Natural Textile Fibres - Optical Activity, Racemization and Epimerization

Dr. Silvio Diana
Central Institute of Restoration - Rome

Prof. Emanuela Marinelli Paolicchi
Doctor in Natural Sciences and Geological Sciences
Collegamento pro Sindone - Rome

Copyright 1997
All Rights Reserved
Reprinted by Permission


Investigation Methods for the Cellulose Supports

The degradation of cellulose textiles in ancient paintings, like linen, hemp, cotton, could be valued through opportune research methods. Cellulose is a natural polymer and is the basis of the fibre and the result of the biosynthesis through condensation of b-glucose molecules. The monomeric units constituting the cellulose macromolecule reach the number of 2500-3000 units in case of raw linen, hemp, cotton and jute fibres. This number expresses the depolymerization degree, that is the (Dp).

The length of the polymeric chain (macromolecules) is not the only parameter referable to the tenacity.

Chemical-textile methods confirmed that the length indicated by the depolymerization degree is the most significant parameter to show the deterioration state, that is the degradation of those fibres. In fact, when the fibre deteriorates, it happens the break-up of the 1,4 b-glucose links between monomer and monomer and the macromolecules break in shorter ones. The fibres resistance depends on the chain length and the shorter macromolecules are, the weaker fibres result. Exceeding a well determinate depolymerization limit, the fibres weakness is so clear that the yarn loses tenacity (Marten method).

As regards the racemic modifications, they could be also obtained through a process just called racemization leading to a mixture containing 50% of each enantiomorph. The racemization could be realised through many ways. It consists in reactions leading to reversible break-up of one of the asymmetrical carbon atom links with formation of a symmetric species. The continue repetition of this process leads to a racemic mixture. Generally the racemization concept is connected with the loss of optical activity.

When only a chiral centre changes its configuration we have the epimerization, where the epimers has several chiral centres which differ only for the configurations of one of them.

The epimerization is a process that involves the conversion of glucose molecules. An example of such chemical method is the spontaneous transformation of a-glucose in a mixture containing 38% of glucose and 62% of b-glucose.

The deterioration degree of a cellulose material could be verified and determinate applying reference parameters of chemical and physical nature in experimental laboratory tests.

To complete the research some comparison elements will be inserted.


Hydrogen Bond's Effects

When a constituent is able to react with the acid or basic centre of a molecule through the formation of hydrogen bonds, significant alterations of the acid or basic strength of the same molecule itself are possible. For instance for some benzoic acids are observed from the pK and from the pH; they are very strong elements and one can establish the parameter that the salicylic acid is 30 times stronger than the p-hydroxide-benzoic acid. However the acids in which the ionisation concerns the carbon-hydrogen bond (called therefore also acids of the carbon) are of particular importance both for their specific characteristics and for the growing importance that the bases have in organic chemistry.

Experimental proofs of laboratory by means of the refractometer COSMO type K-0032 n. 8237 for sugary solutions have given fairly significative results.



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