Analysis of intra-specific variations in the venom of individual snakes based on Raman spectroscopy.

The knowledge of variations in the composition of venoms from different snakes is important from both theoretical and practical points of view, in particular, at developing and selecting an antivenom. Many studies on this topic are conducted with pooled venoms, while the existence and significance of variations in the composition of venoms between individual snakes of the same species are emphasized by many authors. It is important to study both inter- and intra-specific, including intra-population, venom variations, because intra-specific variations in the venom composition may affect the effectiveness of antivenoms as strongly as inter-specific. In this work, based on venom Raman spectroscopy with principal component analysis, we assessed the variations in venoms of individual snakes of the Vipera nikolskii species from two populations and compared these intra-specific variations with inter-specific variations (with regard to the other related species). We demonstrated intra-specific (inter- and intra-population) differences in venom compositions which are smaller than inter-specific variations. We also assessed the compositions of V. nikolskii venoms from two populations to explain inter-population differences. The method used is rapid and requires virtually no preparation of samples, used in extremely small quantities, allowing the venoms of individual snakes to be analyzed. In addition, the method is informative and capable of detecting fairly subtle differences in the composition of venoms.

Dependence of CC stretching wavenumber on polyene length in degraded polyvinyl chloride: a comparative empirical, classical mechanics, and DFT study.

Mathematically describing the length-dependence of vibrational fingerprints of polyenes is challenging, yet crucial in understanding and predicting polyene-associated molecular properties of industrially-important and vital substances. To this end, we develop an analytical relationship between the wavenumbers ν∼C=C of the Raman-active CC stretching mode in polyene sequences (CHCH)n and the polyene length (n) using classical mechanics laws. Noteworthy, this relationship is derived from Newton's equations instead of regression approximations and validated against experimental data for degraded polyvinyl chloride (PVC), t-butyl end-capped all-trans polyenes, ß-carotenes, and carotenoids. Furthermore, given this fundamental tool, we carefully re-examined or validated the up-to-now applied empirical tools; we find that: (i) A phenomenological exponential regression function ν~C=C=1461+151.2×exp-0.07808n proves fairly suitable for describing polyenes with lengths below 24 in degraded PVC. (ii) The derived analytical relationship agrees more closely with a long-established reciprocal-length regression function ν~C=C=1459+720/n+1 for describing carotenoids. Moreover, extensive DFT calculation results on all-trans polyenes H(CHCH)nH (n = 3-30) and polyenes end-capped with terminal vinyl chloride oligomers agree with experiment for shorter polyenes and are similar, showing that complicated calculations of ν∼C=C for infinite degraded PVC chains reduce to the calculations on finite polyene sequences. Noteworthy, unlike other polyene length-determination tools, the proposed analytical polyene length-determination based on intrinsic physical properties could well prove to be an even more versatile tool, as it comes with the added potential for determining or correcting the elasticity constants of carbon bonds in polyene chains.

Raman spectroscopic detection of polyene-length distribution for high-sensitivity monitoring of photo- and thermal degradation of polyvinylchloride.

The degradation of the ubiquitous polyvinylchloride (PVC) material under the influence of various factors is known to result commonly in polyene formation. Such polyene defects occur in the form of conjugated aliphatic chains with different lengths and contents, and their sensitive and length-specific monitoring is important for the assessment of PVC degradation. Here, we report on the resonance-enhanced Raman signatures of polyene sequences of varying lengths in photo- and thermally degraded PVC films. The lengths of polyene segments have been estimated based on their selectively enhanced and spectrally resolved contributions to the Raman bands assigned to the stretching vibrations of conjugated double carbon bonds. Using deconvolution analysis of a characteristic Raman band of polyenes, we especially demonstrate that the spectral signatures of polyene segments corresponding to other various electronic resonances contribute to the Raman spectral envelope observed at a given resonant excitation. In most cases, we observe an asymmetric band profile, indicating an asymmetric length distribution of polyene defects formed in PVC films upon a mild degradation extent less than 1% mass loss. We also demonstrate that the wavenumber (ν1) of the stretching vibrations of single carbon bonds in the polyene sequences of degraded PVC is inversely related to the number (n) of double carbon bonds by an empirical equation n=476·cm-1/ν1-1082·cm-1. To the best of our knowledge, while considering different laser excitations spanning the range of possible electronic resonances from blue to near-infrared for Raman investigations, the present work includes (i) the first Raman spectral deconvolution study for the 532.0 nm excitation wavelength used in most portable Raman probes nowadays and (ii) the screening of polyene defects also beyond the red edge of the visible spectrum and the evidence of a resonance-enhanced polyene signal at 647.1 nm. Important new information has been obtained about polyene lengths and spectral distribution for PVC, whose critical physical properties ranging from flexibility to electrical resistance are severely affected by polyene formation.

