Changes in the secondary structure of apolipoprotein B-100 after Cu2+-catalysed oxidation of human low-density lipoproteins monitored by Fourier transform infrared spectroscopy.

Fourier transform infrared (FTIR) spectra have been obtained of human low-density lipoproteins (LDL) in H2O and 2H2O buffers. The absorption bands are assigned to vibrations of the lipid and apolipoprotein B-100 components. The analysis of second-derivative spectra allowed an assignment of individual protein bands to alpha-helical, random, coil or beta-structure and beta-turn conformations. Changes in the FTIR spectra after Cu2+-catalysed oxidation of the LDL particles indicate that the structure of apolipoprotein B-100 becomes less ordered, with some alterations of alpha-helical and beta-turn conformation. The main beta-structure absorption at 1620 cm-1 is unaffected by oxidation. Taking into account the resistance to oxidation and the slow H-2H exchange it is suggested that the beta-structure is hidden from external factors whereas other structures are mostly present on the surface of the LDL particle. Oxidation affects mainly the surface region of apolipoprotein B-100 and leads to a structural rearrangement which consequently changes the receptor specificity of the LDL.