[Determination of the secondary structure of proteins from their circular dichroism spectra. I. Protein reference...

It is shown that to obtain the protein-derived basic CD spectra for alpha-helical, beta-structural and irregular regions it is necessary to use a common criterion for isolation of secondary structures for all the reference proteins. Using the "rigid" criterion proposed by Finkelstein, Ptitsyn, Kozytsyn and the "mild" one (as proposed by Levitt and Greer) isolation of the alpha- and beta-structural regions for 5 reference proteins (myoglobin, lysozyme, ribonuclease A, papain, lactate dehydrogenase) has been carried out. Using the f alpha, f beta and f irregular thus obtained and the experimental CD spectra of these proteins, the basic CD spectra for alpha-helical, beta-structural and irregular regions in the proteins have been calculated. It is shown that the use of criterions common for all the proteins leads to good agreement between the calculated basic CD spectra for alpha-helical, beta-structural and irregular forms and the CD spectra of polylysine in the corresponding conformations. The secondary structure of the proteins studied has been analysed using the new protein-derived basic spectra and fairly good quantitative agreement with the X-ray data was achieved.