No abstract

we combined circular dichroism, optical rotatory dispersion, and infrared data to give evidence for more ß (pleated sheet) structure than a structure in whole membranes of Mycoplasma laidlawii, strain A. The circular dichroism data exhibit a typical -helical pattern. However, when this pattern is subjected to computer analysis, using a curve-fitting program, the best fit to the membrane experimental curve is obtained with 56.0% ß, 30.1% a, and only 13.2% coil. Calculation of the Moffit-Yang a0 and ba parameters from optical rotatory dispersion data yields 30-45 % ß, 26 % a, and 30-45 % coil. Inc V circular dichroism and optical rotatory dispersion spectra of various cellular membranes have generally been interpreted to have shapes indicating substantial amounts of a helix and showing no evidence of ß structure. While circular dichroism spectra may not give evidence for ß structure, it would be difficult to disprove the presence of substantial amounts of beta structure on the basis of such spectra. Gratzer et al. (1968), Hammes and Schullery (1968), and Stevens et al.(1968) have shown that the circular dichroism bands and optical rotatory dispersion maxima of certain ß structures can have much lower intensity and be displaced to the red of the commonly used reference poly-L-lysine in the ß conformation. Thus it is possible that the circular dichroism minimum of the ß structure could actually be placed at a position normally assigned to the negative ( --) transition of -helical peptides (222-225 µ).Wallach et al. (1969) and Graham and Wallach (1969) have recently used this argument to account for evidence of ß structure in their infrared spectra of mitochondrial membranes in the dry-film state. They do this in spite of the fact that several authors (Urry et al., 1967; Stein and Fleisher, 1967; Wrigglesworth and Packer, 1968; Urry and Ji, 1968) have assumed that ß structure is lacking on the basis of optical rotatory dispersion and circular dichroism measurements.