Re: Help: What material has ESR signal?

From: Steve Harris sbharris_at_ROMAN9.netcom.com (sbharris_at_ix.netcom.com)
Date: 10/08/04 

Date: 7 Oct 2004 17:54:32 -0700

Uncle Al <UncleAl0@hate.spam.net> wrote in message news:<41658AB5.86B4C82B@hate.spam.net>...
> Jim wrote:
> >
> > Uncle Al <UncleAl0@hate.spam.net> wrote in message news:<4162D195.9A42B76E@hate.spam.net>...
> > > Jim wrote:
> > > >
> > > > In another thread
> > > > http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&threadm=78034be6.0409301210.404cbc86%40posting.google.com&prev=/groups%3Fnum%3D25%26hl%3Den%26lr%3D%26ie%3DUTF-8%26group%3Dsci.optics%26start%3D25
> > > > the sources of ESR signal were discussed. However I am not quite sure
> > > > of the conclusion, and then move some text there to this new thread to
> > > > continue the discussion.
> > > >
> > > > Uncle Al <UncleAl0@hate.spam.net> wrote in message news:<413E2969.AFF4C9E8@hate.spam.net>...
> > > > > Jim wrote:
>
> [snip]
>
> > > > Is the presence of unpaired electron the only requirement for
> > > > EPR(ESR)? if so, I am really confused as following:
> > > >
> > > > The electron configuration of Fe+2 and Fe+3 ions are [Ar]3d44s2 and
> > > > [Ar]3d5, having 4 and 5 unpaired electron respectively. Then any
> > > > material containing either Fe+2 or Fe+3 should be capable of ESR
> > > > detection, including hemoglobin.
> > > >
> > > > However I found from this paper [J. Peisach, "HEMOGLOBIN A: AN
> > > > ELECTRON PARAMAGNETIC RESONANCE STUDY OF THE EFFECTS OF INTERCHAIN
> > > > CONTACTS ON THE HEME SYMMETRY OF HIGH-SPIN AND LOW-SPIN DERIVATIVES OF
> > > > FERRIC ALPHA CHAINS", Proc Natl Acad Sci U S A. 1969 July; 63 (3):
> > > > 934?939]that, "Hemoglobin A, a protein containing four peptide
> > > > chains of two different types, alpha and beta, contains four heme
> > > > units per molecule, one in each chain. The iron atoms in the
> > > > functioning hemoglobin molecule, whether oxygenated or deoxygenated,
> > > > exhibit no electronparamagnetic resonance (EPR) signal."
> > > > ---The ferrous ion may not generate EPR at all.
> > > >
> > > > Are there more requirements for EPR signal besides unpaired electron?
> > > > Thank you very much!
> > >
> > > Hemoglobin is Fe(II) whether oxygenated or not. Why don't you tell us
> > > all where the unpaired spin is in low-spin Fe(II)?
> > Sorry for bad expressions.
> > I am just a beginner in this area, and am learning to use ESR.
> > Then do you mean that ferrous (Fe II) ion is in low-spin, and can
> > hardly generate EXR signal? And, the elctron configuration of Fe+2
> > ions is not [Ar]3d44s2, right?
>
> Why are you allowed near the instrument without knowing any chemistry?
>
> Fe(0) has 2 s electrons and 6 d electrons. Iron is the sixth
> transition metal. You read it off the Periodic Table. Fe(II) has 6 d
> electrons. Low-spin octahedral Fe(II) has no unpaired electrons as
> evidenced by its absence of allowed optical transitions. The color of
> hemoglobin is ligand to metal charge transfer. The orbital
> equivalences are determined by ligand geometry about the metal.
>
> Does 10 Dq ring a bell?

Comment:

Uncle Al is not being very helpful here, and does not address the root
of your problem. I suspect he doesn't really know the answer.

Indeed, hemoglobin does not undergo oxidation with O2 binding (though
free heme does!). The Fe(II) in hemoglobin has only 6 d electrons, no
s electrons. In any octahedral field (like that in heme) these 6 can
all go paired into the 3 lower d orbitals (so-called t2g orbitals),
with none above in the two eg orbitals (so called low spin).
Oxygenated hemoglobin is this type of low-spin ferrous complex, and
thus should have no EPR/ESR signal.

The problem is that deoxyhemoglobin contains Fe(II) which has the same
6 d electrons, is known to be in a *high spin* state, with at least 2
and possibly 4 unpaired electrons (i.e., either one or two electrons
get promoted from the 3 t2g ground state d orbitals to the 2 eg
orbitals, I don't know which-- probably the latter.) The switch in
spin-state with O2 binding by hemoglobin is one of the classical
differences between deoxy and oxyhemoglobin, and no doubt has been
studied by EPR/ESR. The deoxy version thus should indeed have an EPR
signal, albeit possibly a weak one. I think the abstract you quote is
simply wrong. EPR spectroscopy of deoxyhemoglobin I'm sure has been
done.

In any case, I don't know why the abstract (possibly erronously)
reports such an unexpected result for deoxyhemoglobin, either. But
I'll admit it.

SBH

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