Structurally modified chlorophylls in reaction centre(s)?

Philip Bacon pbacon at catep.demon.co.uk
Sun Jun 5 16:01:58 EST 1994


Could chemically- or enzymically-modified chlorophylls be 
present and have a role in photosynthetic reaction centre(s)?
-------------------------------------------------------------

I have not been active in chloroplast work for some 25 years, so am not
fully conversant with the latest research. I should nevertheless like to 
offload the following thoughts, just in case anyone out there were to 
consider them worth pursuing.

It has been shown (1,2,3,4) that 'changed' chlorophylls having, for example,
an -OH group at the 13^2 position (4), are artifacts arising from various
leaf treatments or chromatographic procedures.

I think, nevertheless, that this would not preclude the possibility of them
being naturally present in chloroplasts as a very small percentage of the 
total chlorophyll complement, and thus not easily detectable by 
chromatographic methods, but nevertheless fullfilling an important role 
in reaction centre(s). Indeed, one might almost expect any exposed 
chlorophylls in the chloroplast structure to be modified chemically or 
enzymically, given the relative ease of formation of 'changed' 
chlorophylls in vitro, and thus to legitimately exist as such in the 
choroplast (note that the in vitro absorption spectrum of the 13^2 -OH 
form is virtually unchanged from that of the parent molecule).

Although, if there is any validity in this assumption, more than one type
of 'changed' chlorophyll could be present (1,2), the 13^2 -OH form might
certainly be a candidate for an active role in the reaction centre,
given that the electron-donating potential of carbonyl oxygen is activated
by an -OH in the alpha position (5) (as is the case with 
13^1 carbonyl/13^2 -OH). Such a strong electron-donor may also be a very 
good acceptor, once an electron as been donated (see 5). The chemical 
reactivity of the 13^2 -OH itself may also be interesting, given that it 
has two carbonyl groups adjacent to it.

The question then arises -- does recent work UNAMBIGUOUSLY preclude the
possibility of e.g. a 13^2 -OH being present in one or more of the reaction
centre chlorophylls? If not, the idea might be worth pursuing by someone.
(I must confess that I do not know if the Deisenhofer et al. X-ray 
crystallography work would clearly distinguish an -OH from an -H, 
nor whether reaction centre chlorophylls can be extracted totally from 
reaction centre preparations for chromatographic or mass spectroscopic
analysis).

Thank you for your attention. I would welcome any comments, either via
this news group (bionet.photosynthesis), or by email, care of
pbacon at catep.demon.co.uk


References:

(1)  M.F. Bacon, Biochem. J., 1966, 101, 34c-36c
(2)  M.F. Bacon & M. Holden, Phytochemistry, 1967, 6, 193-210
(3)  H.H. Strain, J. Sherma & M. Grandolfo, Anal. Chem., 1967, 39, 926-932
(4)  M. Senge, A. Struck, D. Dornemann, H. Scheer & H. Senger,
     Z. Naturforsch., C., BioSci., 1988, 43, 515-518
(5)  A. Szent-Gyorgyi, Proc. Nat. Acad. Sci, 1967, 58, 2012-2014

-- 
Michael Bacon.
"Ojo del Sol"
San Antonio,
La Herradura, (Granada)
SPAIN

Tel: (34) 58 827608
Fax: (Attention M. Bacon) (34) 58 640422
Email: (Attention M. Bacon) pbacon at catep.demon.co.uk



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