Volume 214, number 1 CHEMICAL PHYSICSLETTERS 22 October 1993 Pressure effects on spectra of photosynthetic light-harvesting pigment-protein complexes Arvi Freiberg, Aleksandr Ellervee, Peeter Kukk, Arlentin Laisaar, M&t Tars and IGh Timpmann zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Insiitute ofPhysics, Estonian Academy ofSciences, Riia 142, 2400 Tartu, Estonia Received 9 July 1993 The influence ofhigh (up to 9 kbar) hydrostatic pressure on the absorption and fluorescence emission spectraof photosynthetic light-harvestingpigment-protein complexes isolated from purple bacteria Rhuduspirillum rubrum has been studied at room tem- perature and at 77 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA K. Under pressure at room temperature all spectral bands exhibit a red-shift at a rate of between 30 and 120 cm-‘/kbar for different bands. From these pressure shifts the compressibility of the protein matrix can be estimated. The com- pressibility is remarkably different for the protein surrounding bacteriochlorophyll a molecules ( zyxwvutsrqponmlkjihgfedcba KB 25 i 5 Mhar- ’ ) than for the one surrounding spimloxanthin molecules (ICC 10k 2 Mhar-’ ). This indicates that the elastic properties of a protein are locally specific. 1. Introduction Studies of proteins under pressure have become a continuously growing field of research activities due to the progress in determining the structure of sev- eral important proteins (in particular, the structure of the pigment-protein complexes of reaction centres in photosynthetic bacteria) and a better understand- ing of physical grounds of conformational states and the dynamics of proteins (for a review of pressure studies, see refs. [ 1,2] ) . Recently, a method of spec- tral hole-burning at low temperatures has been in- troduced into the field [ 3,4], which has improved spectral sensitivity by several orders of magnitude and has allowed experiments to be performed at very moderate pressure changes of only some bars U. In this Letter, the first results obtained by studying the influence of relatively high hydrostatic pressures (up to 9 kbar) of the absorption and fluorescence emission spectra of photosynthetic light-harvesting pigment-protein complexes at room temperature and at 77 K are presented. The complexes were isolated from the membranes of the purple bacteria Rhodos- pirillum rubrum. The building block of the complex ag I bak0.99 atm =O.l MPa= IO5N/m2. 10 is a heterodimer of two (a and /3) polypeptides, each consisting of 50-60 amino acid residues [ 5 1. Within the transmembrane segment an up pair binds two bacteriochlorophyll a (BChl) molecules and one car- otenoid (spirilloxanthin ) molecule. Light-harvest- ing complexes include larger aggregates of hetero- dimers, the actual size of aggregates still being under discussion. Yet, an (up)* basic unit seems to satisfy most of the criteria, including spectral characteris- tics similar to those of native photosynthetic mem- branes [6]. The primary goal of our study was to estimate the compressibility of photosynthetic proteins and to find out whether it is a common characteristic of the whole protein or whether the effect is local. BChl and spi- rilloxanthin molecules serve as internal probes of the local changes of the protein environment under pres- sure. Further we were interested in the possible changes of compressibility with pressure and tem- perature, By intuition one might expect both effects to be present. Protein is a glass-like structure with a lot of intermolecular free space. The loss of inter- molecular space in a compressed protein would lead to a less compressible medium. The same would happen on lowering the temperature. There are some investigations of pressure effects Elsevier Science Publishers B.V.