Colloids and Surfaces A: Physicochemical and Engineering Aspects 209 (2002) 185 – 192 Temperature-induced changes in the surface electric properties of thylakoids and photosystem II membrane fragments A. Dobrikova a , I. Petkanchin b , S.G. Taneva a, * a Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G. Bonche Str., bl.21, Sofia 1113, Bulgaria b Institute of Physical Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonche Str., bl. 11, Sofia 1113, Bulgaria Abstract The effect of heat treatment on the surface electric properties of thylakoid membranes and photosystem II-enriched membrane fragments is studied by electric light scattering. The electric dipole moments and the dimensions of both thylakoid and photosystem II-enriched membranes are shown to depend strongly on the temperature. The thermal-in- duced decrease in the electric dipole moments of thylakoids and photosystem II fragments correlates with heat-in- duced inactivation of the photochemical activity of photosystem II and structural reorganization of the macroassemblies in thylakoid membranes. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Thylakoid membranes; PS II-enriched fragments; Heat stress; Electric light scattering; Electric dipole moments www.elsevier.com/locate/colsurfa 1. Introduction The thylakoid membranes, which make up the chloroplast inner membrane system of higher plants, consist of multicomponent pigment – protein complexes participating in transformation of light energy to chemical energy. The major photosynthetic complexes are two photosynthetic reaction centers (PS I and PS II) and their closely associated light-harvesting complexes (LHC), cy- tochrome b 6 /f complex and ATP-synthase (CF 0 – CF 1 ) [1–3]. The photosynthetic membranes are highly sen- sitive to environmental factors such as tempera- ture, electrolyte concentration, light intensity etc. [4,5]. Heat treatment of thylakoid membranes at temperatures above 30 °C leads to changes in their structural arrangement and functional activ- ity [4,6]. The water-splitting system (18, 24 and 33 kDa proteins), which is exposed on the inner thylakoid surface is the most sensitive component. The primary cause of thermal inactivation of the oxygen evolution has been assigned to release of 33 kDa extrinsic protein [7,8]. It has been sug- gested that the denaturation temperature of iso- lated PS II core complex is around 60 °C [9], while the proteins of LHC II become denatured at Abbreiations: Chl, chlorophyll; LHC II, light-harvesting complex II; p  , permanent dipole moment; PS I,II, photosys- tem I,II;   , electric polarizability. * Corresponding author. Fax: +359-2-971-2493 E-mail address: sgtaneva@obzor.bio21.bas.bg (S.G. Taneva). 0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0927-7757(02)00179-6