Jockusch R A, Paech K, Williams E R
Department of Chemistry, University of California, Berkeley, California 94720.
J Phys Chem A. 2000 Apr 13;104(14):3188-96. doi: 10.1021/jp993147p.
Photodissociation kinetics of the protonated pentapeptide leucine enkephalin measured using a cw CO(2) laser and a Fourier-transform mass spectrometer are reported. A short induction period, corresponding to the time required to raise the internal energy of the ion population to a (dissociating) steady state, is observed. After this induction period, the dissociation data are accurately fit by first-order kinetics. A plot of the log of the unimolecular dissociation rate constant, k(uni), as a function of the log of laser power is linear at low laser powers (<9 W, k(uni) <0.05 s(-1)), but tapers off at high laser power (9-33 W, k(uni) = 0.05-7 s(-1)). The entire measured dissociation curve can be accurately fit by an exponential function plus a constant. The experiment is simulated using a master equation formalism. In the model, the laser radiation is described as an energetically flat-topped distribution which is spatially uniform. This description is consistent with experimental results which indicate that ion motion within the cell averages out spatial inhomogeneities in the laser light. The model has several adjustable parameters. The effect of varying these parameters on the calculated kinetics and power dependence curves is discussed. A procedure for determining a limited range of threshold dissociation energy, E(o), which fits both the measured induction period and power dependence curves, is presented. Using this procedure, E(o) of leucine enkephalin is determined to be 1.12-1.46 eV. This result is consistent with, although less precise than, values measured previously using blackbody infrared radiative dissociation. Although the blackbody dissociation results were used as a starting point to search for fits of the master equation model to experiment, these results demonstrate that it is, in principle, possible to determine a limited range of E(o) from slow infrared multiphoton dissociation data alone.
报道了使用连续波CO(2)激光和傅里叶变换质谱仪测量质子化五肽亮氨酸脑啡肽的光解离动力学。观察到一个短的诱导期，这对应于将离子群体的内能提高到（解离）稳态所需的时间。在这个诱导期之后，解离数据能被一级动力学准确拟合。单分子解离速率常数k(uni)的对数作为激光功率对数的函数图在低激光功率（<9 W，k(uni)<0.05 s(-1)）时是线性的，但在高激光功率（9 - 33 W，k(uni)=0.05 - 7 s(-1)）时逐渐变缓。整个测量的解离曲线可以通过一个指数函数加一个常数准确拟合。使用主方程形式对实验进行了模拟。在模型中，激光辐射被描述为能量上平顶的分布且在空间上是均匀的。这种描述与实验结果一致，实验结果表明细胞内的离子运动使激光光场中的空间不均匀性平均化。该模型有几个可调参数。讨论了改变这些参数对计算出的动力学和功率依赖曲线的影响。提出了一种确定有限范围的阈值解离能E(o)的方法，该方法能同时拟合测量的诱导期和功率依赖曲线。使用该方法，亮氨酸脑啡肽的E(o)被确定为1.12 - 1.46 eV。这个结果与之前使用黑体红外辐射解离测量的值一致，尽管不如其精确。虽然黑体解离结果被用作寻找主方程模型与实验拟合的起点，但这些结果表明原则上仅从慢红外多光子解离数据确定有限范围的E(o)是可能的。
