The performance of solar energy conversion into alternative energy sources in artificial systems highly depends on the thermostability of photosystem I (PSI) complexes Terasaki et al. (2007), Iwuchukwu et al. (2010), Kothe et al. (2013). To assess the thermostability of PSI complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus heating induced perturbations on the level of secondary structure of the proteins were studied. Changes were monitored by Fourier transform infrared (FT-IR) spectra in the mid-IR region upon slow heating (1 degrees C per minute) of samples in D2O phosphate buffer (pD 7.4) from 20 degrees C to 100 degrees C. These spectra showed distinct changes in the Amide I region of PSI complexes as a function of the rising temperature. Absorbance at the Amide I maximum of PSI monomers (centered around 1653 cm(-1)), gradually dropped in two temperature intervals, i.e. 60-75 and 80-90 degrees C. In contrast, absorbance at the Amide I maximum of PSI trimers (around 1656 cm(-1)) dropped only in one temperature interval 80-95 degrees C. The thermal profile of the spectral shift of alpha-helices bands in the region 1656-1642 cm(-1) confirms the same two temperature intervals for PSI monomers and only one interval for trimers. Apparently, the observed absorbance changes at the Amide I maximum during heating of PSI monomers and trimers are caused by deformation and unfolding of alpha-helices. The absence of absorbance changes in the interval of 20-65 degrees C in PSI trimers is probably caused by a greater stability of protein secondary structure as compared to that in monomers. Upon heating above 80 degrees C a large part of alpha-helices both in trimers and monomers converts to unordered and aggregated structures. Spectral changes of PSI trimers and monomers heated up to 100 degrees C are irreversible due to protein denaturation and non-specific aggregation of complexes leading to new absorption bands at 1618-1620 cm(-1). We propose that monomers shield the denaturation sensitive sides at the monomer/monomer interface within a trimer, making the oligomeric structure more stable against thermal stress. (C) 2017 Elsevier B.V. All rights reserved.