Myong Yong Choi

Spectrum and infrared intensities of OH-stretching bands of water dimers

Water dimers have been assembled in He droplets and studied by infrared laser depletion spectroscopy. All four OH stretching bands of the dimer have been identified in the spectral range 3590-3800 cm(-1). Infrared intensities of the bands are also reported. The results are compared with previous measurements and theoretical calculations.

Hydrated HCl clusters, HCl(H2O)(1-3), in helium nanodroplets: studies of free OH vibrational stretching modes.

In this work, infrared laser spectroscopy in helium droplets is used to study the solvation of HCl with small water clusters. Clusters of HCl(H(2)O)(n) with n = 1-3 and (HCl)(2)H(2)O have been identified in the free OH stretching spectral range of 3680-3820 cm(-1). The assignment of the infrared vibrational bands of the clusters is aided by ab initio calculations.

Nonplanarity of adenine: vibrational transition moment angle studies in helium nanodroplets.

Mid-infrared spectra are reported for adenine monomer in helium nanodroplets. We show that there is only one tautomer of adenine, the global minimum structure, observed in helium nanodroplets and characterized by using ab initio calculations and the measurement of vibrational transition moment angles (VTMAs) for the various vibrational modes of the adenine monomer. On the basis of the VTMA analysis on the amino group of the global minimum tautomer, which gives insights into its nonplanarity, a detailed VTMA study of three lowest-energy amino tautomers of adenine is discussed in this study.

Conformationally resolved structures of jet-cooled acetaminophen by UV–UV hole-burning spectroscopy

The conformational structures of jet-cooled acetaminophen were investigated in the gas phase by resonant 2-photon ionization and UV-UV hole-burning spectroscopy. In contrast to the results from a previous study, two nearly isoenergetic conformers were distinctly found in a supersonic molecular beam expansion and positively identified as the cis and trans isomers of acetaminophen by UV-UV hole-burning spectroscopy. The 0-0 bands of the cis and trans isomers were found at 33518.7 and 33485.6 cm(-1), respectively. The vibronic bands of the two isomers are close-lying and/or partially overlapping due to the small energy difference (33 cm(-1)) between the two 0-0 bands. As a consequence, the recorded resonant 2-photon ionization spectrum is highly congested in the low excitation energy region, which develops continuously into a featureless, broadened spectrum in the high energy region.

Interchange-tunneling splitting in HCl dimer in helium nanodroplets

Midinfrared spectra of HCl dimers have been obtained in helium nanodroplets. The interchange-tunneling (IT) splitting in the vibrationally excited state of the bonded H-Cl stretching band (nu(2)) in (H(35)Cl-H(37)Cl) dimers was measured to be 2.7+/-0.2 cm(-1), as compared to 3.7 cm(-1) in free dimer. From the splitting, the strength of the IT coupling in liquid helium of 0.85+/-0.15 cm(-1) was obtained, which is about a factor of 2 smaller than in the free dimer. The results are compared with the previous spectroscopic study of (HF)(2) in He droplets as well as the theoretical study of (HF)(2) and (HCl)(2) dimers in small He clusters.

Fluorescence imaging for Fe3+ in Arabidopsis by using simple naphthalene-based ligands

A main source of Fe3+ exposure for mammals is through plant consumption. Thus, sensitive and selective Fe3+ detection in plant tissue is a significant and an urgent need. Although fluorescence probes have been reported for Fe3+ in water, the detection of endogenous biological Fe3+ in plant tissue remains to be refined due to the high background signal and the thickness of the plant tissue that can hamper the effective application of traditional one-photon excitation. To address these issues, we have synthesized naphthalene-based probes, 1 and 1A. Upon an addition of Fe3+ in water–methanol (1 : 1, v/v, pH 7), the fluorescence probes of 1 and 1A were found to dramatically decrease, but no other metal ions had this effect. More interestingly, 1A, which had no diethyl 2,2′-(phenylazanediyl)diacetate moiety, also exhibited high selectivity for Fe3+. These results clearly indicate that the Fe3+ was bound to the nitrogen and oxygen atoms located near the naphthalene moiety. Furthermore, chemical probes of 1 and 1A were embedded into nanofibrous membrane films NF-1 and NF-1A, respectively, prepared by the electrospinning method for use as a portable chemical probe. Fluorescence changes were examined by immersing the films into solutions of various metal ions. The strong fluorescence intensity of both NF-1 and NF-1A dramatically decreased in accordance with concentration of Fe3+ onto the film, which was a “turn-off” system. In contrast, no significant changes of fluorescence intensity were observed compared that of other metal ions, such as Na+, K+, Zn2+, Pb2+, Mn2+, Cu2+, Co2+, Ca2+, Fe2+, and Cd2+. The results indicate that both NF-1 and NF-1A could be used to selectively detect Fe3+. We also investigated the practicality of both 1 and 1A as imaging probes for Fe3+ to operate within living systems like plants. Chemical probes of both 1 and 1A were tested for fluorescence imaging of Fe3+ in Arabidopsis. As expected, the fluorescence probes displayed high fluorescence imaging for Fe3+ in Arabidopsis.

