Maciej Roman

Recent Advances in Raman Analysis of Plants: Alkaloids, Carotenoids, and Polyacetylenes

This paper demonstrates the special potential of Raman spectroscopy for the study of selected plant metabolites. Carotenoids, which are beneficial components in fruits and vegetables, have been shown to be a significant factor in lowering the risk of various types of cancer and ischemic heart diseases. On the other hand, alkaloids may have various effects on human health, e.g. caffeine is a mild stimulant of the central nervous system and as a result it can influence human behaviour. Polyacetylenes are highly cytotoxic against numerous cancer cell lines and demonstrate antifungal, anti-inflammatory and anti-platelet-aggregatory properties. In most cases, vibrational measurements can be performed directly on plant tissues as well as on fractions isolated from the plant material by hydro-distillation or solvent extraction. Raman spectroscopy techniques allow obtaining spectra which present some characteristic key bands of individual components. Based on such markers related to individual plant substances, spectroscopic analyses in principle allow the discrimination of different species, and even chemotypes among the same species. Moreover, Raman microspectroscopy provides 2- and 3-dimensional images of the investigated plant samples. These maps can be directly compared to the corresponding visual images obtained from a light microscope and offer additional detailed information regarding the local distribution of specific compounds in the surface layers of the analyzed plant tissue.

Spectroscopic Studies on Bioactive Polyacetylenes and Other Plant Components in Wild Carrot Root

Polyacetylenes and other common plant components, such as starch, pectin, cellulose, and lignin, were studied in roots of the wild carrot (Daucus carota) subspecies D. carota subsp. gummifer and D. carota subsp. maximus by Raman spectroscopy. The components were measured in situ, directly in the plant tissue and without any preliminary sample preparation. The analysis was performed on the basis of the intense and characteristic key bands observed in the Raman spectrum. The two main carrot polyacetylenes falcarinol (1) and falcarindiol (2) have similar molecular structures, but their Raman spectra exhibit a small band shift in the symmetric -C≡C-C≡C- mode from 2258 cm⁻¹ to 2252 cm⁻¹. Quantum chemical calculations confirmed that the differences observed between the samples may be due to conformational and environmental changes. The polyacetylenes were also detected by Raman mapping, which visualized the distribution of the compounds across sections of carrot roots. The mapping technique was also applied to assess the distribution of lignin and polysaccharide compounds. The results showed the tissue-specific accumulation of starch and cell wall components such as lignin, pectin, and cellulose.

Nondestructive Raman analysis of polyacetylenes in apiaceae vegetables.

Food plants from the Apiaceae family have been found to demonstrate health-promoting properties. Polyacetylenes are bioactive compounds that are considered to contribute substantially to the beneficial properties of Apiaceae plants. This study applied a Raman mapping technique in the investigation of polyacetylene spatial distribution in fresh roots of some Apiaceae species. Fresh root sections were measured directly without any preliminary preparation. For three Apiaceae species, that is, parsnip ( Pastinaca sativa L.), celeriac ( Apium graveolens var. rapaceum L.), and parsley ( Petroselinum crispum ), the presence of polyacetylenes was confirmed due to the detection of strong and well-separated Raman signals of symmetric -C ≡ C-C ≡ C- stretching vibration in the range of 2200-2300 cm(-1). The spectra were used for generation of two-dimensional maps applying the integration and cluster analysis methods. The Raman maps visualized the distribution of total polyacetylenes as well as individual compounds. Heterogeneous and tissue-specific occurrence of polyacetylenes in roots is shown.

Theoretical modeling of molecular spectra parameters of disubstituted diacetylenes.

Symmetrically disubstituted diacetylenes, X-C≡C-C≡C-X, were studied computationally by using the DFT B3LYP/aug-cc-pVDZ method. For more than 35 substituents the bond lengths, charge density and Laplacian in bond critical points, C≡C stretching vibrational frequencies, (13)C NMR chemical shifts and spin-spin CC coupling constants through diacetylene moiety were calculated and examined by using the substituent sEDA and pEDA descriptors. It is demonstrated that in disubstituted diacetylenes the triple bond length increases with the electron donating and decreases with the electron withdrawing properties of the substituents. The σ-electron repulsion is likely to be responsible for this phenomenon. The electron density of the C-X bond critical point decreases linearly with an increase of the σ-electron donating properties, whereas the Laplacian of electron density in the C≡C bond critical point increases as the sEDA descriptor i.e., σ-electron donating properties of the substituent, are increased. Thus, ρ(C≡C) is locally reduced with an increase of sEDA. The ν(as)(C≡C) and ν(s)(C≡C) mode frequencies decrease with the electron donating and increase with the electron withdrawing properties of the substituents. The calculated chemical shift of the C1-atom, to which the substituent is attached, does not correlate with the substituent descriptors, whereas the δ(C2) deshielding increases when the σ-electron donating properties and C≡C distance are increased. The calculated (1)J(CC) coupling constants decrease with an increase of the triple bond length, the sEDA parameter, and ρ(C-X), whereas they decrease with Laplacian in the C≡C BCP.

