Electron transfer and ligand binding to cytochrome c' immobilized on self-assembled monolayers

We have successfully immobilized Allochromatium vinosum cytochrome c' on carboxylic acid-terminated thiol monolayers on gold and have investigated its electron-transfer and ligand binding properties. Immobilization could only be achieved for pH's ranging from 3.5 to 5.5, reflecting the fact that the protein is only sufficiently pos. charged below pH 5.5 (pI = 4.9). Upon immobilization, the protein retains a near-native conformation, as is suggested by the obsd. potential of 85 mV vs SHE for the heme FeIII/FeII transition, which is close to the value of 60 mV reported in soln. The electron-transfer rate to the immobilized protein depends on the length of the thiol spacer, displaying distance-dependent electron tunneling for long thiols and distance-independent protein reorganization for short thiols. The unique CO-induced dimer-to-monomer transition obsd. for cytochrome c' in soln. also seems to occur for immobilized cytochrome c'. Upon satn. with CO, a new anodic peak corresponding to the oxidn. of an FeII-CO adduct is obsd. CO binding is accompanied by a significant decrease in protein coverage, which could be due to weaker electrostatic interactions between the self-assembled monolayer and cytochrome c' in its monomeric form as compared to those in its dimeric form. The obsd. CO binding rate of 24 M-1 s-1 is slightly slower than the binding rate in soln. (48 M-1 s-1), which could be due to electrostatic protein-electrode interactions or could be the result of protein crowding on the surface. This study shows that the use of carboxyl acid-terminated thiol monolayers as a protein friendly method to immobilize redox proteins on gold electrodes is not restricted to cytochrome c, but can also be used for other proteins such as cytochrome c'

Influence of dsDNA fragment length on particle binding in an evanescent field biosensing system

Particle labels are widely used in affinity-based biosensing due to the high detection signal per label, the high stability, and the convenient biofunctionalization of particles. In this paper we address the question how the time-course of particle binding and the resulting signals depend on the length of captured target molecules. As a model system we used fragments of dsDNA with lengths of 105 bp (36 nm), 290 bp (99 nm) and 590 bp (201 nm), detected in an evanescent-field optomagnetic biosensing system. On both ends the fragments were provided with small-molecule tags to allow binding of the fragments to protein-coated particles and to the capture molecules at the sensor surface. For isolated single particles bound to the surface, we observe that the optical scattering signal per particle depends only weakly on the fragment length, which we attribute to the pivoting motion that allows the particles to get closer to the surface. Our data show a strong influence of the fragment length on the particle binding: the binding rate of particles to the sensor surface is an order of magnitude higher for the longest dsDNA fragments compared to the smallest fragment studied in this paper. We attribute the enhanced binding rate to the length and motional freedom of the fragments. These results generate a new dimension for the design of assays and systems in particle-based biosensing

Variation of linker length in ratiometric fluorescent sensor proteins allows rational tuning of Zn(II) affinity in the picomolar to femtomolar range

Ratiometric fluorescent sensor proteins with a very high and tunable affinity for Zn(II) were created by connecting two fluorescently labeled metal binding domains, CFP-Atox1 and WD4-YFP, using a series of flexible peptide linkers. A simple random-coil model describing the conformational distribution of the linker allowed a quant. understanding of the effect of the linker length on both the change in emission ratio and the Zn(II) affinity