Secondary structure of the lac repressor headpiece. Possibilities and limitations of a joint infrared and circular dichroism study

The secondary structure of the short tryptic headpiece of the lac repressor has been investigated by the analysis of its infrared and circular dichroic spectra. For the latter we used the method of Provencher and Glöckner [Biochemistry (1981) 20, 33-37], which seems to be at present the most successful for the determination of the beta content of proteins. Nevertheless our results indicate that in the case of the lac repressor headpiece this method overestimates the amount of beta structure. We find that the headpiece contains an important helical content of about 50%, depending slightly on the ionic strength. A decomposition of the infrared spectrum in a sum of Gaussian curves reveals clearly the absence of a vibrational band around 1630 cm-1, excluding thus the presence of a multi-stranded beta-pleated sheet. The only beta structure compatible with the infrared results seems to be a two-stranded antiparallel beta sheet, as judged from our results on the beta-sheet model-compound gramicidin S. The unusually strong intensity of the amide I' band is in favour of the existence of such a structure. The quantitative analysis of both infrared and circular dichroism spectra indicates the presence of a certain (but different) amount of beta structure. Comparing these results with several secondary structure predictions, part of the helical residues should be located between Leu-45 and (at least) Arg-35, and an eventual two-stranded beta sheet should be situated in the N-terminal part of the headpiece.