part 4 analysis of polymers Flashcards
DEF polymers
A polymer is a chemical substance composed of macromolecules. A macromolecule is a molecule of high relative molecular mass and its structure is composed of multiple repetition of units of low relative molecular mass (monomers).
- Repeating unit
- Main chain (backbone)
- Side chain
Polymers can be linear, cross-linked, or branched.
DEF polysaccharides
Polysaccharides are naturally occuring polymers made up of many monosaccharide units per molecule.
Homopolysaccharides contain the same monosaccharide repeating unit. In glucans, like cellulose and starch, the repeating unit is D-glucose (cellulose: beta glycoside linkage, starch: alpha glycoside linkage).
Heteropolysaccharides containg more than one type of monosaccharides. Hemicellulose is one of the main component of wood, and it is made of different monosaccharides bond together. Gum is also a type of heteropolysaccharide.
DEF carbohydrates
Carbohydrates are polyhydroxyaldehydes and ketones and their derivatives. Carbohydrates chains are made of monosaccharides disaccharides (with a glycosidic bond), and polysaccharides.
How are monosaccharides composing carbohydrates categorized?
They are classified either by the position of their carbonyl group (other carbons are linked to hydroxy functionality)
- if simple sugar has an aldehyde group (carbonyl C last one in the chain) -> aldose
- if carbonyl internal to the chain, it forms a ketone group, and the simple sugar is called ketose
Or by the optical properties/family: D (hydroxy group on the right) and L (OH group on the left)
Glucose can also exist in a ring structure when the alcohol and the aldehyde are reacting together. The two possible conformation of these rings are alpha and beta.
DEF proteins
Proteins are long polyamide chains derived by the condensation of L-a-amino carboxylic acids (AA
amino acids).
The most common AAs are twenty and differ in the structure of R.
AA residues are joined by amide linkages, -NH-CO- often referred to as the peptide bond.
Many amino acids bond together = protein
A few amino acids = oligomers (peptide)
Proteins are composing egg, milk, silk, wool, animal/human tissue. They can be used in CH as paint binders, adhesives, textiles, parchment, leather, and tempera technique.
Categories of the structure of proteins + classification of proteins
The primary structure is the way in which the AAs are joined to one other in the chain.
The secondary structure is the spatial arrangement of the backbone (e.g. alpha helix).
The tertiary structure is 3D arrangement of the entire protein, including side chains.
Proteins are divided into two main groups. Fibrous proteins serve as structural materials, are long and thread-like with a tendency to lie side by side to form fibers insoluble in water. ex: collagen (chains arranged in triple helix), keratin, myosin in muscle. Globular proteins demonstrate a variety of functions such as enzymes, hormones, folded into compact spheroidal units, with the lipophilic part turned inward, soluble in aqueous solutions. ex: hemoglobin, albumin, insulin…
DEF Collagen
Collagen is the most abundant protein in vertebrates where it composes the connective tissues (bones, teeth, veins, cartilage, tendons). The collagen molecule consists of three polypeptide
chains arranged in a triple helical structure. Collagen has a distinctive composition of AAs rich in glycine, proline and 4-hydroxyproline (Hyp).
The 4-hydroxyproline is a non common AA. Hyp from hydroxylation of Pro in an enzymatic process.
Degradation of collagen leads to selective loss of proline and hydroxyproline, and an increase in glycine content.
Molecular marker:
- collagen
- wood
- cellulose acetate
- silk
- milk
- collagen: pyrrole, hydroxyproline (+ glycine, proline, usually condensation of proline and glycine in pyrolysis products ; proline residues…)
- wood: levoglucosan (main fragment after pyrolysis of cellulose)
- cellulose acetate: acetic acid CH3COOH (but common marker so not helping identification)
- silk: fibroin made of gly repeating units with Ala, Ser, Thr, Val
- milk: beta-lactoglobulin (specific of milk, half of AA residues are specie-specific, resistant to degradation)
Degradation of proteins
Aged proteins are denaturized. Denaturation is the disruption of the protein structure without the rupture peptide bonds, change of conformation affecting solubility and aggregation. It may be caused by heat and loss of water.
Protein may react with carbohydrates in a process called Maillard reactions (occur at temp higher than 140°C).
Hydrolysis may also reduce the MW and change the chemical structure of AAs.
Racemization converts L- into D-AAs. Microbic degradation is fast in the burial environment, much slower in painting layers.
