Triterpenoid resins

Dammar resin was introduced into Europe in the nineteenth century, mainly as a paint varnish It is still used today since it has good optical properties and low acidity. It is derived from various species (the genus Hopea and Shorea of the Dipterocarpaceae family). It is characterised by tetracyclic triterpenoids of the dammarane series and contains minor amounts of pentacyclic triterpenoids of the series of olanane, ursane and hopane. It also contains a polymeric fraction named polycadinene or [S-resene. Dammar resin triterpenoids undergo oxidation with ageing, as described for the components of mastic resin. 

Apart from the use and need as fragrances, the resins are marketed for medicinal use as antiarthritic and antiinflammatory pharmaceutical products. The pharmacological effects are mainly attributed to the presence of the nonvolatile pen-tacyclic triterpenoid boswellic acids. This class of ingredients is not present in the actual valuable B. frereana species hence, B. frereana plays no part in the pharmaceutical area. 

Mono- and sesquiterpenoids are of limited use for the identification and classification of aged resins. Due to their volatility, they are rarely found in ancient samples except when they have been conserved in very particular conditions [88,98], On the other hand, the di-and triterpenoids enable us to identify resins thereby identifying their botanical origin [2,99]. Figures 1.1 and 1.2 show the main diterpenoid and triterpenoid structures. 

Diterpenoid and triterpenoids in natural resins generally lead to one, two or three oxygen atoms in the form of acidic, carboxylic or alcoholic functionality and a variable degree of unsaturation. 

Figure 1.2 General molecular formulae of triterpenes present in triterpenoid resins...

Frankincense, also known as olibanum, is obtained from trees belonging to the genus Boswellia (Burseraceae family). It is one of the best-known ancient plant resins. The ancient Egyptians were the first to use it as incense in embalming practices and in the preparation of medicines, cosmetics and perfumes, and today it is still used therapeutically. It contains pentacyclic triterpenoids belonging to oleanane, ursane or lupane type molecules and in particular of a- and p-boswellic acids, and their O-acetates [104 111], 11 -Oxo-p-boswellic acid and its acetyl derivative, identified in several Boswellia species, are also diagnostic for frankincense [112]. 

Resins older than 40 000 years are considered to be fossil resins. The fossilization of resins begins with polymerisation and forms ambers and copals. Most of the ambers are derived from components of diterpenoid resins with a labdanoid structure other ambers are based on polymers of sesquiterpene hydrocarbons such as cadinene, and may include triterpenoids less common ambers from phenolic resins derive from polymers of styrene. Figure 1.4 shows the skeletal structures of the components which make up the polymers occurring in fossil resins [141]. 

M.P. Colombini, F. Modugno, E. Ribechini, Chemical study of triterpenoid resinous materials in archaeological findings by means of direct exposure electron ionisation mass spectrometry and gas chromatography/mass spectrometry, Rapid Communications in Mass Spectrometry, 20, 1787 1800 (2006). 

G.A. van der Doelen, K.J. van den Berg, J.J. Boon, N. Shibayama, E.R. de la Rie, W.J.L. Genuit, Analysis of fresh triterpenoid resins and aged triterpenoid varnishes by high performance liquid chromatography atmospheric pressure chemical ionisation (tandem) mass spectrometry, Journal of Chromatography A, 809, 21 37 (1998). 

The first two PCs account for 85% of the total variance of the data set. The PCA highlights that the sample is placed in the area of the mastic resin cluster. This confirmed the hypothesis made on the basis of a qualitative examination of the mass spectra, even though the sample is clearly shifted at lower values of PC2 with respect to the centroid of the mastic cluster. This was attributed to the presence of other substances mixed with the resin, and/or to a different quantitative distribution of the triterpenoids present in the censer with respect to the reference raw mastic resin. 

Despite the complexity of the chemical composition of the resinous materials, in a few minutes such techniques provide a mass spectral fingerprint, which highlights the compounds that are the main components in the sample. They avoid any sampling treatment before analysis. They have thus enabled diterpenoid resinous materials from Coniferae, and several triterpenoid materials to be clearly identified. In particular, the DE-MS technique is able to distinguish between different triterpenoid materials such as mastic resin, frankincense resin and birch bark tar. In fact, using PCA on DE-MS mass... 

