QA determination

Judith Gallant judith_gallant at
Wed Jul 31 19:27:18 EST 1996

Dear Photosynthesizers,

I wish to make an accurate determination of QA with a PS II core complex
preparation.  I only have access to a photodiode array spectrophotometer
with continuous illumination.  Using a chlorophyll concentration of 17 uM,
I observe a nice reproducible difference of around 0,004 absorbance units
at 325 nm between an illuminated and a dark-adapted sample.  But this
difference rises with illumination time ranging from 10 to 60 seconds (no
DCMU nor Tris wash).  Since there is no more plastoquinone pool, should I
use DCMU in order to prevent oxidation of QA by any plastoquinone still
anchored in the QB site?  When I do, the absorbance difference at 325 nm
does not change with illumination time, but I observe strange changes in
the difference spectrum around 270 nm which vary with illumination time.
If I use this inhibitor, should I perform a Tris wash in order to eliminate
the manganese cluster and prevent interference from any possible S state
transitions?  What illumination time should I use before measuring the
reduced absorbance spectrum?  In the van Gorkom article (BBA, 1974,
347:439-442), it is written:  Its (QA) reoxidation in the dark was
inhibited by DCMU, except in the presence of DPPH.  This DPPH
(1,1-diphenyl-2-picrylhydrazyl) was used to measure the light minus dark
difference spectrum in the 250 to 450 nm region.  What is the effect of
this reducing product on PS II?

Thanks in advance for all your help.

Judith Gallant
Graduate student

Departement de Chimie-Biologie
Universite du Quebec a Trois-Rivieres
3351, boulevard des Forges
C. P. 500, Trois-Rivieres
Quebec, Canada
G9A 5H7

Tel: (819) 376-5170 ext 3333
Fax: (819) 376-5057
e-mail: judith_gallant at

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