Preresonance Raman spectra of crystals of retinal isomers

Fast outdoor screening and discrimination of carotenoids of halophilic microorganisms using miniaturized Raman spectrometers

Eight miniaturized Raman spectrometers were used to perform a fast outdoor screening and discrimination of carotenoids of a series of halophilic and non-halophilic microorganisms on a set of eight lyophilized samples, each containing high concentrations of a specific dominant carotenoid pigment. Raman spectra were acquired using different excitations (532, 785, sequentially shifted excitation of 785 and 853, and 1064 nm), based on the model of each Raman spectrometer, in order to ascertain the feasibility of individual wavelengths. The wavenumber positions of diagnostic Raman bands of carotenoids were observed for the different carotenoid species. Characteristic carotenoid Raman bands of the pigment bacterioruberin were reported (using the 532 nm excitation) at 1504-1509 cm^-1, salinixanthin at 1510-1513 cm^-1, spirilloxanthin at 1509-1513 cm^-1, decaprenoxanthin at 1519 cm^-1, β-carotene at 1526 cm^-1, and sarcinaxanthin at 1526-1528 cm^-1. A 532 nm excitation consistently provided best results due to the significant resonance signal enhancement (both quantitative and qualitative carotenoid detection). Good results were also obtained using the sequentially shifted excitation combining two lasers in the near infrared spectral region, and similarly good results were acquired using a standard 1064 nm excitation. The least suitable was a 785 nm excitation, with the carotenoid Raman signal almost always weaker compared to major fluorescence signal arising from other types of pigments or biomolecules in the samples. A thorough light shielding was essential in order to acquire good quality data. This study shows that miniaturized Raman spectrometers, some even equipped with longer wavelength excitation, are able to detect different carotenoid pigments under non-laboratory conditions in a fast way, and discriminate between them, to a certain degree. The implications of this type of research are especially useful in astrobiology, where the searching, detection and discrimination of biomarkers such as carotenoids is receiving significant attention.

The Diversity of Linear Conjugated Polyenes and Colours in Nature: Raman Spectroscopy as a Diagnostic Tool

This review is centered on the linear conjugated polyenes, which encompasses chromatic biomolecules, such as carotenoids, polyunsaturated aldehydes and polyolefinic fatty acids. The linear extension of the conjugated double bonds in these molecules is the main feature that determines the spectroscopic properties as light-absorbing. These classes of compounds are responsible for the yellow, orange, red and purple colors which are observed in their parent flora and fauna in nature. Raman spectroscopy has been used as analytical tool for the characterization of these molecules, mainly due to the strong light scattering produced by the delocalized pi electrons in the carbon chain. In addition, conjugated polyenes are one of the main target molecular species for astrobiology, and we also present a brief discussion of the use of Raman spectroscopy as one of the main analytical tools for the detection of polyenes extra-terrestrially.

Three ancient documents solve the jigsaw of the parchment purple spot deterioration and validate the microbial succession model

The preservation of cultural heritage is one of the major challenges of today's society. Parchments, a semi-solid matrix of collagen produced from animal skin, are a significant part of the cultural heritage, being used as writing material since ancient times. Due to their animal origin, parchments easily undergo biodeterioration: the most common biological damage is characterized by isolated or coalescent purple spots, that often lead to the detachment of the superficial layer and the consequent loss of written content. Although many parchments with purple spot biodegradative features were studied, no common causative agent had been identified so far. In a previous study a successional model has been proposed, basing on the multidisciplinary analysis of damaged versus undamaged samples from a moderately damaged document. Although no specific sequences were observed, the results pointed to Halobacterium salinarum as the starting actor of the succession. In this study, to further investigate this topic, three dramatically damaged parchments were analysed; belonging to a collection archived as Faldone Patrizi A 19, and dated back XVI-XVII century A.D. With the same multidisciplinary approach, the Next Generation Sequencing (NGS, Illumina platform) revealed DNA sequences belonging to Halobacterium salinarum; the RAMAN spectroscopy identified the pigment within the purple spots as haloarchaeal bacterioruberin and bacteriorhodopsine, and the LTA technique quantified the extremely damaged collagen structures through the entire parchments, due to the biological attack to the parchment frame structures. These results allowed to propose a model of the progressive degradation pattern of the parchment collagen. Overall, these data validate a multi-phase microbial succession model. This demonstration is pivotal to possible new restoration strategies, important for a huge number of ancient documents.

Resonance Raman spectroscopy in biochemistry and biology

Resonance Raman (RR) spectroscopy provides detailed information on the vibrational and electronic properties of biochemical and biological chromophores. The analysis of RR spectra, using for example model compounds or a group frequency approach, enables us to form an accurate structural picture of the chromophore in its natural biological site. Moreover, the insight gained into the electronic states of a biological chromophore can be crucial to our understanding of its function. Thus the RR technique represents a powerful means of eliciting precise structural and electronic data from a coloured species and of focusing upon key aspects of its function. It has even been possible to obtain RR spectra from some natural chromophores invivo, giving spectra detailed and informative enough to please a spectroscopist from a system complex enough to satisfy a biologist.

Modeling the resonance Raman spectrum of a metarhodopsin: implications for the color of visual pigments

Resonance Raman spectra of an invertebrate rhodopsin are reported. The spectrum of squid acid metarhodopsin is compared with the spectra of model compounds of the retinylidene chromophore in the all-trans conformation. Correlations made between acid metarhodopsin and these crystalline model compounds with known x-ray structures indicate that the chromophore in this intermediate is an all-trans protonated Schiff base. The data suggest a mechanism for the red shift in rhodopsin.