P

PAGE (polyacrylamide gel electro-
phoresis) A type of *electrophore-
sis used to determine the size and
composition of proteins. Proteins are
placed on a matrix of polyacrylamide
gel and an electricÜeld is applied.
The protein molecules migrate to-
wards the positive pole, the smaller
molecules moving at a faster rate
through the pores of the gel. The pro-
teins are then detected by applying a
stain.

PAH Polyaromatic hydrocarbon. See
particulate matter.

palladium Symbol Pd. A soft white
ductile *transition element (see also
platinum metals); a.n. 46; r.a.m.
106.4; r.d. 12.02; m.p. 1552°C; b.p.
3140 ± 1 °C. It occurs in some copper
and nickel ores and is used in jew-
ellery and as a catalyst for hydro-
genation reactions. Chemically, it
does not react with oxygen at normal
temperatures. It dissolves slowly in
hydrochloric acid. Palladium is ca-
pable of occluding 900 times its own
volume of hydrogen. It forms few
simple salts, most compounds being
complexes of palladium(II) with some
palladium(IV). It was discovered by
William Woolaston (1766–1828) in
1803.

A

• Information from the WebElements site

palmitate(hexadecanoate)A salt
or ester of palmitic acid.

palmitic acid (hexadecanoic acid)
A 16-carbon saturated fatty acid,
CH 3 (CH 2 ) 14 COOH; r.d. 0.85; m.p.
63 °C; b.p. 390°C. Glycerides of

palmitic acid occur widely in plant

and animal oils and fats.

pantothenic acidA vitamin of the

*vitamin B complex. It is a con-

stituent of coenzyme A, which per-

forms a crucial role in the oxidation

of fats, carbohydrates, and certain

amino acids. DeÜciency rarely occurs

because the vitamin occurs in many

foods, especially cereal grains, peas,

egg yolk, liver, and yeast.

OH

C

H 2

N

H

C

H 2

C

H 2

OH

H OH

3 CH CH 3 O O

Pantothenic acid

papain A protein-digesting enzyme

occurring in the fruit of the West In-

dian papaya tree (Carica papaya). It is

used as a digestant and in the manu-

facture of meat tenderizers.

paper chromatography A tech-

nique for analysing mixtures by

*chromatography, in which the sta-

tionary phase is absorbent paper. A

spot of the mixture to be investigated

is placed near one edge of the paper

and the sheet is suspended vertically

in a solvent, which rises through the

paper by capillary action carrying the

components with it. The components

move at different rates, partly be-

cause they absorb to different extents

on the cellulose and partly because of

partition between the solvent and

the moisture in the paper. The paper

is removed and dried, and the differ-

ent components form a line of spots

along the paper. Colourless sub-

stances are detected by using ultra-