Optical properties of polypeptides in the β‐conformation

The rotational strengths and oscillator strengths of the n π* band and ππ* exciton bands have been calculated for antiparallel and parallel β‐structures of varying length and width. The results are compared with experiment and with previous theoretical treatments of β‐structures. The generally good agreement of calculations on the antiparallel β‐structure with experimental results on poly‐ L ‐lysine and poly‐ L ‐serine indicates that these systems are indeed in the antiparallel conformation. It is found that the exciton component strongest in absorption shifts to longer wavelengths as the width of an antiparallel structure increases, and it is suggested that the position of the ππ* absorption band may be a useful criterion of sheet width. The results also reconcile the linear dichroism measurements of Rosenheck and Sommer on poly‐ L ‐lysine films with an anti‐parallel structure. Calculations on parallel β‐structures indicate that the CD spectra of this form will be rather similar to that of the antiparallel form. However, the major absorption band in the antiparallel form is associated with a small positive CD band, while in the parallel form it coincides with a large negative CD band. Finally, it is pointed out that the large positive CD bands predicted for single‐stranded parallel and antiparallel β‐structures at about 200 mμ render unlikely the suggestion that random‐coil polypeptides contain a substantial fraction of extended chain.