[Features of the structure of apolipoprotein A1 as an HDL and complexes with dimyristoylphosphatidylcholine. A method for low temperature ESR-spectroscopy].

Apolipoprotein A1 (apoA1) was labelled with the excess of 2,2,6,6-tetramethylpiperidinyl-N-oxyl-4-(2,4-dichloro-1,3,5- triazinyl-6)-amine at pH 9.8. The products, containing 7.2 (+/- 1) paramagnetic labels per 1 molecule of apoA1, showed spin-spin and dipole-dipole exchange interactions. The ESR spectra of the spin labelled A1 (at 77K) had d1/d values 0.76 (at pH 7.4), 0.59 (in 3 M NaCl), 0.55 (in 3 M guanidinium hydrochloride), 0.44 (in 40% 2-chloroethanol). Micellar complexes of spin labelled apoA1/DMPC (1 : 20 mol/mol and 1 : 190 mol/mol) and HDL containing spin labelled apoA1 were prepared. Comparison of ESR spectra at 77 K showed that apoA1 structure varies in the complexes with different stoichiometry and in spin labelled HDL. These data show the importance of hydrophobic protein-protein interactions for the structure of HDL and synthetic complexes of apoA1 with phosphatidyl choline.