Raman Spectroscopy Study of Structurally Uniform Hydrogenated Oligomers of α-Olefins.

The expansion of the range of physico-chemical methods in the study of industrially significant α-olefin oligomers and polymers is of particular interest. In our article, we present a comparative Raman study of structurally uniform hydrogenated dimers, trimers, tetramers, and pentamers of 1-hexene and 1-octene, that are attractive as bases for freeze-resistant engine oils and lubricants. We found out that the joint monitoring of the disorder longitudinal acoustic mode (D-LAM) and symmetric C-C stretching modes allows the quantitative characterization of the number and length of alkyl chains (i.e., two structural characteristics), upon which the pour point and viscosity of the hydrocarbons depend, and to distinguish these compounds from both each other and linear alkanes. We demonstrated that the ratio of the contents of CH2 and CH3 groups in these hydrocarbons can be determined by using the intensities of the bands in the spectra, related to the asymmetric stretching vibrations of these groups. The density functional theory (DFT) calculations were applied to reveal the relations between the wavenumber and bandshape of the symmetric C-C stretching mode and a conformation arrangement of the 1-hexene and 1-octene dimers. We found that the branched double-chain conformation results in the splitting of the C-C mode into two components with the wavenumbers, which can be used as a measure of the length of branches. This conformation is preferable to the extended-chain conformation for hydrogenated 1-hexene and 1-octene dimers.

Symmetric C-C stretching mode splitting versus CH2-chain conformation order in sodium montmorillonite modified by cetyltrimethylammonium bromide.

Exploiting Raman spectroscopy and computational modeling, for the first time, we report and explain an interesting phenomenon in clay modified by cetyltrimethylammonium bromide. A splitting of the CH(2)-chain's symmetric C-C stretching Raman mode found at ~1128 cm(-1) in cetyltrimethylammonium bromide into two bands at 1128 and 1139 cm(-1) in clay modified by cetyltrimethylammonium bromide is observed. We demonstrate that this splitting appears if two types of trans-segments with nonequivalent lengths and terminal groups coexist in the CH(2)-chain of the alkylammonium ion embedded into the clay interlayer space. We report Raman experimental evidence for a CH(2)-chain bending within the clay galleries, resulting in the symmetric C-C stretching band splitting, as was also suggested by computational modeling. Noteworthy, we postulate that this unique behavior based on CH(2)-chain bending provides a general understanding of conformation reorganization and switching within long CH(2)-chain molecules confined within modified clay interlayer galleries. For all modifier concentrations, we show that the intercalated cetyltrimethylammonium ions exist in a liquid-like state, consisting mainly of trans conformations (~86%) of two types in approximately equal proportions. Moreover, we demonstrate that the integral Raman intensity ratio I(1295)(CH(2))/I(705)(clay) provides a rapid nondestructive quantification of the relative content of alkylammonium ions in modified clays. These results demonstrate that a simple direct monitoring of specific modifier-dependent interlayer conformational states is possible, which is of great importance for a tunable fabrication of modified clays-based nanocomposites with desired properties.

Raman spectroscopic characterization of the interlayer structure of Na+-montmorillonite clay modified by ditetradecyl dimethyl ammonium bromide.

Raman spectroscopy has been applied for the rapid and nondestructive monitoring of the interlayer structure of sodium montmorillonite (MMT) clay modified by ditetradecyl dimethyl ammonium (DDA+) bromide. This work demonstrates that a detailed analysis of Raman spectra in the fingerprint region (600-1600 cm(-1)), in combination with model simulation, allows one to distinguish different conformational states of DDA+ in the interlayer space of the modified clay, namely, a liquidlike state but rich in trans conformers, disordered conformational states, and a crystallike conformation appearing at increasing modifier content. These conformations differ in the angle between their alkyl chains, the relative content of trans and gauche conformers and the relative length of trans segments. The shape and width of the Raman band at 1300 cm(-1) and the peak intensity ratio I1088/I1064 can be used for a qualitative analysis of the ratio of gauche/trans conformers. The integral intensity ratios I*1064/I*1300 and I*1300/I*705 help to determine the proportion of trans conformers and the content of the modifier in the clay, respectively, thereof providing quantitative characterization of the modified clay (conformational reorganization and modifier content). Noteworthy, the transition from a liquidlike to crystal-like conformation is further supported by the splitting of the symmetric C-C stretching Raman band of the trans segments within the alkyl chains at 1133 cm(-1) (liquidlike conformation) into two modes at 1124 and 1135 cm(-1) corresponding to two parallel trans chains of nonequivalent lengths (crystal-like conformation).