The assignment of Q_y(1,0) vibrational structure and Q _x for chlorophyll a

We used non-photochemical persistent spectral hole-burning at 1.4 K to investigate the Q_y(1,0) vibrational structure of Chl a in a water-soluble chlorophyll-binding protein (WSCP) which exhibits resolved structure in its broadband optical spectra. Franck-Condon vibrational overlap factors were determined from the vibrational hole-burning data and used to simulate the Q_y(1,0) spectra. The simulations were not able to accurately reproduce the details of the Q_y(1,0) spectrum. This indicates a breakdown of the approximations used for the analysis and demonstrates that vibrationally induced mixing of electronic states (vibronic coupling) is active for Chl a. By considering the inhomogeneous broadening and vibrational hole-burning phenomena in the Q_x and Q_y(1,0) region of Chl-WSCP in addition to magnetic circular dichroism data, we favor the traditional placement of Q_x at λ∼570-590 nm rather than the alternate assignment underneath the Q_y(1,0) absorption near λ∼615-630 nm.