Lhiaubet-Vallet Virginie, Belmadoui Noureddine, Climent Maria J, Miranda Miguel A
Instituto de Tecnología Química UPV-CSIC and Departamento de Química, Universidad Politécnica de Valencia, Avda de Los Naranjos s/n, 46022 Valencia, Spain.
J Phys Chem B. 2007 Jul 19;111(28):8277-82. doi: 10.1021/jp071524p. Epub 2007 Jun 23.
The aim of the present work was to find a ketoprofen (KP) equivalent suitable for time-resolved studies on the interactions of its KP-like triplet state with biomolecules or their simple building blocks, under physiologically relevant conditions. Such a compound should fulfill the following requirements: (i) it should be soluble in aqueous media; (ii) its triplet lifetime should be longer than that of KP, ideally in the microsecond range; and (iii) its photodecarboxylation should be slow enough to avoid interference in the time-resolved studies associated with formation of photoproducts. Here, the glycine derivative of ketoprofen (KPGly) has been found to fulfill all the above requirements. In a first stage, the attention has been focused on the photophysical and photochemical properties of KPGly, and then on its excited-state interactions with key amino acids and nucleosides. In acetonitrile, the typical benzophenone-like triplet-triplet absorption (3KPGly) with lambda(max) at 520 nm and a lifetime of 5.3 micros was observed. This value is very close to that of 3KP (5.6 micros) obtained under the same conditions. In methanol, the 3KPGly features were also close to those of 3KP with detection of a short-lived triplet state that evolves to give a ketyl radical. By contrast with the behavior of KP, in deaerated aqueous solutions at pH = 7.4, the transient detected in the case of KPGly displayed two bands at lambda(max) at 330 and 520 nm, very similar to those observed in acetonitrile solution but with a lifetime of 7.5 micros at 520 nm. Hence, it was assigned to the KPGly triplet. In the case of KP, efficient decarboxylation occurs in the subnanosecond time scale, via intramolecular electron transfer. This process gives rise to a detectable carbanion intermediate (lifetime approximately 250 ns) and prevents detection of the shorter-lived 3KP signal. In a second stage, the attention has been focused on the excited-state interactions between 3KPGly and amino acids or nucleosides; for this purpose, 2'-deoxyguanosine (dGuo), thymidine (Thd), tryptophan (Trp), and tyrosine (Tyr) have been chosen as photosensitization targets. In general, efficient quenching (rate constant kq > 109 M(-1) x s(-1)) was observed; it was attributed for dGuo, Tyr, and Trp to a photochemical reaction involving initial electron transfer from the biological target to 3KPGly, followed by proton transfer from the amino acid or the nucleoside radical cation to KPGly-*. As a matter of fact, ketyl radical together with guanosinyl, tyrosinyl, or tryptophanyl radicals were detected; this supports the proposed mechanism. The results with Thd were somewhat different, as the efficient 3KPGly quenching was ascribed to oxetane formation by a Paterno Büchi photocycloaddition.
本研究的目的是找到一种酮洛芬（KP）类似物，适用于在生理相关条件下对其类KP三重态与生物分子或其简单结构单元相互作用进行时间分辨研究。这样的化合物应满足以下要求：（i）它应可溶于水性介质；（ii）其三重态寿命应长于KP，理想情况下在微秒范围内；（iii）其光脱羧反应应足够慢，以避免干扰与光产物形成相关的时间分辨研究。在此，已发现酮洛芬的甘氨酸衍生物（KPGly）满足上述所有要求。在第一阶段，注意力集中在KPGly的光物理和光化学性质上，然后是其与关键氨基酸和核苷的激发态相互作用。在乙腈中，观察到典型的二苯甲酮类三重态-三重态吸收（³KPGly），λmax在520 nm，寿命为5.3微秒。该值与在相同条件下获得的³KP（5.6微秒）非常接近。在甲醇中，³KPGly的特征也与³KP相似，检测到一个短寿命的三重态，其演变成一个酮基自由基。与KP的行为相反，在pH = 7.4的脱气水溶液中，KPGly情况下检测到的瞬态在λmax处显示出330和520 nm的两个谱带，与在乙腈溶液中观察到的非常相似，但在520 nm处的寿命为7.5微秒。因此，它被归为KPGly三重态。在KP的情况下，通过分子内电子转移在亚纳秒时间尺度内发生有效的脱羧反应。这个过程产生了一个可检测的碳负离子中间体（寿命约250纳秒），并阻止了较短寿命的³KP信号的检测。在第二阶段，注意力集中在³KPGly与氨基酸或核苷之间的激发态相互作用上；为此，选择了2'-脱氧鸟苷（dGuo）、胸腺嘧啶核苷（Thd）、色氨酸（Trp）和酪氨酸（Tyr）作为光敏化目标。一般来说，观察到了有效的猝灭（速率常数kq > 10⁹ M⁻¹·s⁻¹）；对于dGuo、Tyr和Trp，这归因于一种光化学反应，该反应涉及从生物目标到³KPGly的初始电子转移，随后是从氨基酸或核苷自由基阳离子到KPGly⁻*的质子转移。事实上，检测到了酮基自由基以及鸟苷基、酪氨酸基或色氨酸基自由基；这支持了所提出的机制。Thd的结果有所不同，因为³KPGly的有效猝灭归因于通过Paterno Büchi光环加成形成氧杂环丁烷。
