Evidence for an .alpha.II-type helical conformation for bacteriorhodopsin in the purple membrane

Abstract

Bacteriorhodopsin in native purple membrane is generally thought to be approximately 80% a-helical. However, all published far-UV circular dichroism spectra for purple membrane suspensions have been found to differ, both in shape and in magnitude, from published spectra of soluble proteins such as myoglobin, whose structure has been established as 80% a-helical by X-ray diffraction techniques. This has been interpreted as evidence that (1) bacteriorhodopsin has considerable 0-sheet content or (2) optical artifacts are significant in the circular dichroism spectrum. It is proposed that this discrepancy is in fact more consistent with a substantial alI-type conformational character for the protein a-helices. Although a-helical proteins are generally envisioned as having a classical aI-type geometry in which the amide planes are all nearly parallel to the helical axis, there is no steric barrier to changing the dihedral angles so that the amide planes become significantly tilted with respect to the helical axis. The hydrogen-bonding character of the helices, however, will be affected, so this transition may require energy input. If the amide planes tilt far enough to eliminate hydrogen bonding along the helix, the structure is considered to be aII. The conclusion that bacteriorhodopsin has significant aII character is strongly supported by oriented film circular dichroism studies presented here. In addition, assuming a substantial aII-type rather than an exclusive aI-type conformation for the bacteriorhodopsin can result in excellent agreement between circular dichroism and infrared linear dichroism techniques as regards estimated helix tilt angles both before and after bleaching of the purple membrane with hydroxylamine in the presence of light.

Isomerization of retinal chromophore and conformational changes in membrane protein bacteriorhodopsin by solubilization with a mild non-ionic detergent, n-octyl-β-glucoside: an FT-Raman and FT-IR spectroscopic study

“…From theoretical model calculations, Krimm and Dwivedi [42] proposed that the anomalous position of the amide I band is attributable to distorted helical structure with weaker hydrogen-bonding in the peptide backbone, referred to as a II helix. The a II helical structure was proposed by other spectroscopic studies utilizing circular dichroism [43,44], and Raman [44] and nuclear magnetic resonance [45]. The shoulder near 1640 cm À1 is due to b-sheet in the loop region on the extracellular side.…”

Rational design of peptide nanotubes for varying diameters and lengths

“…The molar ratios of the left‐ and the right‐handed helix in the nanotubes as estimated from the intensities of the Cotton effects are 1/0.97 for S25D12 / S24L14 , 1/0.95 for S25D12 / S22L16 and 1/0.93 for S25D12 / S25L20 , confirming that the nanotubes are composed of equimolar mixtures of the peptides. Furthermore, the increased ratio of the intensity at 222 nm compared to that at 208 nm indicates a helix bundle structure, which results from the strong association of neighbouring helices24.…”

Cation recognition by self-assembled monolayers of oriented helical peptides having a crown ether unit

“…20 In addition, the complexation of LipoA16Cr with K ϩ strengthens the molecular property of primary amphiphilicity. 21 The amphiphilic property should facilitate aggregation of the peptide, leading to increase in the magnitude of the negative Cotton effect around 222 nm. On the other hand, CrA16lipoa did not change the molar ellipticity around 222 nm linearly with K ϩ concentrations.…”