Lai Yin-Hung, Chen Bo-Gaun, Lee Yuan Tseh, Wang Yi-Sheng, Lin Sheng Hsien
Genomics Research Center, Academia Sinica, Taipei, 115, Taiwan.
Rapid Commun Mass Spectrom. 2014 Aug 15;28(15):1716-22. doi: 10.1002/rcm.6952.
Although several reaction models have been proposed in the literature to explain matrix-assisted laser desorption/ionization (MALDI), further study is still necessary to explore the important ionization pathways that occur under the high-temperature environment of MALDI. 2,4,6-Trihydroxyacetophenone (THAP) is an ideal compound for evaluating the contribution of thermal energy to an initial reaction with minimum side reactions.
Desorbed neutral THAP and ions were measured using a crossed-molecular beam machine and commercial MALDI-TOF instrument, respectively. A quantitative model incorporating an Arrhenius-type desorption rate derived from transition state theory was proposed. Reaction enthalpy was calculated using GAUSSIAN 03 software with dielectric effect. Additional evidence of thermal-induced proton disproportionation was given by the indirect ionization of THAP embedded in excess fullerene molecules excited by a 450 nm laser.
The quantitative model predicted that proton disproportionation of THAP would be achieved by thermal energy converted from a commonly used single UV laser photon. The dielectric effect reduced the reaction Gibbs free energy considerably even when the dielectric constant was reduced under high-temperature MALDI conditions. With minimum fitting parameters, observations of pure THAP and THAP mixed with fullerene both agreed with predictions.
Proton disproportionation of solid THAP was energetically favorable with a single UV laser photon. The quantitative model revealed an important initial ionization pathway induced by the abrupt heating of matrix crystals. In the matrix crystals, the dielectric effect reduced reaction Gibbs free energy under typical MALDI conditions. The result suggested that thermal energy plays an important role in the initial ionization reaction of THAP.
尽管文献中已经提出了几种反应模型来解释基质辅助激光解吸/电离（MALDI），但仍需要进一步研究以探索在MALDI的高温环境下发生的重要电离途径。2,4,6-三羟基苯乙酮（THAP）是一种理想的化合物，可用于在最小副反应的情况下评估热能对初始反应的贡献。
分别使用交叉分子束装置和商用MALDI-TOF仪器测量解吸的中性THAP和离子。提出了一个包含从过渡态理论导出的阿仑尼乌斯型解吸速率的定量模型。使用具有介电效应的GAUSSIAN 03软件计算反应焓。嵌入过量富勒烯分子中的THAP在450nm激光激发下的间接电离给出了热诱导质子歧化的额外证据。
定量模型预测，THAP的质子歧化将通过从常用的单个紫外激光光子转换而来的热能来实现。即使在高温MALDI条件下介电常数降低时，介电效应也会大大降低反应吉布斯自由能。在最小拟合参数的情况下，纯THAP和与富勒烯混合的THAP的观测结果均与预测结果一致。
固态THAP的质子歧化在单个紫外激光光子作用下在能量上是有利的。定量模型揭示了基质晶体突然加热引起的重要初始电离途径。在基质晶体中，介电效应在典型的MALDI条件下降低了反应吉布斯自由能。结果表明，热能在THAP的初始电离反应中起重要作用。
