CONFORMATION OF HUMAN SERUM APOLIPOPROTEIN A-I(166-185) IN THE PRESENCE OF SODIUM DODECYL-SULFATE OR DODECYLPHOSPHOCHOLINE BY H-NMR AND CD - EVIDENCE FOR SPECIFIC PEPTIDE-SDS INTERACTIONS

The segment, YSDELRQRLAARLEALKENG, corresponding to residues 166 to 18 5 of human serum apolipoprotein A-I, was studied by circular dichroism and NMR spectroscopy in sodium dodecyl sulfate and dodecylphosphochol ine micelles. 2-Dimensional NOESY, TOCSY and DQF-COSY spectra of apoA- I(166-185) in perdeuterated sodium dodecyl sulfate (SDS-d(25)) and dod ecylphosphocholine (DPC-d(38)) micelles were collected at a peptide/SD S (DPC) ratio of 1:40. Similar CD spectra and NOE connectivity pattern s were observed for apoA-I(166-185) in SDS and DPC, indicating a simil ar helical conformation in both. Conformations of apoA-I(166-185) in D PC-d(38) micelles, and in SDS-d(25) micelles at two pH values, 6.6 and 3.7, were determined using distance geometry calculations. Backbone s uperposition (N,C-alpha,C=O) for an ensemble of twenty-nine structures in DPC at pH 6.0 gave a RMSD of 0.45+/-0.09 Angstrom for the region D 168 to K182, while for all atoms it was 1.60+/-0.17 Angstrom. In SDS, the ensemble of nineteen structures each at pH 6.6 and 3.7 gave RMSDs of 0.28+/-0.07 Angstrom and 0.35+/-0.10 Angstrom, respectively, for th e region D168 to K182. RMSD for superposition of all atoms was 1.36+/- 0.10 Angstrom and 1.38+/-0.21 Angstrom at the respective pH values. In all cases a highly defined class A amphipathic helical structure was found for the region R171 to K182. Since the same structure occurs in micelles with either negatively charged or zwitterionic head groups it strongly suggests a dominant role for hydrophobic interactions in sta bilizing the complex. The Y166 aromatic ring is bent back upon the hel ix axis at the lower pH. NMR determination of pK(a) values for D168, E 169, E179 and E183 in the presence of SDS or DPC indicated a micro-pH at the micellar surface approximately one pH unit higher than the norm al residue pK(a). SDS interactions with the peptide were examined by c ollecting H-1 NOESY spectra in the presence of protiated SDS. Residues R171, R173, R177, as well as the aromatic ring of Y166, were shown by intermolecular NOE measurements to interact with SDS, hence a key int eraction in stabilizing the complex appears to be between interfacial basic side-chains and SDS alkyl chains.