Vibrational Spectroscopies and Chemometry for Nondestructive Identification and Differentiation of Painting Binders

A comprehensive dataset of vibrational spectra of different natural organic binding media is presented and discussed. The binding media were applied on a glass substrate and analyzed after three months of natural ageing. The combination of Raman and FT-NIR spectroscopies allows for an improved identification of these materials as Raman technique is more informative about the skeletal vibrations, while FT-NIR spectroscopy is more sensitive to the substituents and polar groups. The experimental results are initially discussed in the framework of current spectral assignment. Then, multivariate analysis (PCA) is applied leading to differentiation among the samples. The two major principal components allow for a complete separation of the different classes of organic materials. Further differentiation within the same class is possible thanks to the secondary components. The loadings obtained from PCA are discussed on the basis of the spectral assignment leading to clear understanding of the physical basis of this differentiation process

Microanalysis of Organic Pigments in Ancient Textiles by Surface-Enhanced Raman Scattering on Agar Gel Matrices

We review some new methods based on surface-enhanced Raman scattering (SERS) for the nondestructive/minimally invasive identification of organic colorants in objects whose value or function precludes sampling, such as historic and archeological textiles, paintings, and drawing. We discuss in detail the methodology we developed for the selective extraction and identification of anthraquinones and indigoids in the typical concentration used in textiles by means of an ecocompatible homogeneous nanostructured agar matrix. The extraction system was modulated according to the chemical properties of the target analyte by choosing appropriate reagents for the extraction and optimizing the extraction time. The system has been found to be extremely stable, easy to use and produce, easy to store, and at the same time able to be analyzed even after long time intervals, maintaining its enhancement properties unaltered, without the detriment of the extracted compound. Highly structured SERS band intensities have been obtained from the extracted dyes adopting laser light excitations at 514.5 and 785 nm of a micro-Raman setup. This analytical method has been found to be extremely safe for the analyzed substrates, thus being a promising procedure for the selective analysis and detection of molecules at low concentration in the field of artworks conservation

non covalent interactions in anisole co2 n n 1 2 complexes

Non-covalent interactions are a ubiquitous binding motif and a challenge for theory and experiments

High-Resolution Spectroscopic Studies of Complexes Formed by Medium-Size Organic Molecules

A wealth of structural and dynamical information has been obtained in the last 30 years from the study of high-resolution spectra of molecular clusters generated in a cold supersonic expansion by means of highly resolved spectroscopic methods. The data obtained, generally lead to determination of the structures of stable conformations. In addition, in the case of weakly bound molecular complexes, it is usual to observe the effects of internal motions due to the shallowness of the potential energy surfaces involved and the flexibility of the systems. In the case of electronic excitation experiments, also the effect of electronic distribution changes on both equilibrium structures and internal motions becomes accessible. The structural and dynamical information that can be obtained by applying suitable theoretical models to the analysis of these unusually complex spectra allows the determination and understanding of the driving forces involved in formation of the molecular complex. In this way, many types of non-covalent interactions have been characterized, from pure van der Waals interactions in complexes of rare gases to moderate-strength and weak hydrogen bonds and to the most recent halogen bonds and n-\u3c0 interactions. The aim of this review is to underline how the different experimental and theoretical methods converge in giving a detailed picture of weak interactions in small molecular adducts involving medium-size molecules. The conclusions regarding geometries and energies can contribute to understanding of the different driving forces involved in the dynamics of the processes and can be exploited in all fields of chemistry and biochemistry, from design of new materials with novel properties to rational design of drugs

New Insights on the Raman and SERS Spectra of Luteolin under Different Excitation Conditions: Experiments and DFT Calculations

We have studied, by density functional theory, the interaction between luteolin and Ag, devising two complexes where an Ag14 cluster faces two different sites of the molecule. The two sites are identified as quinoid-like and cathecol-like, and the complexes are CPLX1 and CPLX2, respectively. Raman and SERS spectra of luteolin were measured at different excitation wavelengths. Luteolin solid samples from different suppliers have different Raman spectra, possibly associated with different arrangements in the solids. These spectra are well reproduced by our DFT calculations. Assignment of the vibrational modes of luteolin and of the two luteolin–Ag14 complexes is obtained thanks to decomposition of the normal coordinates in terms of internal coordinates. The calculated Raman spectrum for CPLX1 seems to better reproduce the experimental SERS spectra. CPLX2 furnishes a spectrum still resembling that of luteolin in the high frequency region and is possibly responsible for some weak bands in the 1400–1700 cm−1 range that cannot be accounted by the CPLX1 system. SERS spectra are dependent on the Raman excitation wavelength. The calculation of the electronic spectrum suggests the presence of charge-transfer states, which might be responsible for the changes in the SERS spectra

Linear and Non-Linear Middle Infrared Spectra of Penicillin G in the CO Stretching Mode Region

In this work we report the linear and non-linear IR spectral response characterization of the CO bonds of PenicillinG sodium salt in D2O and in DMSO−d6 solutions. In order to better characterize the spectral IR features in the CO stretching region, broadband middle infrared pump-probe spectra are recorded. The role of hydrogen bonds in determining the inhomogeneous broadening and in tuning anharmonicity of the different types of oscillators is exploited. Narrow band pump experiments, at the three central frequencies of β−lactam, amide and carboxylate CO stretching modes, identify the couplings between the different types of CO oscillators opening the possibility to gather structural dynamic information. Our results show that the strongest coupling is between the β−lactam and the carboxylate CO vibrational modes

The Role of the Hydrogen Bond Network in Maintaining Heme Pocket Stability and Protein Function Specificity of <i>C. diphtheriae</i> Coproheme Decarboxylase

Monoderm bacteria accumulate heme b via the coproporphyrin-dependent biosynthesis pathway. In the final step, in the presence of two molecules of H2O2, the propionate groups of coproheme at positions 2 and 4 are decarboxylated to form vinyl groups by coproheme decarboxylase (ChdC), in a stepwise process. Decarboxylation of propionate 2 produces an intermediate that rotates by 90° inside the protein pocket, bringing propionate 4 near the catalytic tyrosine, to allow the second decarboxylation step. The active site of ChdCs is stabilized by an extensive H-bond network involving water molecules, specific amino acid residues, and the propionate groups of the porphyrin. To evaluate the role of these H-bonds in the pocket stability and enzyme functionality, we characterized, via resonance Raman and electronic absorption spectroscopies, single and double mutants of the actinobacterial pathogen Corynebacterium diphtheriae ChdC complexed with coproheme and heme b. The selective elimination of the H-bond interactions between propionates 2, 4, 6, and 7 and the polar residues of the pocket allowed us to establish the role of each H-bond in the catalytic reaction and to follow the changes in the interactions from the substrate to the product