[LMB] Chlorophyll

Joel Polowin jpolowin at hotmail.com
Sat Dec 25 20:07:17 GMT 2021

Markus Baur <baur at chello.at> wrote:
> i think the colour of the ersatz-chlorophyll would not only depend on 
> the metal itself, but also what exactly the surrounding molecule is 
> doing .. but i do not know enough about organic chemistry to make 
> predictions

My Ph.D. is in organometallic chemistry: structures that have at least
one metal atom in combination with an organic structure.  I'm out of
date, having not been able to work in the field for a couple of decades,
and I know very little biochemistry.  So take my comments with a *bit*
of salt.

Even on Earth, there are several variations of "regular" chlorophyll,
with differing absorption spectra.  I don't think there's any particular
reason why a biochemical "tree of life" -- the complete set of all
living things through history, originating with some basic organism
and branching out through multiple evolutionary paths -- couldn't run
to variations of chlorophyll that modify the absorption spectra to
different ranges.

Many of our plants have other pigments in addition to chlorophyll which
change their colours from the "standard" green.  We've got blue-green
and red algae, which are very distinctly not-green, though they're still
using the regular chlorophyll for their photosynthesis.

Chlorophyll and the heme in our blood have a common substructure,
the porphyrin ring, which is an organic ring with a metal atom at
its center.  Other metal atoms can go in there instead.  That affects
the colour, as do the various other organic bits that are attached to
the ring.  I can't think of a reason why a chlorophyll analogue couldn't
be based on a porphyrin using some metal atom other than magnesium.

There are compounds other than heme that some animals use for carrying
oxygen around -- most commonly, hemocyanin.  That's the blue pigment
that colours the blood of cephalopods.  It isn't porphyrin-based.  There
isn't any particular reason why a biochemical "tree of life" couldn't
come up with some non-porphyrin structure to do photosynthesis with.

As for sky colour... the blue is caused by Rayleigh scattering.  Light
passing through an atmosphere that's essentially transparent to it still
undergoes some scattering, as a fundamental effect.  A photon that isn't
passing directly from sun to eyeball can be deflected; that's why the
entire sky is lit up.  The degree of scattering varies as the fourth
power of the frequency of the radiation.  That is, the blue/violet end
of the visible spectrum is much more likely to be deflected.  But our
sun emits more blue than violet light, and our eyes are more sensitive
to blue than to violet, so we see our sky as blue.  Any planet with a
sun similar to ours, and an atmosphere similar to ours, without a lot of
non-transparent stuff suspended in it, is going to look like our sky.

There are several factors that could change that.  If there are coloured
gases in the atmosphere, that would (obviously) change the appearance.
If the local sun is much cooler or hotter than ours, and therefore
emits a lot less violet/blue than ours or a lot more violet than blue,
there would be different light to scatter.  If there's a lot of dust or
aerosol suspended, that also affects the scattering.  If there's a much
denser atmosphere than ours, too much of the blue/violet is scattered
away, rather than reaching us.

Our setting sun and its sky appear red because of the last effect.
Mars has a very thin atmosphere with lots of suspended dust, so its sky
is red.  When our sky has a lot of particulate pollution, such as smoke,
we also get redder skies.


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