Effects of heat treatment on the protein secondary structure and pigment microenvironment in photosystem 1 complex

The protein secondary structure and pigments' microenvironment In photosystem I (PSI) complexes were studied in the temperature range of 25-80 degrees C using Fourier transform infrared (FT-IR) and circular dichroism (CD) spectroscopy, respectively. Quantitative analysis of the component bands of the amide I band (1700-1 600 cm(-1)) showed no significant change below 50 degrees C. However, apparent conformational changes occurred at 60 T and further continued at 70 and 80 T accompanied with transitions of secondary structure Mainly from alpha-helix to the beta-sheet structures. CD analysis demonstrated that the regular arrangement, viz. protein microenvironment of pigments of PSI complexes, was destroyed by heat treatment which might come from the changes of protein secondary structure of PSI. The CD signals at 645 nm contributed by chlorophyll (Chl) b of light-harvesting complex I (LHC1) were easily destroyed at the beginning of heat treatment (25-60 degrees C). When temperature reached 70 and 80 degrees C, the CD signals at 478 nm contributed mainly by Chl b of LHC1 and 498 nm contributed by carotenoids decreased most rapidly, indicating that LHC1 was more sensitive to high temperature than core complexes. In addition, the oxygen uptake rate decreased by 90.81% at 70 degrees C and was lost completely at 80 degrees C showing that heat treatment damaged the regular function of PSI complexes. This may be attributed to heat-Induced changes of pigment microenvironment and protein secondary structure, especially transmembrane (alpha-helix located in PsaA/B of PS1.