QD:Puf Nanohybrids Are Compatible with Studies in Cells.

Colloidal semiconductor quantum dots (QD), as well as other nanoparticles, are useful in cell studies as fluorescent labels. They may also be used as more active components in various cellular assays, serving as sensors or effectors. However, not all QDs are biocompatible. One of the main problems is their outer coat, which needs to be stable and to sustain hydrophilicity. Here we show that purpose-designed CdSe QDs, covered with a Puf protein, can be efficiently accumulated by HeLa cells. The uptake was measurable after a few hours of incubation with nanoparticles and most of the fluorescence was localised in the internal membrane system of the cell, including the endoplasmic reticulum and the Golgi apparatus. The fluorescence properties of QDs were mostly preserved, although the maximum emission wavelength was slightly shifted, and the fluorescence lifetime was shortened, indicating partial sensitivity of the QDs to the cell microenvironment. QD accumulation resulted in a decrease in cell viability, which was attributed to disturbance of endoplasmic reticulum performance.

Quantum dot clusters as self-assembled antennae with phycocyanine and phycobilisomes as energy acceptors.

In this study, we investigated an experimental and Monte-Carlo computational characterization of self-assembled antennae built using CdTe colloidal quantum dots (QDs). These clusters provide efficient excitation of phycocyanine (PC) or phycobilisomes (PBSs). PBSs are light-harvesting complexes (LHCs) of cyanobacteria, made of several PC units, organized in disks and rods. Each PC contains three separate cofactors. Therefore, we analyzed variations in multi-donor and multi-acceptor systems. The self-assembled QD clusters were formed mostly by electrostatic interactions, possibly due to the introduction of a positive charge on an originally negatively charged nanoparticle surface. Our results suggest that PC may accept energy from multiple nanoparticles localized at a distance significantly longer than the Förster radius. The excitation transfers between particular nanoparticles with possible delocalization. The maximal energy transfer efficiency was obtained for the PC/PBS : QD ratio from 1 to 20 depending on the QD size. This cannot be fully explained using computational simulations; hence, we discussed the hypothesis and explained the observations. Our self-assembled systems may be considered for possible applications in artificial light-harvesting systems because absorption spectra of QDs are different from the absorption characteristics of PC/PBS. In addition, huge clusters of QDs may effectively increase the optical cross-section of so-created nanohybrids.

Heterodimerizing helices as tools for nanoscale control of the organization of protein-protein and protein-quantum dots.

In this study, we tested the possibility of creating complexes of two proteins by fusing them with heterodimerizing helices. We used the fluorescent proteins GFP and mCHERRY expressed with a His-tag as our model system. We added heterodimer-forming sequences at the C- or N- termini of the proteins, opposite to the His-tag position. Heterodimerization was tested for both helices at the C-terminus or at the N- terminus and C-terminus. We observed complex formation with a nanomolar dissociation constant in both cases that was higher by one order of magnitude than the Kds measured for helices alone. The binding of two C-terminal helices was accompanied by an increased enthalpy change. The binding between helices could be stabilized by introducing an additional turn of the helix with cysteine, which was capable of forming disulphide bridges. Covalently linked proteins were obtained using this strategy and observed using fluorescence cross-correlation spectroscopy. Finally, we demonstrated the formation of complexes of protein dimers and quantum dots.