Brancaleon L, Gasparini G, Manfredi M, Mazzini A
Department of Physics, University of Parma, Italy.
Arch Biochem Biophys. 1997 Dec 1;348(1):125-33. doi: 10.1006/abbi.1997.0368.
In the range of temperature 10-35 degrees C, Trp-X dipeptides show an unusual increase of fluorescence intensity in solution at pH 7. This effect has been recently studied by means of steady-state fluorescence. Although a model involving the deprotonation at the ground state of the zwitterion was proposed, the activation energy for that process could not rule out the involvement of excited state. In order to understand the mechanism of the thermal-induced increase of fluorescence, we present here time-resolved fluorescence experiments on Trp-X and X-Trp dipeptides at different pH and excitation wave-length. The fluorescence lifetimes (tau i) decrease in accord to thermal quenching, with activation energies (Ei) ranging from 4.0 to 6.4 kcal/mol. Under those circumstances where the anomaly was detected the preexponential factors of the longer-lived component increased as well as their fractional fluorescence. This component can be assigned to the anion species. Because of its larger (three- to fourfold) fluorescence quantum yield, compared to that of the corresponding zwitterion, the large increase of the concentration of the anion leads to an increase of the overall emission despite the thermal quenching. Also the decay-associated spectra well account for the red shift of the emission fluorescence spectrum, which accompanies the anomaly. Our model well fits the experimental data using a simple equation which combines Van't Hoff and Arrhenius equations; it also explains the presence of the anomalous thermal quenching exclusively in Trp-X dipeptides excited above 290 nm and at pH around neutrality.
在10 - 35摄氏度的温度范围内，色氨酸-X二肽在pH值为7的溶液中荧光强度呈现异常增加。最近通过稳态荧光对这种效应进行了研究。尽管提出了一个涉及两性离子基态去质子化的模型，但该过程的活化能并不能排除激发态的参与。为了理解热诱导荧光增加的机制，我们在此展示了在不同pH值和激发波长下对色氨酸-X和X-色氨酸二肽进行的时间分辨荧光实验。荧光寿命（τi）根据热猝灭而降低，活化能（Ei）范围为4.0至6.4千卡/摩尔。在检测到异常的情况下，寿命较长组分的预指数因子以及它们的荧光分数也会增加。该组分可归因于阴离子物种。由于与相应两性离子相比，其荧光量子产率大（三到四倍），尽管存在热猝灭，阴离子浓度的大幅增加仍导致总发射增加。此外，衰变相关光谱也很好地解释了伴随异常出现的发射荧光光谱的红移。我们的模型使用一个结合了范特霍夫方程和阿伦尼乌斯方程的简单方程很好地拟合了实验数据；它还解释了仅在290纳米以上激发且pH值接近中性的色氨酸-X二肽中存在异常热猝灭的现象。
