ABSTRACT
The imidazole ring (Im) of histidine side chains plays a unique role in the function of proteins through covalent bonding with metal ions and hydrogen bonding interactions with adjusted biomolecules and water. At biological interfaces, these interactions are modified because of the presence of an electric field. Self-assembled monolayers (SAMs) with the functional Im group mimic the histidine side chain at electrified interfaces. In this study, we applied in-situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) to probe the structure and hydrogen bonding of Im-functionalized SAM on smooth Au at the electrochemical interface. The self-assembly of molecules on the Au induced the proton shift from N1 atom (Tautomer-I), which is the dominant form of Im in the bulk sample, to N3 atom (Tautomer-II). The impact of electrode potential on the hydrogen bonding interaction strength of the Im ring was identified by SHINERS. Temperature-Raman measurements and density functional theory (DFT) analysis revealed the spectral marker for Im ring packing (mode near 1496-1480 cm-1) that allowed us to associate the confined and strongly hydrogen bonded interfacial Im groups with electrode polarization at -0.8 V. Reflection adsorption IR (RAIR) spectra of SAMs with and without Im revealed that the bulky ring prevented the formation of a strongly hydrogen bonded amide group network.
Subject(s)
Gold , Nanoparticles , Amides , Electrodes , Gold/chemistry , Histidine/chemistry , Hydrogen , Imidazoles , Protons , Spectrum Analysis, Raman/methods , WaterABSTRACT
Graphene research and technology development requires to reveal adsorption processes and understand how the defects change the physicochemical properties of the graphene-based systems. In this study, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and graphene-enhanced Raman spectroscopy (GERS) coupled with density functional theory (DFT) modeling were applied for probing the structure of riboflavin adsorbed on single-layer graphene substrate grown on copper. Intense and detailed vibrational signatures of the adsorbed riboflavin were revealed by SHINERS method. Based on DFT modeling and detected downshift of prominent riboflavin band at 1349 cm-1 comparing with the solution Raman spectrum, π-stacking interaction between the adsorbate and graphene was confirmed. Different spectral patterns from graphene-riboflavin surface were revealed by SHINERS and GERS techniques. Contrary to GERS method, SHINERS spectra revealed not only ring stretching bands but also vibrational features associated with ribityl group of riboflavin and D-band of graphene. Based on DFT modeling it was suggested that activation of D-band took place due to riboflavin induced tilt and distortion of graphene plane. The ability to explore local perturbations by the SHINERS method was highlighted. We demonstrated that SHINERS spectroscopy has a great potential to probe adsorbed molecules at graphene.
ABSTRACT
Studying the biochemistry of yeast cells has enabled scientists to understand many essential cellular processes in human cells. Further development of biotechnological and medical progress requires revealing surface chemistry in living cells by using a non-destructive and molecular structure sensitive technique. In this study shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was applied for probing the molecular structure of Metschnikowia pulcherrima yeast cells. Important function of studied cells is the ability to eliminate iron from growth media by precipitating the insoluble pigment pulcherrimin. Comparative SERS and SHINERS analysis of the yeast cells in combination with bare Au and shell-isolated Au@SiO2 nanoparticles were performed. It was observed that additional bands, such as adenine ring-related vibrational modes appear due to interaction with bare Au nanoparticles; the registered spectra do not coincide with the spectra where Au@SiO2 nanoparticles were used. SHINERS spectra of M. pulcherrima were significantly enhanced comparing to the Raman spectra. Based on first-principles calculations and 830-nm excited Raman analysis of pulcherrimin, the SHINERS signatures of iron pigment in yeast cells were revealed. Being protected from direct interaction of metal with adsorbate, Au@SiO2 nanoparticles yield reproducible and reliable vibrational signatures of yeast cell wall constituents.
