Biology Reference
In-Depth Information
1. He showed that in addition to the red-coloured, well-known
rhodopsin, there was yet another, purple-coloured rod photopigment
which he aptly named porphyropsin (Wald, 1937a).
It turned out that the land vertebrates and marine-fish species
possessed the rhodopsin system and freshwater vertebrates, the
porphyropsin system, while species which could live both in fresh
water and in one of the other habitats frequently possessed both
photopigments, mixed or in temporal succession.
As might have been expected on this evidence, the chromophores
(retinal) of the two rod photopigments were found to be very similar
in structure. The principle difference was an extra carbon-carbon
double bond in the porphyropsin chromophore (situated in the ring
chain), displacing the absorption spectrum about 22 nm towards the
red-end of the spectrum. It was also found that this change in structure
was accompanied by very little change in chemical behaviour; the
porphyropsin system constituted a bleaching and regeneration cycle
of precisely the same form as that of rhodopsin (see Wald,
1937a
,
1938
/1939,
1949a
,
1968
).
2. Wald also discovered the existence of a
cone
photopigment
in the chicken retina (Wald,
1937b
). Obviously, the concentration of
cone photopigments had to be very low relative to that of rhodopsin,
since the outer segment of the cones, in general, appeared quite
colourless. Wald, therefore, in his attempt to extract cone photopig-
ments selected the chicken retina which contained mostly cones.
Firstly, he irradiated the retinal extract of the chicken with
deep-red light (wavelengths above 650 nm) to which rhodopsin is
relatively insensitive, and when the deep-red light produced no further
bleaching effect, he exposed the residue to white light and thereby
produced a renewed bleaching. The substances which bleached in the
red and white light were assumed to be, respectively, cone and rod
(rhodopsin) photopigments. The cone photopigment appeared violet
in colour (maximum change in spectral absorption due to the red
irradiation was obtained at about 570 nm) and Wald therefore named
it iodopsin (visual violet).
