Brookman Jennifer, Chacón Jorge N, Sinclair Roy S
Division of Chemistry and Chemical Engineering, University of Paisley, High Street, Paisley Renfrewshire, Scotland, UK.
Photochem Photobiol Sci. 2002 May;1(5):327-32. doi: 10.1039/b201621d.
Urocanic acid, an important human skin chromophore, undergoes a variety of photochemical transformations when exposed to the near-UV portion of sunlight and natural daylight, the principal reaction being the transformation from the stable trans- or (E)-form of the chromophore (trans-UA) to the biologically active cis- or (Z)-form (cis-UA), which is claimed to induce immunosuppression linked to the onset of skin cancer. This study is concerned with the comparative photophysical behaviour of the two urocanic acid isomers in aqueous solution using both continuous irradiation and pulsed irradiation techniques. The UV absorption maximum for both isomers occurs in the region of 270 nm with the absorption shape varying characteristically with pH, the cis-isomer showing a lower overall molar absorptivity. Both isomers exhibit weak fluorescence (quantum yields estimated to be less than 10(-4)) with each isomer showing small differences in the way in which pH and excitation wavelength influence the fluorescence emission characteristics. Pulsed nanosecond laser irradiation at 266 nm of aqueous solutions at pH 7 shows that both isomers undergo photo-ionisation with a quantum yield of 0.02 for the hydrated electron production, a quantum yield value comparable with that for photoisomerisation at this wavelength. Laser flash studies also show that the photo-ionised species reacts efficiently with oxygen (quenching rate kQ = 1.3 x 10(9) M(-1) s(-1)), while some preliminary experiments indicate that both cis- and trans-urocanic acids react with the semiquinone radical of L-3,4-dihydroxyphenylalanine (L-DOPA) with a fast reaction rate constant of approximately 5 x 10(7) M(-1) s(-1). The photophysical characteristics of trans-UA and cis-UA reported here are discussed in the context of other recent pulsed irradiation studies on urocanic acid over nanosecond and picosecond time scales, in an attempt to clarify the complex photo-behaviour of this interesting biomolecule.
尿刊酸是人体皮肤中的一种重要发色团，当暴露于阳光和自然光的近紫外部分时，会发生多种光化学转变，主要反应是发色团从稳定的反式或（E）-形式（反式-UA）转变为具有生物活性的顺式或（Z）-形式（顺式-UA），据称这种转变会引发与皮肤癌发病相关的免疫抑制。本研究利用连续照射和脉冲照射技术，研究了两种尿刊酸异构体在水溶液中的比较光物理行为。两种异构体的最大紫外吸收峰出现在270 nm区域，吸收形状随pH值有特征性变化，顺式异构体的总体摩尔吸光率较低。两种异构体均表现出弱荧光（量子产率估计小于10^(-4)），每种异构体在pH值和激发波长影响荧光发射特性的方式上存在微小差异。在pH值为7的水溶液中，用266 nm的脉冲纳秒激光照射表明，两种异构体都会发生光电离，产生水合电子的量子产率为0.02，该量子产率值与该波长下的光异构化量子产率相当。激光闪光研究还表明，光离子化物种与氧气反应效率很高（猝灭速率kQ = 1.3×10^(9) M^(-1) s^(-1)），而一些初步实验表明，顺式和反式尿刊酸都与L-3,4-二羟基苯丙氨酸（L-DOPA）的半醌自由基反应，反应速率常数约为5×10^(7) M^(-1) s^(-1)。本文报道的反式-UA和顺式-UA的光物理特性，是在最近其他关于尿刊酸在纳秒和皮秒时间尺度上的脉冲照射研究的背景下进行讨论的，旨在阐明这种有趣生物分子复杂的光行为。
