Response of villin headpiece-capped gold nanoparticles to ultrafast laser heating.

The integrity of a small model protein, the 36-residue villin headpiece HP36, attached to gold nanoparticles (AuNP) is examined, and its response to laser excitation of the AuNPs is investigated. To that end, it is first verified by stationary IR and CD spectroscopy, together with denaturation experiments, that the folded structure of the protein is fully preserved when attached to the AuNP surface. It is then shown by time-resolved IR spectroscopy that the protein does not unfold, even upon the highest pump fluences that lead to local temperature jumps on the order of 1000 K of the phonon system of the AuNPs, since that temperature jump persists for too short a time of a few nanoseconds only to be destructive. Judged from a blue shift of the amide I band, indicating destabilized or a few broken hydrogen bonds, the protein either swells, becomes more unstructured from the termini, or changes its degree of solvation. In any case, it recovers immediately after the excess energy dissipates into the bulk solvent. The process is entirely reversible for millions of laser shots without any indication of aggregation of the protein or the AuNPs and with only a minor fraction of broken protein-AuNP thiol bonds. The work provides important cornerstones in designing laser pulse parameters for maximal heating with protein-capped AuNPs without destroying the capping layer.

Amyloid-derived peptide forms self-assembled monolayers on gold nanoparticle with a curvature-dependent ß-sheet structure.

Using a combination of Fourier transform infrared (FTIR) spectroscopy and solid-state nuclear magnetic resonance (SSNMR) techniques, the secondary structure of peptides anchored on gold nanoparticles of different sizes is investigated. The structure of the well-studied CALNN-capped nanoparticles is compared to the structure of nanoparticles capped with a new cysteine-terminated peptide, CFGAILSS. The design of that peptide is derived from the minimal amyloidogenic sequence FGAIL of the human islet polypeptide amylin. We demonstrate that CFGAILSS forms extended fibrils in solution. When constrained at a nanoparticle surface, CFGAILSS adopts a secondary structure markedly different from CALNN. Taking into account the surface selection rules, the FTIR spectra of CFGAILSS-capped gold nanoparticles indicate the formation of ß-sheets which are more prominent for 25 nm diameter nanoparticles than for 5 nm nanoparticles. No intermolecular (13)C-(13)C dipolar coupling is detected with rotational resonance SSNMR for CALNN-capped nanoparticles, while CALNN is in a random coil configuration. Coupling is detected for CFGAILSS-capped gold nanoparticles, however, consistent with an intermolecular (13)C-(13)C distance of 5.0 ± 0.3 Å, in agreement with intermolecular hydrogen bonding in a parallel ß-sheet structure.

PEGylation modulates the interfacial kinetics of proteases on peptide-capped gold nanoparticles.

Fluorescence unquenching measurements of protease-dependent release of fluorescent biomolecules from peptide-capped gold nanoparticles reveal the effect of the monolayer composition on enzyme kinetics.