INFRARED-SPECTROSCOPY OF HUMAN APOLIPOPROTEIN FRAGMENTS IN SDS D2O - RELATIVE LIPID-BINDING AFFINITIES AND A NOVEL AMIDE-I ASSIGNMENT/

Infrared absorption spectra are reported for six apolipoprotein fragme nts in SDS/D2O. Five of the peptides correspond to proposed lipid-bind ing domains of human apolipoproteins [apoC-I(7-24), apoC-I(35-53), apo A-II(18-30)+, apoA-I(166-185), apoE(267-289)], and the sixth is the de novo lipid associating peptide LAP-20. The amide I infrared absorptio n patterns are generally consistent with predominantly helical structu res (as determined previously by NMR spectroscopy and distance geometr y calculations) and further suggest that apoA-I(166-185) and apoE(267- 289) are bound to SDS relatively weakly in comparison to the other fou r peptides. The latter conclusion is also supported by the temperature dependence of the infrared spectra, as increasing temperature promote s a distinct increase in random coil structure only for apoA-I(166-185 ) and apoE(267-289). In addition to features readily ascribed to helic es, the infrared spectra of all the peptides show absorptions in the s pectral region 1630-1635 cm(-1) that is usually associated with beta-s tructure, a motif that is clearly absent from the NMR-derived structur es. Parallel difficulties also arose in the analyses of the circular d ichroism spectra. We suggest that both the low-frequency infrared abso rptions and the ambiguities in interpreting the CD spectra may be due to unusual structures at the peptide C-termini, involving C=O groups t hat form hydrogen bonds simultaneously either with two solvent molecul es or with donors from the backbone (NH) and the solvent (OH). Analogo us absorptions may be a general feature of solvent-exposed helices, wh ich suggests a need for caution in assigning amide I bands below 1640 cm(-1).