Abstract


                                  p-Hydroxyphenacylphenylacetate:

                            Can the Release Quantum Yield be Increased?


   Jamaludin Al Anshori, Ľubica Klíčová, Tomáš Šolomek, Peter Šebej, Dominik Heger and Petr Klán



     Photoremovable protecting groups (PPGs) are a key tool for investigation of biochemical
processes in living cells due to their ability to release the substrates in time and space of
intererest.^1,2

     p-Hydroxyphenacyl moiety has been proved to be an efficient photoremovable protecting group
(PPG) for various substrates, for example, halides, phenols and acids. The mechanism of their
photodeprotection involves a rapid ISC to the triplet state followed by a subsequent release of the
substrate and concomitant formation of a triplet biradical that closes to the putative spirodione
intermediate. Hydrolysis of the putative spirodione intermediate formed p-hydroxyphenylacetic
acid.^3 The release of good leaving groups such as diethyl phosphate, tosylate and mesylate, was
efficient with high quantum yields (~1) and ultrafast release rates (k ~ 10^10 s^-1). However, the
quantum yield drops significantly for poor leaving group (~ 0.05 for phenol). An adiabatic triplet
state tautomerization^4 might represent the energy wasting paths decreasing the quantum yield of
the pHP PPG.

     In the present work, we studied how the concentration of a general base and pH (acetate buffer
at pH 3.6 and 6) affects the overall disappearance quantum yields of pHP phosphate. Five different
phenyl ring substituted pHP-PA derivatives have also been synthesized to investigate the effects of
phenyl ring substituents to the overall quantum yields. The results were correlated with those of
laser flash photolysis and quantum chemical calculations.

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^1  Wirz J., Photochem. Photobiol. Sci., 2012, 11, 445

^2  Klán P., Šolomek T., Bochet C., Blanc A., Givens R. S., Rubina M., Popik V., Kostikov A., Wirz
J. Chem. Rev. 2013, 113, 119–191.

^3 Givens R. S., Heger D., Wirz J,  J. Am. Chem. Soc. 2008, 130, 3307, and references therein.

^4 Klíčova L, Šebej P, Šolomek T, Hellrung B, Slavíček P, Klán P, Heger D,  and Wirz J., J.
Phys. Chem. A 2012, 116, 2935–2944