No abstract

We present new observations of the infrared (IR) spectrum of neutral methanol and neutral and protonated methanol clusters employing IR plus vacuum ultraviolet (vuv) spectroscopic techniques. The tunable IR light covers the energy ranges of 2500-4500 cm(-1) and 5000-7500 cm(-1). The CH and OH fundamental stretch modes, the OH overtone mode, and combination bands are identified in the vibrational spectrum of supersonic expansion cooled methanol (2500-7500 cm(-1)). Cluster size selected IR plus vuv nonresonant infrared ion-dip infrared spectra of neutral methanol clusters, (CH(3)OH)(n) (n=2,[ellipsis (horizontal)],8), demonstrate that the methanol dimer has free and bonded OH stretch features, while clusters larger than the dimer display only hydrogen bonded OH stretch features. CH stretch mode spectra do not change with cluster size. These results suggest that all clusters larger than the dimer have a cyclic structure with OH groups involved in hydrogen bonding. CH groups are apparently not part of this cyclic binding network. Studies of protonated methanol cluster ions (CH(3)OH)(n)H(+) n=1,[ellipsis (horizontal)],7 are performed by size selected vuv plus IR photodissociation spectroscopy in the OH and CH stretch regions. Energies of the free and hydrogen bonded OH stretches exhibit blueshifts with increasing n, and these two modes converge to approximately 3670 and 3400 cm(-1) at cluster size n=7, respectively.

Small peptide ions are studied by time-resolved photodissociation (TRPD). Laser desorption of neutral peptides is combined with laser photoionization in an ion trap followed by thermalization, laser photodissociation, and time-of-flight mass analysis. Ionization and excitation take place through an aromatic chromophore at the C-terminus of the peptide, whereas dissociation produces the immonium ion at the N-terminus. The purpose is to uncover the role of intramolecular vibrational redistribution (IVR) in unimolecular fragmentations of peptide radical cations the excitation of which is site-selective. Whereas previous experiments concentrated on mass spectra, the avenue taken here is the determination of microcanonical rate constants. The rate constants are measured at a fairly well-defined internal energy E for two peptides possessing the same chromophore, undergoing the same fragmentation but having a different number of degrees of freedom. Experimental rate measurements in the range of ∼10 2 -10 5 s -1 will be presented for the peptides leucyl tyrosine (LeuTyr) and leucyl leucyl tyrosine (LeuLeuTyr). One-color (280.5 nm) two-photon ionization, thermalization for 1980 ms, and excitation at 579 nm of LeuTyr and LeuLeuTyr yield (4.8 ( 1.8) × 10 3 and (2.9 ( 1.9) × 10 2 s -1 inverse time constants, that is, rate constants, respectively. The rate constants provide a clear indication that the peptide length (i.e., its number of degrees of freedom) strongly correlates with the dissociation rate. This has been tested further through measurements at different photodissociation energies and through Rice-Ramsperger-Kassel-Marcus/quasi equilibrium theory (RRKM/QET) calculations that are demonstrated to be in good agreement with the experimental observations, indicating that the internal energy, E, is randomized. In other words, these peptides do not circumvent IVR.

The aliphatic amino acids glycine, valine, leucine, and isoleucine are thermally placed into the gas phase and expanded into a vacuum system for access by time of flight mass spectroscopy and infrared (IR) spectroscopy in the energy range of 2500-4000 cm(-1) (CH, NH, OH, and stretching vibrations). The isolated neutral amino acids are ionized by a single photon of 10.5 eV energy (118 nm), which exceeds by less than 2 eV their reported ionization thresholds. As has been reported for many hydrogen bonded acid-base systems (e.g., water, ammonia, alcohol, acid clusters, and acid molecules), the amino acids undergo a structural rearrangement in the ion state (e.g., in simplest form, a proton transfer) that imparts sufficient excess vibrational energy to the ion to completely fragment it. No parent ions are observed. If the neutral ground state amino acids are exposed to IR radiation prior to ionization, an IR spectrum of the individual isomers for each amino acid can be determined by observation of the ion intensity of the different fragment mass channels. Both the IR spectrum and fragmentation patterns for individual isomers can be qualitatively identified and related to a particular isomer in each instance. Thus, each fragment ion detected presents an IR spectrum of its particular parent amino acid isomer. In some instances, the absorption of IR radiation by the neutral amino acid parent isomer increases a particular fragmentation mass channel intensity, while other fragmentation mass channel intensities decrease. This phenomenon can be rationalized by considering that with added energy in the molecule, the fragmentation channel populations can be modulated by the added vibrational energy in the rearranged ions. This observation also suggests that the IR absorption does not induce isomerization in the ground electronic state of these amino acids. These data are consistent with theoretical predictions for isolated amino acid secondary structures and can be related to previous IR spectra of amino acid conformers.