Notizen: Conformation Analysis of the Polypeptides in the Thylakoid Membrane
Zeitschrift für Naturforschung. B, A journal of chemical sciences
The ultraviolet circular dichroism spectrum of a chloroplast fraction of Antirrhinum ma jus has recently been published The fraction was obtained from stroma-freed chloroplasts by ultrasonic treatment and fractioning centrifugation 2 -3 . The spectrum shows extrema of ellipticity at 194 ( + ), 208 (-) and 222 ( -) nm. Thus it is similar to a protein with a considerable a-helix content. GREENFIELD and FASMAN 4 have calculated circular dichroism spectra for proteins of different conformations by
... t conformations by linear superposition of reference spectra. These authors used poly-L-lysine in a, ß and random coil conformations for reference. A comparison with the spectra of GREENFIELD and FAS-MAN 4 shows that the spectrum of the fragments of the thylakoid membrane closely resembles that of a protein with 42 percent a-helix, 40 percent random coil and 18 percent ^-structure ( Fig. 1 ). The biggest dif- Fig. 1. Comparison of the experimental (• • •) with a calculated ( ) spectrum obtained by linear superposition of polylysine reference spectra according to GREENFIELD and FASMAN. The 121 points which mark the experimental curve are arithmetic means of 5 -10 registrations from 5 preparations. ference between the experimental and calculated spectra consists in a red shift of the positive extremum from 191 to 194 nm. In order to exclude experimental errors in this comparison, we have measured the spectra of poly-L-lysine in its three conformations with the same apparatus 1 in which we measured the spectra of the thylakoid fragments. The preparative methods have been described previously 2> 3 . A comparable red shift, besides other distortions, has been described several times for optical rotation dispersion and circular dichroism spectra of mem-Requests for reprints should be sent to Prof. Dr. W. MENKE, MPI für Züchtungsforschung, D-5000 Köln 30. branes. This red shift has recently been attributed to light scattering at the membranes 5-12 . However, the approximate average particle size in the suspensions used in this investigation is only 100 Ä. Consequently, light scattering should not be the main cause of the observed red shift. An attempt to more accurately determine the conformation parameters, by means of a linear-least-squares-fit, did not yield a better fit to the experimental spectrum, because its characteristic shape was not maintained in the calculated spectrum (Fig. 2) . However, a closer fit is obtained if one allows a shift of the reference spectra on the wavelength-scale (Fig. 3) . Fig. 2. Approximation of the experimental spectrum by means of a linear-least-squares-fit. The conformation parameters Pi of the theoretical function y (A) = Pa • Ra (2) + Pß-Rß (A) + Py-Ry(2) are varied to give the minimum sum of error squares. Ra, R/3, Ry are the reference spectra for a-helix, ßstructure and random coil. The fit was obtained by computer. 4> o 20 «V E o o © Fig. 3. Approximation of the experimental spectrum by means of a non-linear-least-squares-fit of the chloroplast circular dichroism spectrum. The theoretical function is: y (A) = Pa-Ra(A-Aa)+Py-RyU-Ay). Ai are the shift parameters. Aj<0 means a shift to longer, Aj>0 means a shift to shorter wavelengths.