Thermodynamic profile for urea photo-release from a N-(2-nitrobenzyl) caged urea compound

Abstract

Photoactivable bioactive molecules, often termed “caged” compounds, have attracted significant attention as useful tools for photo-regulating enzymatic activity. Here we examine the mechanism associated with photo-release of urea from a caged urea compound, N-(2-nitrobenzyl)urea, using photothermal beam deflection and time-resolved absorption spectroscopy. Photodissociation of the caged urea results in the prompt formation of an aci-nitro intermediate that decays to nitrosobenzaldehyde by releasing urea with the rate constant of 4.5 × 10³ s⁻¹. Appearance of the aci-nitro intermediate is associated with a volume contraction of −13 ± 1 mL mol⁻¹ and a negligible change in enthalpy (ΔH = 6 ± 4 kcal mol⁻¹). On the microsecond time-scale, the conversion of the aci-nitro intermediate and concomitant release of urea result in a volume expansion of 6 ± 2 mL mol⁻¹ and a negative enthalpy change of −25 ± 5 kcal mol⁻¹. No additional processes were observed on the timescale up to 100 ms suggesting that the breakdown of the aci-nitro intermediate is the rate-limiting step for urea photo-release. These results suggest a similar mechanism for caged urea photo release as determined previously for the caged ATP compound.