Abstract
Colloidal quantum dots (QDs) are nanoparticles that are able to photoreduce redox proteins by electron transfer (ET). QDs are also able to transfer energy by resonance energy transfer (RET). Here, we address the question of the competition between these two routes of QDs’ excitation quenching, using cadmium telluride QDs and cytochrome c (CytC) or its metal-substituted derivatives. We used both oxidized and reduced versions of native CytC, as well as fluorescent, nonreducible Zn(II)CytC, Sn(II)CytC, and metal-free porphyrin CytC. We found that all of the CytC versions quench QD fluorescence, although the interaction may be described differently in terms of static and dynamic quenching. QDs may be quenchers of fluorescent CytC derivatives, with significant differences in effectiveness depending on QD size. SnCytC and porphyrin CytC increased the rate of Fe(III)CytC photoreduction, and Fe(II)CytC slightly decreased the rate and ZnCytC presence significantly decreased the rate and final level of reduced FeCytC. These might be partially explained by the tendency to form a stable complex between protein and QDs, which promoted RET and collisional quenching. Our findings show that there is a net preference for photoinduced ET over other ways of energy transfer, at least partially, due to a lack of donors, regenerating a hole at QDs and leading to irreversibility of ET events. There may also be a common part of pathways leading to photoinduced ET and RET. The nature of synergistic action observed in some cases allows the hypothesis that RET may be an additional way to power up the ET.
Highlights
In nature, electron transfer (ET)plays an important role in cellular processes
Biosensors with quantum dots (QDs) are mostly based on Förster resonance energy transfer (FRET) between them and fluorescent dyes.[3−5] In some such assays, ET between redox-active molecules and QD is responsible for analyte detection
The results revealed that the quenching mechanism and its efficiency depend on both the metal occupancy of cytochrome c (CytC) and the size of the QDs
Summary
Plays an important role in cellular processes. When some artificial particles, such as colloidal quantum dots (QDs), are introduced in natural systems, they may compete with natural electron donors and acceptors. A low level of CytC photoreduction under pulse illumination related to the bright emission of QDs (Figure S4A, fluorescence of QD630 with ∼100 ns time span) presented a substantial obstacle for the observation of spectral changes during the process of FeCytC reduction (Figure S4B, 416 nm wavelength chosen for the expected photoinduced absorption related to a 410−416 nm shift of the FeCytC Soret peak). As previously shown by gel filtration and the dynamic light scattering method, native Fe(III)CytC does not form stable complexes with CdTe QDs in a phosphate buffer.[12] It was not clear if CytC derivatives would share a similar behavior because metal substitution may change the overall charge distribution of the protein. We were still able to obtain data to help answer our main questions
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