absorbance may be visible in the LD spectrum, notwithstanding the fact that it is not detectable in the absorption spectrum. Another explanation might be the presence of N-formylkynurenine, a degradation product of Trp, which absorbs at 337 nm with E = 1900 M-'.cm-' [28]. This might be formed by the presence of radicals during the polymerization of the gel or by photolysis of Trp residues.…”