Evening star., 1937-07-10

New light is thrown on the complex
chemistry Of seeing in the progress re-
port of a ten-year physiological research
by DR.. Selig Hecht of Columbia Univer-
sity and his associates.
In the rods and cones of the retina
Of the eye is substance known as visual
purple. This is "bleached" when light
falls upon it and in the process impulses
are set up over the optic nerve which
travel to the back of the brain. This
visual purple. however, has the peculiar
property of building up its pigment again
in the dark so that the same process can
be repeated. This 1t does indefinitely,
as long as life lasts in the seeing Or
ganism.
extracted from the eye, but Dr. Hecht
and his co-Workers found that visual
purple could be made to act in the same
way upon exposure to light in solution
contained in test tube. Thus they can
duplicate experimentally the organic
process.
They made an intensive chemical study
of the material and have been able to
classify it definitely as one of the pro-
teinS.
DR.. Hecht and his co-workers OBJ
tanned visual purple from the eyes of
frogs and measured the rate at which
the substance in solution passed through
a porous glass. In this way they meas
ured the approximate dimensions of its
molecule. It's diameter turns out to be
about three -millionths of an inch
This shows visual purple Professor
Hecht says, Ito be large molecule of
the size hitherto found only for pro-
teins, one of those substances rich in
nitrogen which form the basic con-
stituents of all living material. It's
molecular weight is 10000 times that
of the hydrogen atom. Ordinary sub
stances like salts have molecular weights
less than 500. The smallest protein, egg
albumin, has a molecular weight of $4,000.
This is powerful evidence that visual
purple is really protein. Adding the
recent finding that visual purple Hber-
ates a carotenoio when it is bleached by
light or acted upon by chloroform, the
substance is definitely placed among the
proteins."
Bleaching solution Of visual purple by
placing them in the light, Professor
Hecht points out, long has been possible
in the laboratory, but only recently were
he and his colleagues able to work out
the precise conditions under which the
substance RECEIVERS its sensitivity to color
in the dark.
"A visual purple solution freshly pre-
pared from frogs" he says, lis pinkish
purple. When exposed to strong light
the color changes rapidly to yellow and
then very slowly becomes colorless. If
the bleached yellow solution is placed in
the dark at Is degrees centigrade and
examined an hour later it is found to be
distinctly pinkish again and this color
fades rapidly on exposure to light. This
procedure may be repeated several times.
I'VE have measured the course of this
regeneration in the solution by following
its concentration which highly sensi-
tive photoelectric spectrophotomete
which uses so little light that it leaves
the visual purple unaffected. Regen-
Grafton begins Immediately and proceeds
rapidly More than half the increase
in density is accomplished in the first
ten minutes, although the concentration
continues to increase for over an hour,
reaching = value of about 20 per cent
or more of the original solution.
There are two criteria for recognizing
visual purple. One is it's absorption
spectrum which In the case of frog visual
purple is q nearly symmetrical curve
The second is its sensitivity to light.
Within the errors Of the experiment.