Composition and (in)homogeneity of carotenoid crystals in carrot cells revealed by high resolution Raman imaging

Three categories of roots differing in both β/α-carotene ratio and in total carotenoid content were selected based on HPLC measurements: high α- and β-carotene (HαHβ), low α- and high β-carotene (LαHβ), and low α- and low β-carotene (LαLβ). Single carotenoid crystals present in the root cells were directly measured using high resolution Raman imaging technique with 532nm and 488nm lasers without compound extraction. Crystals of the HαHβ root had complex composition and consisted of β-carotene accompanied by α-carotene. In the LαHβ and LαLβ roots, measurements using 532nm laser indicated the presence of β-carotene only, but measurements using 488nm laser confirmed co-occurrence of xanthophylls, presumably lutein. Thus the results show that independently on carotenoid composition in the root, carotenoid crystals are composed of more than one compound. Individual spectra extracted from Raman maps every 0.2-1.0μm had similar shapes in the 1500-1550cm(-1) region indicating that different carotenoid molecules were homogeneously distributed in the whole crystal volume. Additionally, amorphous carotenoids were identified and determined as composed of β-carotene molecules but they had a shifted the ν1 band probably due to the effect of bonding of other plant constituents like proteins or lipids.

Vibrational and theoretical study of selected diacetylenes.

Six commonly used disubstituted diacetylenes with short side-chains (RCCCCR, where R=CH2OH, CH2OPh, C(CH3)2OH, C(CH3)3, Si(CH3)3, and Ph) were analyzed using vibrational spectroscopy and quantum-chemical calculations to shed new light on structural and spectroscopic properties of these compounds. Prior to that the conformational analysis of diacetylenes was performed to search the Potential Energy Surface for low-energy minima. Theoretical investigations were followed by the potential energy distribution (PED) analysis to gain deeper insight into FT-Raman and FT-IR spectra that, in some cases, were recorded for the first time for the studied compounds. The analysis was focused mainly on spectral features of the diacetylene system sensitive to the substitution. Shifts of the characteristic bands and changes in bond lengths were observed when changing the substituent. Furthermore, Fermi resonance was observed in the vibrational spectra of some diacetylenes. FT-IR spectra were measured by using two methods, i.e. transmission (with KBr substrate) and Attenuated Total Reflection (ATR), showing the latter adequate and fast tool for IR measurements of diacetylenes. Additionally, Surface Enhanced Raman Spectroscopy (SERS) was applied for phenyl derivative for the first time to study its interaction with metallic nanoparticles that seems to be perpendicular.

Natural monoacetylenes studied by quantum-chemical chemistry

This study is a part of the project focused on the vibrational analysis of natural mono- and polyacetylenes by using Raman spectroscopy and theoretical calculations. Their vibrational spectra show strong and polarized –C≡C– bands in the region of about 2200 cm–1. Mono- as well as polyacetylenes are supposed to be active in plants yet not available in an isolated form, so theoretical simulation of their vibrational spectra and comparison with the registered ones seems to be an excellent way to confirm or exclude the presence of these compounds in the investigated plants. Such an approach was applied here to analyze polyacetylenes in roots of Coreopsis grandiflora. According to literature, this plant should contain a monoacetylene substituted by a thiophene ring. Theoretical calculations allowed to confirm this assumption.

Vibrational and theoretical study of diacetylenic acids

Three selected diacetylenic acids (DAs) with side-chains of various length (CH3-(CH2)m-C≡C-C≡C(CH2)n-COOH, where m=7, 9, 11, and n=3, 8) were analyzed using vibrational spectroscopy and quantum-chemical calculations. The conformational analysis was followed by potential energy distribution (PED) calculations to gain deeper insight into their FT-Raman and FT-IR spectra. The analysis was focused on spectral features of the diacetylene system sensitive to the substitution. In particular, the electron donor-acceptor properties of the substituent and the influence of side-chain length were studied. FT-IR spectra were measured by using two techniques, i.e. transmission (with KBr substrate) and Attenuated Total Reflection (ATR), and the latter seems to be less adequate for DAs measurements because the bands in the fingerprint region as well as the ν(C≡C)as mode are relatively of low intensity. Additionally, polymerization process of DAs was recognized using FT-Raman spectroscopy and strong and well-separated bands of diacetylenic polymers. Temperature and exposure to the sunlight are the factors of an important influence on the polymerization process of DAs. Since the investigated DAs are carboxylic acids, the interpretation of experimental spectra was performed on the basis of monomer and dimer calculations.

General Overview on Vibrational Spectroscopy Applied in Biology and Medicine

This chapter contains a short introduction to vibrational spectroscopy followed by an overview on its biological and biomedical applications. The spectroscopic techniques discussed in the book and their special advantages are briefly listed, i.e. Surface-Enhanced Raman Spectroscopy (SERS), Raman Optical Activity (ROA), Vibrational Circular Dichroism (VCD), Electronic Circular Dichroism (ECD) and matrix isolation.