Two slightly different laser desorption/ionisation methods were developed simultaneously by Karas and Hillenkamp [1] and Tanaka et al. [2]. Whereas Karas and Hillenkamp used small organic matrix molecules to assist and facilitate the desorption and ionisation of analytes (MALDI), Tanaka et al. used ultra-fine metal powders and glycerol. Zumbiihl et al. first analysed natural triterpenoid resins, dammar and mastic, both... 

Studying the Ageing of Natural Triterpenoid Resin Varnishes... 

Various authors have studied the ageing of triterpenoid resins to understand and possibly slow their deterioration [3, 4, 12, 13, 17 36]. The main degradation pathway is autoxida-tion, an oxidative radical chain reaction [37, 38] after formation of radicals, oxygen from the air is inserted, leading to peroxides. The peroxides can be homolytically cleaved, resulting in new radicals that continue the chain reaction. The cleavage of peroxide bonds can be induced thermally or photochemically. 

A study of the photoageing of single triterpenes was undertaken to establish the suitability of the GALDI-MS method [39]. The triterpenes served as a simplified reference system for the more complex natural triterpenoid resins, dammar and mastic. Seven triterpenes were... 

Triterpenoid resins used as varnishes on paintings were first studied with GALDI-MS by Zumbiihl et al. [3,4], He realised that freshly bought commercial resin, usually considered fresh , is actually in an advanced stage of oxidation. Additional experiments were performed later on with improved mass resolution that allowed more detailed conclusions [34 36],... 

The resin composition was indeed found to influence the ageing processes in mastic, the polymer fraction was shown to reduce the oxidation of triterpenoids (Figure 5.8). This was found for both natural ageing [33, 36] and artificial ageing. Because the polymer, a polymyrcene, contains many double bonds that can easily be oxidised [42], it might act as a radical stabiliser. The mastic samples shown in Figure 5.8 were obtained from three aliquots of the same mastic solution, the polymer part of the first aliquot was removed and added to the third. 

Besides the well-established chromatographic/mass spectrometric or spectroscopic methods there is always a need for complementary methods for the study of organic materials from art objects. The application of laser desorption/ionisation mass spectrometry (LDI-MS) methods to such materials has been reported only sporadically [12, 45 48] however, it is apparently increasing in importance. After GALDI-MS had been applied to triterpenoid resins, as described in Section 5.2, this relatively simple method was evaluated for a wider range of binders and other organic substances used for the production or conservation of artwork. Reference substances as well as original samples from works of art were analysed. 

D. Scalarone, M.C. Duursma, J.J. Boon, and O. Chiantore, MALDI TOF Mass Spectrometry on Cellulosic surfaces of Fresh and Photo aged Di and Triterpenoid Varnish Resins, J. Mass Spectrom., 40, 1527 1535 (2005). 

P. Dietemann, C. Higgitt, M. Kalin, M.J. Edelmann, R. Knochenmuss, and R. Zenobi, Aging and Yellowing of Triterpenoid Resin Varnishes Influence of Aging Conditions and Resin Composition, J. Cult. Heritage, 10, 30 40 (2009). 

P. Dietemann, S. Zumbuhl, R. Knochenmuss, R. Zenobi, Aging of triterpenoid resin varnishes on paintings studied by graphite assisted laser desorption/ionization mass spectrometry, Adv. Mass Spectrom., 15, 909 910 (2001). 

The GC-MS chromatogram obtained with the same substance after acid methanolysis and silylation is presented in Figure 10.10. The major compounds are fatty acid methyl esters corresponding to a mixture of an animal fat (attested by the presence of E15 0 and E17 0 with ante and iso isomers) and castor oil (attested by the presence of methyl ricinoleate E18 1,120H) [32]. Diterpenoid or triterpenoid resin components are not observed. 

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