Question

What do the R_f values indicate about the relative solubilites of the pigments in the water and solvent phases?

Explain the relative solublities of chl b and chl a in the water and solvent phases on the basis of molecular structure.

Explain the relative solublities of b-carotene, lutein, and violaxanthin in the water and solvent phases on the basis of molecular structure.

Answer 1

Rf value is the distance travelled by a given component divided by the distance travelled by the solvent front.

Non-polar compounds move up the plate most rapidly (higher Rf value),

whereas polar substances travel up the TLC plate slowly or not at all (lower Rf value).

Factors that affect the Rf value is --solvent system

If different solvent systems are used, there will be different Rf values for each pigment because the solubility would change for each pigment as a result of a change in solvent type (and thus a change in the polarity of the solvent will changes solubility of the pigment).

Most photosynthetic pigments are insoluble in water but they are soluble in non-polar solvents such as ether, acetone and chloroform.,pigments travel ay different rates depending on the solubility .

It is possible to measure the solubility of a pigment in a solvent by measuring how far the pigment travels versus how far the solvent travels.this is called retention factor.

Rf values are characteristics of specific pigments and can be used to identify them,higher the Rf value less polar the compound

A compound not migrating from the origin would have an Rf of 0

A compound migrating with the solvent front would have Rf of 1.0

A compound migrating half as far as the solvent front would have an Rf of 0.5

Chlrophyll is not soluble in water ,the molecule contains other elements, including magnesium ,the long hydrocarbon (phytol) tail attached to the porphyrin ring makes chlorophyll fat-soluble and insoluble in water.Chlorophyll can be easily dissolved in acetone and alcohol.

Chlorophyll a and b differs in the functional groups attached to them .

Chlrophyll b is more soluble than chlorophyll a in polar solvents because of its carbonyl group.

Relative solublities of b-carotene, lutein, and violaxanthin:

it is a cyclohexenyl ring system,it is insoluble in the water ,Insoluble in ethanol, glycerol, propylene glycol; slightly soluble in boiling organic solvents such as ether benzene , carbon disulfide , methylene chloride.

Lutein:

it is insoluble in the water

it is mostly soluble in hydrophobic solvents mostly the tetrahydrofuran

It is biosynthesized from zeaxanthin by epoxidation,soluble in hydrophobic solvents like methanol.

The Rf values differ for b-carotene,lutein,violaxanthin due the variation in their structure and their solubilty .