Auramine‑O as a Fluorescence Marker for the Detection of Amyloid Fibrils

There is an indispensable need for a fluorescence marker for the detection of amyloid fibrils, where, at present, the most used marker is thioflavin-T (ThT). Here, we present the use of auramine-O (AuO) as a possible alternative to ThT. As with ThT, the increase in the emission of AuO upon binding to amyloid fibrils is the result of inhibition of the free rotation of the two dimethylamino arms of the molecule. This inhibition prevents the excited-state electronic wave function from moving from the emissive locally excited state to the dark charge-transfer state. We further show that not only AuO is comparable to ThT as a fluorescent marker for amyloid fibrils but also it has a unique spectroscopic signature. AuO has distinct two modes that are characterized by a large shift in the absorption and emission peak positions between its unbound and bound states (before and after the fibrils formation, respectively). In this context, we show that, whereas the emission band position is red-shifting, the absorption peak shifts to the blue and the spectrum exhibits an isosbestic point. The large shifts in emission and absorption peak positions can be explained by the photoacid activity of AuO exhibiting an excited-state proton-transfer process

Electron Transport via Cytochrome C on Si–H Surfaces: Roles of Fe and Heme

Monolayers of the redox protein Cytochrome C (CytC) can be electrostatically formed on an H-terminated Si substrate, if the protein- and Si-surface are prepared so as to carry opposite charges. With such monolayers we study electron transport (ETp) via CytC, using a solid-state approach with macroscopic electrodes. We have revealed that currents via holo-CytC are almost 3 orders of magnitude higher than via the heme-depleted protein (→ apo-CytC). This large difference in currents is attributed to loss of the proteins’ secondary structure upon heme removal. While removal of only the Fe ion (→ porphyrin-CytC) does not significantly change the currents via this protein at room temperature, the 30–335 K temperature dependence suggests opening of a new ETp pathway, which dominates at high temperatures (>285 K). These results suggest that the cofactor plays a major role in determining the ETp pathway(s) within CytC

Doping Human Serum Albumin with Retinoate Markedly Enhances Electron Transport across the Protein

Electrons can migrate via proteins over distances that are considered long for nonconjugated systems. The nanoscale dimensions of proteins and their enormous structural and chemical flexibility makes them fascinating subjects for exploring their electron transport (ETp) capacity. One particularly attractive direction is that of tuning their ETp efficiency by “doping” them with small molecules. Here we report that binding of retinoate (RA) to human serum albumin (HSA) increases the solid-state electronic conductance of a monolayer of the protein by >2 orders of magnitude for RA/HSA ≥ 3. Temperature-dependent ETp measurements show the following with increasing RA/HSA: (a) The temperature-independent current magnitude of the low-temperature (<190 K) regime increases significantly (>300-fold), suggesting a decrease in the distance-decay constant of the process. (b) The activation energy of the thermally activated regime (>190 K) decreases from 220 meV (RA/HSA = 0) to 70 meV (RA/HSA ≥ 3)