UV Transition Moments of Tyrosine

To assist polarized-light spectroscopy for protein-structure analysis, the UV spectrum of <i>p</i>-cresol, the chromophore of tyrosine, was studied with respect to transition moment directions and perturbation by solvent environment. From linear dichroism (LD) spectra of <i>p</i>-cresol aligned in stretched matrices of poly­(vinyl alcohol) and polyethylene, the lowest π–π* transition (L_b) is found to have pure polarization over its entire absorption (250–300 nm) with a transition moment perpendicular to the symmetry axis (C₁–C₄), both in polar and nonpolar environments. For the second transition (L_a), polarized parallel with the symmetry axis, a certain admixture of intensity with orthogonal polarization is noticed, depending on the environment. While the L_b spectrum in cyclohexane shows a pronounced vibrational structure, it is blurred in methanol, which can be modeled as due to many microscopic polar environments. With the use of quantum mechanical (QM) calculations, the transition moments and solvent effects were analyzed with the B3LYP and ωB97X-D functionals in cyclohexane, water, and methanol using a combination of implicit and explicit solvent models. The blurred L_b band is explained by solvent hydrogen bonds, where both accepting and donating a hydrogen causes energy shifts. The inhomogeneous solvent-shift sensitivity in combination with robust polarization can be exploited for analyzing tyrosine orientation distributions in protein complexes using LD spectroscopy