The synthesis of high n~olecular weight (100 000 to 200 000) polymers and copolymers of L-tryptophan and y-benzyl-L-glutamate is reported. The optical rotatory dispersion (0.r.d.) of these polypeptides is recorded in the wavelength range 540-320 mp and the bo values of the Moffitt equation, using X O = 212 mp, are listed. Poly-L-tryptophan has a bo value of f 570 in dimethylforman~ide (DMF). A linear relationship exists between this value, bo values of copolymers of various ratios of L-tryptophan and y-benzyl-L-glutamate, and the value of -670 found for poly-r-benzyl-~-glutamate. T h e 0.r.d. curve of a poly-L-tryptophan film, in the 330-200 mp wavelength range, reveals two positive Cotton effects in the 370-290 m p region and a large negative Cotton effect a t 233 mp. Thus, despite the positive bo value, these data prove that poly-L-tryptophan, in DMF, has the right-handed helical conformation. Hypochromicity was found for the tryptophanyl residue in the helical polypeptide. T h e rotatory contribution of chromophores, such as tryptophan or coenzymes, when bound asymmetrically to a protein, can be very significant, and caution is advised in the interpretation of such 0.r.d. curves.Optical rotatory dispersion (0.r.d.) measurements have become extensively used to determine the conformation of proteins (1, 2, 3). The anomalous rotatory dispersion curves for many synthetic poly-a-amino acids and proteins in the helical conforn~ation have been adequately interpreted by the Moffitt equation (4, 5) [nz1]x = aoXo2/(X" Xo2) + bOXo4/(X2 -kg2)?. Although it is acknowledged that this treatment of 0.r.d. data is of a highly empirical nature, the constant bo in this equation has been successfully used as a measure of the helical content in polypeptides and proteins. The bo value of approximately -630 to -670 (XO = 212 mp) has been found for the 100% right-handed helical conformation (5, 6), while the random coil form has a bo value of zero (6). The sign of the bo has been taken to indicate the sense of helix, and has been corroborated by X-ray studies (7). Estimates of helical content derived from 0.r.d. data (8, 9) agree remarkably well with X-ray data for myoglobin (10). More recently a modified Drude equation (11) has been proposed to estimate helical contents in polypeptides based on 0.r.d. studies penetrating further into the ultraviolet region. However, this treatment is theoretically inadequate, as a new Cotton effect has since been discovered (Holzwarth and Doty (12) have shown that the 233 mp Cotton effect is a composite of two such bands and that two circular dichroism bands are responsible for this Cotton effect, one caused by a n -+ n* transition and the other caused by a n -+ n* transition), which liltewise places
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