Para-amino benzoic acid (PABA) is considered to be in the
B-complex vitamin family.
The human body can make it from folic acid, since
PABA forms the middle part of that vitamin:
folic acid, showing PABA inside
This means that in the strict sense, it is not a vitamin,
since the body can make it. In this sense, however, Vitamin A
is not a vitamin, since the body makes that from
Bacteria in the intestines produce folic acid if there is
PABA present. In fact, since PABA is needed by microbes,
but is not strictly necessary for humans, a class of drugs
that simulate PABA can fool the microbes into trying to use
then instead of PABA, which causes them to die. This class
of drugs is the sulfa drugs, and that is why people taking them
are advised not to take PABA at the same time.
PABA is taken orally in vitamin supplements. However, its most
widely known use is as a sunscreen. Taking it orally will not
protect from the sun, as the sunscreen function is purely a matter
of PABA acting as a dye that absorbs ultraviolet light. You need
to paint it on the skin for it to block the sun.
PABA is acidic, and can sting if it gets in the eyes. Some people
are sensitive to PABA when it is applied to the skin. PABA also
darkens to stain clothing. For these reasons modifications to PABA
have become popular as sunscreens. Reacting PABA with long chain
alcohols to form PABA esters like
polyethylene glycol 25 PABA
PEG-25 PABA ester
can eliminate the irritation and staining problems. Other PABA
esters are glycerol PABA, padimate A, and padimate O.
Molecules absorb light when the light's wavelength is just the
right size to cause electrons in the molecule to vibrate in time
with the light. The electrons resonate in the molecule.
In some molecules, the electrons are not bound to a single atomic
nucleus, but instead roam free across several nuclei, in what are called
resonance bonds. You may notice that sometimes a six-sided ring
is shown with alternating double and single bonds, while at other
times it is shown with a circle inside. Both forms are showing
the same thing. The circle just draws our attention to the fact
that the bonds don't really alternate between double and single,
but they are more like 1½ bonds -- the electron spends
half of the time in one place, and half of the time in the other,
In para amino benzoic acid, there is another resonance structure
right next to the six-sided ring. It is a carboxyl group,
shown with a single bond between carbons, and a double bond between
the carbon and the oxygen. This is also a place where the electron
can bounce around between the three nuclei.
It is even possible for an electron to move back and forth from
one end of the molecule to another, across all of the resonance
bonds. This lets it slosh back and forth, like water in a bathtub.
In the case of para amino benzoic acid, the rhythm of the sloshing
matches the frequency of UV-B light. The electron can move in time
with the light wave, and absorb its energy. The energy is later
released as photons of longer wavelength light, such as heat.
Knowing this, we can design other molecules that have resonance
structures that will catch UV-B rays.
One such molecule is benzophenone,
and a derivative called benzophenone-3, also known as oxybenzone.
Notice that both molecules have the six-sided ring (called a
benzine ring, and a carboxyl group (the carbon-oxygen
double bond). These molecules also protect against UV-A.
You can see that they have longer chains of resonance bonds,
and so will resonate at longer wavelengths.
Getting more creative with our chemistry, we can add other
desirable features to our molecules. We can add long chains
of hydrocarbons to make it mix better with suntan oils, and
spread easily on the skin. We can modify the resonance bonds
a little bit, to get a broader absorption range, into the
longer wave UV-A region.
Such a designer molecule is octocrylene and a similar
one known as avobenzone
Other molecules with the benzine ring and carboxyl group also
have side chains added to make them better sunscreens. Two
popular ones are
Homomethyl salicylate, also known as homosalate
Other salicylates used as sunscreens are
4-isopropylbenzyl salicylate, and
triethanolamine salicylate, which goes by the tradename
Another class of UV-B absorbers is the cinnamates.
These molecules are not water soluble, and stick to the skin
well, and are good for water resistant sunscreens. Others in
this family are
Isopentenyl-4-methoxycinnamate also known as
Isoamyl 4-methoxycinnamate, and
As you can see, there are many molecules that will absorb
ultraviolet light. There are many more in use than I have
described here. But they all have resonance bonds in common.
PABA: InChI=1/C7H7NO2/c8-6-3-1-5(2-4-6)7(9)10/h1-4H,8H2,(H,9,10)/f/h9H octyl methoxycinnamate: InChI=1/C18H26O3/c1-4-6-7-15(5-2)14-21-18(19)13-10-16-8-11-17(20-3)12-9-16/h8-13,15H,4-7,14H2,1-3H3/b13-10+ folic acid: InChI=1/C19H19N7O6/c20-19-25-15-14(17(30)26-19)23-11(8-22-15)7-21-10-3-1-9(2-4-10)16(29)24-12(18(31)32)5-6-13(27)28/h1-4,8,12,21H,5-7H2,(H,24,29)(H,27,28)(H,31,32)(H3,20,22,25,26,30)/t12-/m0/s1/f/h24,26-27,31H,20H2 benzine: InChI=1/C6H6/c1-2-4-6-5-3-1/h1-6H triethanolamine: InChI=1/C6H15NO3/c8-4-1-7(2-5-9)3-6-10/h8-10H,1-6H2