The predicted UV-vis spectrum of (M )-oP 7 is an excellent match for the shapes of the experimental spectra of (S )-oP 7 ( t Bu) and (R)-oP 7 ( i Pr) (Figure ). The calculations show that a positive Cotton effect at 250 nm is associated with an M absolute twist sense of the helix, indicating that (S )-oP 7 ( t Bu) favors the M twist sense and (R)-oP 7 ( i Pr) favors P.