Proteins achieve efficient energy storage and conversion through electron transfer along a series of redox cofactors. Multiheme cytochromes are notable examples. These proteins transfer electrons over distance scales of several nanometers to >10 μm and in so doing they couple cellular metabolism with extracellular redox partners including electrodes. Here, we report pump-probe spectroscopy that provides a direct measure of the intrinsic rates of heme–heme electron transfer in this fascinating class of proteins. Our study took advantage of a spectrally unique His/Met-ligated heme introduced at a defined site within the decaheme extracellular MtrC protein of Shewanella oneidensis. We observed rates of heme-to-heme electron transfer on the order of 10⁹ s⁻¹ (3.7 to 4.3 Å edge-to-edge distance), in good agreement with predictions based on density functional and molecular dynamics calculations. These rates are among the highest reported for ground-state electron transfer in biology. Yet, some fall 2 to 3 orders of magnitude below the Moser–Dutton ruler because electron transfer at these short distances is through space and therefore associated with a higher tunneling barrier than the through-protein tunneling scenario that is usual at longer distances. Moreover, we show that the His/Met-ligated heme creates an electron sink that stabilizes the charge separated state on the 100-μs time scale. This feature could be exploited in future designs of multiheme cytochromes as components of versatile photosynthetic biohybrid assemblies.

蛋白质通过沿着一系列氧化还原辅因子的电子转移实现有效的能量储存和转换。多血红素细胞色素是值得注意的例子。这些蛋白质在几纳米到 > 10 μm 的距离范围内传输电子，因此它们将细胞代谢与包括电极在内的细胞外氧化还原伙伴结合起来。在这里，我们报告了泵-探针光谱，它提供了对这一类迷人蛋白质中血红素-血红素电子转移的固有速率的直接测量。我们的研究利用了在Shewanella oneidensis的十血红素细胞外 MtrC 蛋白内的特定位点引入的光谱独特的 His/Met 连接血红素。我们观察到血红素到血红素的电子转移速率约为 10 ⁹ s ^-1（3.7 到 4.3 Å 边到边距离），与基于密度泛函和分子动力学计算的预测非常一致。这些速率是生物学中基态电子转移报告的最高速率之一。然而，有些比 Moser-Dutton 标尺低 2 到 3 个数量级，因为在这些短距离上的电子转移是通过空间的，因此与通常在更长距离上的通过蛋白质隧道方案相比，与更高的隧道势垒相关。此外，我们展示了 His/Met 连接的血红素产生了一个电子汇，可以在 100 微秒的时间尺度上稳定电荷分离状态。这一特征可以在多血红素细胞色素的未来设计中被利用，作为多功能光合生物杂交组件的组成部分。