Electronic Circular Dichroism Spectroscopy of Jet-Cooled Phenylalanine and Its Hydrated Clusters

We obtained resonant two-photon ionization circular dichroism (R2PICD) spectra of jet-cooled phenylalanine (Phe) and its hydrated clusters (Phe­(H₂O)_<i>n</i>, <i>n</i> = 1–2) near the origin band of the S₀–S₁ transition. The R2PICD spectra of Phe exhibit well-resolved CD bands of six different conformers present in the jet, which vary in sign and magnitude depending on their conformations. We revised the previous structural assignments of the Phe conformers based on the comparison between the experimental and theoretical CD signs, infrared spectra, and rotational band contours. The R2PICD spectra of Phe­(H₂O)_<i>n</i> reveal that hydration with one or two water molecule(s) does not affect the CD signs of Phe conformers but significantly increases their CD magnitudes. Furthermore, conformational selection by solvation alters relative populations of Phe conformers, leading to a sign inversion in the CD spectra of Phe­(H₂O)_<i>n</i> compared with that of Phe monomer

Isomer-Specific Induced Circular Dichroism Spectroscopy of Jet-Cooled Phenol Complexes with (−)-Methyl l‑Lactate

Induced circular dichroism (ICD) is the CD observed in the absorption of an achiral molecule bound to a transparent chiral molecule through noncovalent interactions. ICD spectroscopy has been used to probe the binding between molecules, such as protein–ligand interactions. However, most ICD spectra have been measured in solution, which only exhibit the averaged CD values of all conformational isomers in solution. Here, we obtained the first isomer-selective ICD spectra by applying resonant two-photon ionization CD spectroscopy to jet-cooled phenol complexes with (−)-methyl l-lactate (PhOH-(−)­ML). The well-resolved CD bands in the spectra were assigned to two conformers, which contained different types of hydrogen-bonding interactions between PhOH and (−)­ML. The ICD values of the two conformers have different signs and magnitudes, which were explained by differences both in the geometrical asymmetries of PhOH bound to (−)­ML and in the electronic coupling strengths between PhOH and (−)­ML