Effect of the electric field generated by the helix dipole on photoinduced intramolecular electron transfer in dichromophoric alpha-helical peptides
Effect of the electric field generated by the helix dipole on photoinduced intramolecular electron transfer in dichromophoric alpha-helical peptides
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DOI：
10.1021/ja951555a
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发表时间：
1996-03-06
影响因子：
15
通讯作者：
Fox, MA
中科院分区：
文献类型：
--
作者：
Galoppini, E;Fox, MA
在自然光合作用中，嵌入蛋白质基质中的供体-受体对之间的长程电子转移在长距离 (> 10 Å) 内快速发生。 1 据推测，与光合反应中心周围蛋白质 R 螺旋部分的永久偶极子相关的电场影响初级电子转移事件的速率。 2a R 螺旋的偶极子（每个氨基酸残基约 3.5 D）沿螺旋轴产生 109 V/m 的静电场，2 在氨基端产生有效正电荷，在羧基端产生有效负电荷，每个大小为 0.8×10-19 C。 2b,c 这种强电场在蛋白质的结构和功能中起着重要作用。 2a, 3 在这项工作中，我们研究了探针发色团相对于螺旋产生的电场方向的位置对分子内电子转移反应速率的影响。具有悬垂电子供体 (D) 和受体 (A) 发色团的螺旋寡肽 1 和 2（图 1）的区别仅在于供体-受体 (DA) 对沿偶极螺旋的位置反转。 D 和 A 之间的光致电子转移生成电荷分离对 (D•+ A•-)，其方向与 2 中的内部电场相反，与 1 中的电场相反。在存在电场的情况下，电荷分离供体受体对的势能较高或较低，具体取决于该场方向。因此，当中性前体之间的电子转移产生D•+ A•-时，ΔG以及因此的电子转移速率取决于该定位。即当D•+ A•-逆场时，驱动力较大，速率较快（若ΔG在正常范围内）。 4, 5 因此，我们预计 1 中电场与光诱导电子转移方向的排列应会导致 1 中的电子转移速率比 2 中更快。肽 1 和 2 通过标准溶液相肽偶联反应合成，6 寡肽主链由 L-丙氨酸 (Ala) 和氨基异丁酸 (Aib) 制备。 7, 8 供体（N,N-二甲基苯胺）和受体（芘）作为 L-丙氨酸残基上的取代基并入主链，六个残基分隔发色团。由于解折叠经常发生在肽末端，因此 D 和 A 通过三个残基与末端分开，以固定它们的方向。还制备了仅包含供体 (3) 或受体 (4) 的两种参考肽用于对照实验。为了定义 1 和 2 中附加的 D 和 A 之间的距离和相对角方向，有必要建立主链的构象。通过分析它们在乙腈和甲醇中的圆二色光谱，证实了1和2的右手螺旋构象，它们各自在190 nm处显示出强正带，在210和220 nm附近显示出两个负带。 9 在两种溶剂中，1 和 2 的 CD 光谱相同，这表明在实验的时间尺度上，这两种肽具有非常相似的二级结构。为了区分 R 或 310 螺旋构象，分析了 1 和 2 的 2D 1H NMR 谱（COSY、NOESY、ROESY）。观察到两种 NOE 相互作用，CH (i)-RCH (i+ 3) 和 NH (i)-NH (i+ 3)，它们是 R 螺旋的特征，在 310 螺旋中观察不到。 10, 11 我们得出结论，1 和 2 的优先构象是 R 螺旋，并且两个发色团彼此大致平行并且与垂直于螺旋轴的平面相距约 10 Å。 12 在 R 螺旋中，六个残基对应于两匝线圈。这 …
In natural photosynthesis, long-range electron transfer between donor-acceptor pairs embedded within a protein matrix occurs rapidly over long (> 10 Å) distances. 1 It has been postulated that the electric field associated with the permanent dipole of R-helical sections of the proteins surrounding the photosynthetic reaction center influences the rate of the primary electron transfer event. 2a The dipole of an R-helix (about 3.5 D per amino acid residue) generates an electrostatic field along the helix axis of 109 V/m, 2 producing an effective positive charge at the amino end and an effective negative charge at the carboxyl end, each of magnitude 0.8× 10-19 C. 2b, c This strong electric field plays an important role in the structure and function of proteins. 2a, 3 In this work, we have investigated the effect of the position of probe chromophores relative to the direction of the electric field generated by the helix on the rate of intramolecular electron transfer reactions. Helical oligopeptides 1 and 2 with pendant electron donor (D) and acceptor (A) chromophores (Figure 1) differ only by the positional reversal of the donor-acceptor (DA) pair along the dipolar helix. Photoinduced electron transfer between D and A generates a charge-separated pair (D•+ A•-) which is oriented with the internal electric field in 2 and against the field in 1. The potential energy of a charge-separated donoracceptor pair in the presence of an electric field is higher or lower depending on this field orientation. Thus, when D•+ A•-is generated by electron transfer between neutral precursors, ΔG and, therefore, the electron transfer rate depend on this positioning. Namely, the driving force will be larger and the rate faster (if ΔG lies in the normal region) when D•+ A•-is against the field. 4, 5 Hence, we anticipate that the alignment of electric field in 1 against the direction of photoinduced electron transfer should induce a faster rate of electron transfer in 1 than in 2. Peptides 1 and 2 were synthesized by standard solution-phase peptide coupling reactions, 6 the oligopeptide backbone having been prepared from L-alanine (Ala) and aminoisobutyric acid (Aib). 7, 8 The donor (N, N-dimethylaniline) and the acceptor (pyrene) were incorporated into the backbone as substituents on L-alanine residues, and six residues separate the chromophores. As unfolding often occurs at peptide termini, D and A were separated from the terminus by three residues in order to fix their orientation. Two reference peptides, containing only the donor (3) or the acceptor (4), were also prepared for control experiments.To define the distance and the relative angular orientation between the appended D and A in 1 and 2, it was necessary to establish the conformation of the backbones. The right-handed helical conformation of 1 and 2 was confirmed by analysis of their CD spectra in acetonitrile and in methanol, each of which showed a strong positive band at 190 nm and two negative bands near 210 and 220 nm. 9 In both solvents, the CD spectra of 1 and 2 were identical, suggesting that, on the time scale of the experiments, the two peptides have a very similar secondary structure. To distinguish between an R-or 310-helical conformation, 2D 1H NMR spectra (COSY, NOESY, ROESY) of 1 and 2 were analyzed. Two kinds of NOE interactions were observed, CH (i)-RCH (i+ 3) and NH (i)-NH (i+ 3), which are characteristic of an R-helix and not observable in 310-helices. 10, 11 We conclude that the preferential conformation of 1 and 2 is R-helical and that the two chromophores, roughly parallel to one another and to a plane perpendicular to the helix axis, are∼ 10 Å apart. 12 In an R-helix, six residues correspond to two turns of coil. The …