Helix sense of poly- -benzyl-L-aspartate

Determination of molecular order in supported lipid membranes by internal reflection Fourier transform infrared spectroscopy

When polarized internal reflection infrared spectroscopy is used to determine molecular order in supported lipid membranes, the results are critically dependent on the accuracy of assumptions made about the evanescent electric field amplitudes in the membrane. In this work, we examine several expressions used for calculating evanescent electric field amplitudes in supported lipid monolayers and bilayers, and test their validity by measuring the infrared dichroism of poly-gamma-benzyl-L-glutamate and poly-beta-benzyl-L-aspartate under conditions in which their molecular order is known. Our results indicate that treating such systems as a simple single interface between two semi-infinite bulk phases is more accurate than the commonly employed thin-film approximation. This implies that earlier conclusions about molecular order in supported lipid membranes may require substantial revision.

Structure of synthetic peptide analogues of an eggshell protein of Schistosoma mansoni

The peptide (Gly-L-Tyr-L-Asp-L-Lys-L-Tyr)6, referred to as F4-6, was synthesized as a model for a schistosome eggshell protein in which the Gly-Tyr-Asp-Lys-Tyr consensus sequence is repeated over 40 times. Analysis by CD, Fourier transform infrared spectroscopy, potentiometric and spectrophotomertric titrations, NMR, and molecular modeling suggests that F4-6 forms some type of left-handed structure. A likely possibility appears to be a left-handed alpha-helix stabilized by Lysi-Aspi +4 salt bridges and possibly Aspi-Tyri +4 hydrogen bonding and Tyr-Tyr interactions. Spectroscopic studies of a number of F4-6 analogues support this conclusion. For example, substitution of D-Ala for Gly produces a peptide with enhanced left-handed helical spectral characteristics, whereas an L-Ala substitution results in a peptide with minimal structure. These studies suggest that the F4 protein from Schistosoma mansoni may be the first example of a naturally occurring protein devoid of proline and carbohydrate that forms a left-handed helix composed of L-amino acids, although alternative forms of other left-handed structures have yet to be rigorously excluded.

Infrared spectrum of the purple membrane: clue to a proton conduction mechanism?

The infrared spectrum of the purple membrane of Halobacterium halobium has amide I and amide A frequencies that are anomalously high for standard alpha-helical structures. Normal mode calculations indicate that these and other unusual features of the spectrum can be attributed to alpha 11-helices. Such structures suggest that the helix backbone may provide the framework through which proton transport takes place.