Crovetto Luis, Martínez-Junza Víctor, Braslavsky Silvia E
Max-Planck-Institut für Bioanorganische Chemie (formerly Strahlenchemie), Mülheim an der Ruhr, Germany.
Photochem Photobiol. 2006 Jan-Feb;82(1):281-90. doi: 10.1562/2005-03-22-RA-468.
The thermodynamic parameters for the formation of the free radicals upon electron transfer quenching of the flavin triplet state (3FMN) by tryptophan and tyrosine, Delta(FR)H and Delta(FR)V, were obtained in aqueous solution by the application of laser-induced optoacoustic spectroscopy at various temperatures. The Delta(FR)H and Delta(FR)V values include the electron transfer and charge separation steps plus the protonation of the FMN anion radical and the deprotonation of the amino-acid cation radical. A linear correlation was found between the Delta(FR)H and Delta(FR)V values for each of the amino acids in phosphate buffers of [CH3(CH2)3]4N+, Li+, NH4+, K+ and Cs+. The compensation between Delta(FR)H and Delta(FR)V within the salt series, and the independent evaluation of the Gibbs energy for electron transfer Delta(ET)G(o) afforded the entropy change, Delta(FR)S, for the reaction, different for the two amino acids. The values of Delta(FR)H, Delta(FR)V and Delta(FR)S in each buffer are mainly determined by the changes in strength and probably number of hydrogen bonds between the reacting partners and water produced along all steps leading to the radicals FMNH* and A*. The Delta(FR)V values linearly correlate with the tabulated entropy of organization of the water structure for the five cations, DeltaS(o)(cat). The entropy change upon formation of the free radicals, Delta(FR)S, quantitatively correlated to the Delta(FR)V value, drives the separation of the ion pair after the electron transfer reaction in the case of highly organizing cations. The ratio X = T Delta(FR)S/Delta(FR)V = (55 +/- 9) kJ cm(-3) for Trp as 3FMN quencher is smaller than X = (83 +/- 9) kJ cm(-3) for Tyr as quencher. These values are discussed in conjunction with the Marcus reorganization energy, as calculated from the Gibbs activation energy of the electron transfer process, which is independent of the salt present but different for each of the two quenchers.
通过在不同温度下应用激光诱导光声光谱法，在水溶液中获得了色氨酸和酪氨酸对黄素三重态（3FMN）进行电子转移猝灭形成自由基时的热力学参数Δ(FR)H和Δ(FR)V。Δ(FR)H和Δ(FR)V值包括电子转移和电荷分离步骤，以及FMN阴离子自由基的质子化和氨基酸阳离子自由基的去质子化。在[CH3(CH2)3]4N+、Li+、NH4+、K+和Cs+的磷酸盐缓冲液中，发现每种氨基酸的Δ(FR)H和Δ(FR)V值之间存在线性相关性。盐系列中Δ(FR)H和Δ(FR)V之间的补偿，以及电子转移吉布斯自由能Δ(ET)G(o)的独立评估，得出了反应的熵变Δ(FR)S，两种氨基酸的该值不同。每个缓冲液中Δ(FR)H、Δ(FR)V和Δ(FR)S的值主要由反应伙伴之间以及沿所有导致自由基FMNH和A的步骤产生的水之间氢键强度的变化以及可能的数量变化决定。Δ(FR)V值与五种阳离子的水结构组织的列表熵ΔS(o)(cat)呈线性相关。自由基形成时的熵变Δ(FR)S与Δ(FR)V值定量相关，在高度有序的阳离子情况下，驱动电子转移反应后离子对的分离。对于作为猝灭剂的色氨酸，X = TΔ(FR)S/Δ(FR)V = (55 ± 9) kJ cm(-3)，小于作为猝灭剂的酪氨酸的X = (83 ± 9) kJ cm(-3)。结合根据电子转移过程的吉布斯活化能计算得出的马库斯重组能对这些值进行了讨论，该吉布斯活化能与存在的盐无关，但两种猝灭剂各自不同。
