No abstract

Proteins and peptides play a predominant role in biochemical reactions of living cells. In these complex environments, not only the constitution of the molecules but also their three-dimensional configuration defines their functionality. This so-called secondary structure of proteins is crucial for understanding their function in living matter. Misfolding, for example, is suspected as the cause of neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Ultimately, it is necessary to study a single protein and its folding dynamics. Here, we report a first step in this direction, namely ultrasensitive detection and discrimination of in vitro polypeptide folding and unfolding processes using resonant plasmonic nanoantennas for surface-enhanced vibrational spectroscopy. We utilize poly-l-lysine as a model system which has been functionalized on the gold surface. By in vitro infrared spectroscopy of a single molecular monolayer at the amide I vibrations we directly monitor the reversible conformational changes between α-helix and β-sheet states induced by controlled external chemical stimuli. Our scheme in combination with advanced positioning of the peptides and proteins and more brilliant light sources is highly promising for ultrasensitive in vitro studies down to the single protein level.

Surface-enhanced infrared absorption spectroscopy (SEIRA) is applied to study protein conformational changes. In general, the appropriate functionalization of metal surfaces with biomolecules remains a challenge if the conformation and activity of the biomolecule shall be preserved. Here we present a SEIRA study to monitor pH-induced conformational changes of poly-l-lysine (PLL) covalently bound to a thin gold layer via self-assembled monolayers (SAMs). We demonstrate that the composition of the SAM is crucial. A SAM of 11-mercaptoundecanonic acid (MUA) can link PLL to the gold layer, but pH-driven conformational transitions were hindered compared to poly-l-lysine in solution. To address this problem, we devised a variety of SAMs, i.e., mixed SAMs of MUA with either octanethiol (OT) or 11-mercapto-1-undecanol (MUoL) and furthermore SAMs of MT(PEG)4 and NHS-PEG10k-SH. These mixed SAMs modify the surface properties by changing the polarity and the morphology of the surface present to nearby PLL molecules. Our experiments reveal that mixed SAMs of MUA-MUoL and SAMs of NHS-PEG10k-SH-MT(PEG)4 are suitable to monitor pH-driven conformational changes of immobilized PLL. These SAMs might be applicable for chemoselective protein immobilization in general.

Hexagonally arranged Au nanoparticles exhibiting a broad Gaussian-shaped size distribution ranging from 30 nm to 80 nm were deposited on Si substrates and irradiated with Ar(+) and Ga(+) ions with various energies from 20 to 350 keV and 1 to 30 keV, respectively. The size and energy dependence of the sputter yield were measured using high-resolution scanning electron microscopy image analysis. These results were compared to simulation results obtained by iradina, a Monte Carlo code, which takes the specifics of the nano geometry into account. The experimental sputter yields are significantly higher than simulated sputter yields for both bulk and the nano geometry. The difference can be clearly attributed to thermally driven effects, which significantly increase the measured sputter yields.

To accommodate the high demand for gold nanoparticles, which is generated by the extraordinary optical properties of plasmonic metamaterials, a focus has been placed on their large-scale synthesis for several years. In this work, a simple method for the preparation of nearly monodisperse gold nanoparticles with diameters of up to 80 nm is presented. For this purpose, gold nanoparticles with an average diameter of 9 nm were synthesized in toluene by using oleylamine both as a reducing and stabilizing agent. These gold nanoparticles [a]