A few examples of natural fibers
Natural fibers come from plants, animals, and minerals. Fibers are usually short, except for silk, which is continuous. Fibers are spun into yarns by spinning, which become fabrics by weaving.
- Linen: obtained from the stem of the flax plant and it is a crop. It was a primary fiber in Europe in the Middle Ages and the Renaissance was used extensively for clothing.
- Cotton: fibers growing in the boll (capsule) of the seeds of cotton plants.
- Keratin is one of the main components of wool. Keratin is the main component of appendages of some epidermal layers of vertebrates such as hair, horn, nails in mammals, and feathers in birds. Many keratin variants exist. In general two keratine polypeptides are arranged into alfa-helix
and they are twisted around each other to form a left-handed coil: an assemblage named coiled-coil. Several paired coiled coils interact to form packed structures of micro and macrofibrils. Adjacent polypeptide chains are linked together by disulfide bonds of cysteine residues. - Fibroin is the main component of silk. Fibroin is produced by some insects and arachnids to form cocoons, nests, webs, egg stalks. Silk fibroin is produced by the larvae (silkworms) of the moth Bombyx mori. The protein is rich in glycine units separated by serine and alanine residues (repeat units of of Gly-Ser-Gly-Ala-Gly-Ala). Beta-sheets are stacked forming layer with Gly opposing Ser and Ala. The fiber is strong (micorcrystalline), not extensible (breaking covalent bonds of chains) and flexible (adjacent layers associated by weak Van der Waals bonds).
A few examples of artificial and synthetic fibers
Synthetic fibers are man-made, usually from different sources (wood, oil, coal). They are continuous filament fibers. They are either regenerated by chemical-physical reactions followed by the restoration of cellulose (viscosa rayon, cupro, modal) or modified by permanent chemical transformation. This modification is a substitution of H in OH groups by chemical groups such as acyl groups (CH3CO), with different degrees of substitution per glucose unit (1-3). ex: cellulose nitrate (explosive) and acetate.
What is analytical pyrolysis?
Analytical pyrolysis is the characterization, in an inert atmosphere, of a material or a chemical process by a chemical degradation reaction induced by thermal energy. The thermal energy needs to be sufficient to cleave bonds: temperature above 300°C. The pyrolyzer is linked with the GC-MS system. Pyrolysis is generally carried out at temperatures around 500-600°C.
How is pyrolysis GC-MS carried out?
1) sample is inserted into a capsule (quartz tube)
2) microfurnace or electrically heated platinum filament are heated up in an inert atm (N2) to eliminate all traces of oxygen
3) pyrolysis of the sample then GC analysis
- GC = separation
- MS = detection
- pyrogram
The chemical structure of the pyrolysis products formed by thermal degradation are giving info on the chemical nature of the organic material. The detection of various products of pyrolysis can help to identify the original polymer.
What are the two main types of reactive pyrolysis?
Pyrolysis is performed in the presence of a reagent to produce the formation of diagnostic compounds amenable to GC-MS analysis.
- Pyrolysis methylation: Methylation with a methylating reagent, such as tetramethylammonium hydroxide (TMAH) is a common application combined with pyrolysis leading to the formation of methylated derivatives (thermally assisted hydrolysis and methylation THM-TMAH). Ex: TMH-TMAH a vegetable oil produced a series of FAMEs (methylated fatty acids). This technique may lead to some analytical errors due to pigment interferences in the analysis of siccative oils.
- Pyrolysis silylation: pyrolysis in the presence of a silylating reagent. This type of pyrolysis is used to decrease the polarity of the compound and improve chromatographic behavior. For example, pyrolysis can be conducted in the presence of hexamethyldisilazane (HMDS), which leads to the silylation of the OH group.
Identification of lignin
Pyrolysis methylation is particularly important for the analysis of lignin in wood. Lignin is a component of cell walls. Chemically, lignin is considered a polymer of substituted phenylpropane units. The main units composing lignin are: hydroxybenzene (Phenol ; H), Guaiacol (2-methoxyphenol ; G), and Siringol (2,6-dimethoxyphenol ; S).
Softwood, originating from gymnosperms (no flowers: conifers, gynko) is only composed of G units. Hardwood from dicotyledons angiosperms (mainly trees & shrubs) is composed of HSG lignin. Hardwood from monocotyledons (mainly herbaceous) is composed of HG lignin.
It is not possible to analyze lignin directly by gas chromato but those structural units are, after degradation.
