Preparation and photophysical properties of a caged kynurenine
We have prepared L-kyurenine 4-hydroxyphenacyl ester, a caged derivative of L-kynurenine. Nα-tBOC-L-tryptophan was reacted with 4-hydroxyphenacyl bromide in DMF with K2CO3 as the base to give the Nα-tBOC 4-hydroxyphenacyl ester. The ester was then treated with O3 in MeOH at -20 °C, followed by trifluoroacetic acid in CH2Cl2, then aqueous HCl to obtain the caged kynurenine as the dihydrochloride salt. The caged kynurenine is stable as a dry solid in the dark at -78 °C, but in aqueous solutions in phosphate buffer at pH 7-8 hydrolyzes rapidly (t1/2 ∼ 5 minutes). Solutions in Tris at pH 7 are more stable (t1/2 > 30 minutes), and solutions in 1 mM HCl are stable for several hours. As expected, the ester is cleaved in μseconds with laser pulses at 355 nm. The caged kynurenine may be useful for preparation of substrate complexes for crystallography or in biological studies on kynurenine.