Self-Assembled Tb3+ Complex Probe for Quantitative Analysis of ATP during Its Enzymatic Hydrolysis via Time-Resolved Luminescence in Vitro and in Vivo

To more accurately assess the pathways of biological systems, a probe is needed that may respond selectively to adenosine triphosphate (ATP) for both in vitro and in vivo detection modes. We have developed a luminescence probe that can provide real-time information on the extent of ATP, ADP, and AMP by virtue of the luminescence and luminescence lifetime observed from a supramolecular polymer based on a C3 symmetrical terpyridine complex with Tb3+ (S1-Tb). The probe shows remarkable selective luminescence enhancement in the presence of ATP compared to other phosphate-displaying nucleotides including adenosine diphosphate (ADP), adenosine monophosphate (AMP), guanosine triphosphate (GTP), thymidine triphosphate (TTP), H2PO4- (Pi), and pyrophosphate (PPi). In addition, the time-resolved luminescence lifetime and luminescence spectrum of S1-Tb could facilitate the quantitative measurement of the exact amount of ATP and similarly ADP and AMP within living cells. The time-resolved luminescence lifetime of S1-Tb could also be used to quantitatively monitor the amount of ATP, ADP, and AMP in vitro following the enzymatic hydrolysis of ATP. The long luminescence lifetime, which was observed into the millisecond range, makes this S1-Tb-based probe particularly attractive for monitoring biological ATP levels in vivo, because any short lifetime background fluorescence arising from the complex molecular environment may be easily eliminated.

NMR detection of chirality and enantiopurity of amines by using benzene tricarboxamide-based hydrogelators as chiral solvating agents

Enantiomeric excess of chiral compounds is a key parameter that can influence their activity or therapeutic action. Current approaches to the rapid measurement of enantiomeric excess using 1H NMR is based on the formation of diastereomeric complexes between chiral analytes and a chiral host, leading to two species with no symmetry relationship. Here, we demonstrate that a gelator host system can provide a means to elicit distinct chemical shifts in the 1H nuclear magnetic resonance (NMR) of a mixture of diamines or monoamines depending on the corresponding enantiomeric excess of the guest mixture of chiral amines. These gelator hosts provide a unique example of NMR based assessment of the chirality and enantiopurity of guest amines.

Helium Nanodroplet Study of the Hydrogen-Bonded OH Vibrations in HCl–H2O Clusters

Mixed (HCl)N(H2O)M clusters have been assembled in He droplets from the constituting molecules. Spectra of the clusters were obtained in the range of hydrogen-bonded OH vibrations (3100-3700 cm(-1)) by infrared laser depletion spectroscopy. The observed bands were assigned to cyclic hydrogen-bonded aggregates containing up to two HCl and three H2O molecules. The obtained frequencies are in good agreement with the results of harmonic quantum chemical calculations upon appropriate uniform shifts mimicking anharmonic corrections. Although larger clusters containing up to six water molecules were also produced in the droplets, their spectra were found to contribute to the unresolved signal in the range 3250-3550 cm(-1). The fact that no narrow bands could be unambiguously assigned to the mixed clusters containing more than three water molecules may indicate that such clusters exist in many isomeric forms that lead to overlapped and unresolved bands giving rise to broad structureless features. Another possible explanation includes the formation of elusive zwitterionic clusters, whose bands may have considerable breadth due to electrostatic coupling of different vibrational modes and concomitant intramolecular vibrational relaxation.

Watching the growth of aluminum hydroxide nanoparticles from aluminum nanoparticles synthesized by pulsed laser ablation in aqueous surfactant solution

Aluminum nanospheres were prepared by a pulsed Nd-YAG laser ablation method in aqueous surfactant, cetyltrimethylammonium bromide (CTAB), solutions. In the absence of CTAB but with aging, fast hydrolysis reaction between Al and water changed the composition and structures of Al nanospheres to Al oxides and then further to Al hydroxides, bayerite and gibbsite, via a dissolution–recrystallization process. Thus, the control of the hydrolysis rate, producing pure Al and Al hydroxide nanoparticles selectively was attempted by varying the concentration of CTAB in the pulsed laser ablation solution; the transformation processes from Al to Al hydroxides were monitored. The resulting nanoparticles at each development stage were analyzed by X-ray diffraction measurements, field emission scanning electron microscopy, high-resolution transmission electron microscope observations, fast Fourier transform analysis, and energy dispersive spectrometer analysis. Possible mechanisms for the dissolution–recrystallization process of Al hydroxides are proposed.