Electronic circular dichroism spectroscopy of cold, isolated chiral molecules and clusters

 Nam Joon Kim
Department of Chemistry, Chungbuk National University, Republic of Korea

I will introduce two main research topics of our laboratory. One is cold ion spectroscopy combined with electrospray ionization (ESI) quadrupole ion trap reflectron time-of-flight (QIT-reTOF) mass spectrometry and the other is circular dichroism (CD) spectroscopy of jet-cooled chiral molecules and clusters. We constructed an ESI QIT-reTOF mass spectrometer for cold ion spectroscopy and developed a UV-UV hole burning (HB) spectroscopic technique that can be applied to ions stored in a quadrupole ion-trap (QIT). Although UV-UV HB spectroscopy has been extensively used to obtain conformationally selective electronic spectra of neutral molecules produced in a supersonic jet, it has never been applied to ions in an ion trap due to the difficulty in distinguishing the ions produced by the pump laser from those by the probe laser. We found that some photofragment ions in the QIT do not get trapped but escape from the QIT right after the irradiation of the pump pulse and thus can be detected separately from those produced by the probe pulse by using a reflectron time-of-flight mass spectrometer attached next to the ion trap. Using this technique, we obtained the UV-UV HB spectra of dibenzo-18-crown-6 and with alkali metal cations (M+-DB18C6, M=K, Rb, and Cs) and confirmed the presence of only single conformational isomers for all of the complex ions in the cold QIT. We also obtained the UV photodissociation (PD) spectra of adenine complexes with alkali metal cations (M+A). The UVPD spectrum near the origin band of the S0-S1 transition exhibits well-resolved vibronic bands, all of which are assigned to be from a single isomer by UV-UV HB spectroscopy. Comparing the UVPD spectrum with those simulated using the time-dependent density functional theory, we determined the single isomer to be the complex ion where M+ ion is bound to not the canonical 9H-adenine (A9) but the 7H-adenine tautomer (A7). We suggest that the single isomer, M+A7, is formed in solution through tautomerization, which is promoted by the presence of M+ ion in kinetics and/or thermodynamics depending on its binding site to A9. 
For the second topic, we developed a new CD spectroscopic technique using a photoelastic modulator and successfully applied to obtain the resonant two-photon ionization CD (R2PICD) spectra of jet-cooled pseudoephedrine and ephedrine. Here, to understand the solvation effects on CD values, we obtained the R2PICD spectra of 2-amino-1-phenylethanol (APE) and their hydrated clusters produced in a supersonic jet using the CD spectroscopic technique. We identified the origin bands of two different conformers of APE and their hydrated clusters and measured their CD values. It is found that the solvation by one or two water molecules does not alter the CD sign of bare APE but increases its CD magnitude. The possible reasons for the increase of the CD magnitude by solvation will be discussed. We also obtained the R2PICD spectra of jet-cooled phenylalanine (Phe) and its hydrated clusters (Phe(H2O)n, n=1,2) near the origin bands of the S0-S1 transition to understand the contribution of different conformations of Phe to the near-UV CD spectra of proteins and peptides. The origin bands of six different conformers of Phe were identified, and their CD values were compared with theoretical ones predicted using the time-dependent density functional theory. The R2PICD spectra of Phe(H2O)n reveal that the hydration of Phe with one or two water molecules does not alter the CD sign but greatly increases the CD magnitude. This study manifests that the CD spectroscopy of jet-cooled chiral compounds is a powerful tool to determine conformations of large, flexible amino acids and their contribution to the near-UV CD spectra of proteins.