Time-resolved imaging of bound and dissociating nuclear wave packets in strong-field ionized iodomethane.

We report the results of a time-resolved coincident ion momentum imaging experiment probing nuclear wave packet dynamics in the strong-field ionization and dissociation of iodomethane (CH3I), a prototypical polyatomic system for photochemistry and ultrafast laser science. By measuring yields, kinetic energies, and angular distributions of CH3+ + I+ and CH3+ + I++ ion pairs as a function of the delay between two 25 fs, 790 nm pump and probe pulses, we map both, bound and dissociating nuclear wave packets in intermediate cationic states, thereby tracking different ionization and dissociation pathways. In both channels, we find oscillatory features with a 130 fs periodicity resulting from vibrational motion (C-I symmetric stretch mode) in the first electronically excited state of CH3I+. This vibrational wave packet dephases within 1 ps, in good agreement with a simple wave packet propagation model. Our results indicate that the first excited cationic state plays a key role in the dissociative ionization of CH3I and that it represents an important intermediate in the sequential double and multiple ionization at moderate intensities.

Infrared multiple photon dissociation spectrum of protonated bis(2-methoxyethyl) ether obtained with a tunable CO2 laser.

A moderate-resolution infrared multiple photon dissociation (IRMPD) spectrum of protonated bis(2-methoxyethyl) ether (diglyme) was obtained using a grating-tuned CO2 laser. The experimental spectrum compares well with one calculated theoretically at the MP2 level and exhibits defined peaks over the span of the CO2 laser output lines as opposed to a relatively featureless spectrum over this wavelength range obtained using free electron laser infrared radiation. The lowest energy structure corresponding to the calculated vibrational spectrum is consistent with structures previously calculated at the same level of theory. Alternative structures were calculated at lower levels of theory for comparison and investigation of the energetics of proton-heteroatom interactions. Broadening of the IRMPD action spectrum due to energetic phenomena characteristic of proton bridges was not observed and thus did not obscure the correlation between theoretical calculations and experimentally determined spectra as it may have in previous studies.

Expression of a functional Kir4 family inward rectifier K+ channel from a gene cloned from mouse liver.

1. A low stringency polymerase chain reaction (PCR) homology screening procedure was used to probe a mouse liver cDNA library to identify novel inward rectifier K+ channel genes. A single gene (mLV1) was identified that exhibited extensive sequence homology with previously cloned inward rectifier K+ channel genes. The mLV1 gene showed greatest sequence identity with genes belonging to the Kir4 subfamily. The amino acid sequence of mLV1 was 96 % identical to a Kir channel cloned from human kidney (hKir4.2), and approximately 60 % identical to the Kir4.1 channel cloned from human and rat, so that mLV1 was classified as mKir4.2. 2. Xenopus oocytes injected with cRNA encoding mKir4.2 displayed a large inwardly rectifying K+ current, while control oocytes injected with H2O displayed no similar K+ current. The current was blocked by Ba2+ and Cs+ in a voltage-dependent fashion and displayed inward rectification that was intermediate between that of the strong inward rectifier Kir2.1 and the weak inward rectifier Kir1.1. The current was weakly blocked by TEA in a voltage-independent fashion. 3. mKir4.2 current was subject to modulation by several distinct mechanisms. Intracellular acidification decreased mKir4.2 current in a reversible fashion, while activation of protein kinase C decreased mKir4.2 current in a manner that was not rapidly reversible. Incubation of oocytes in elevated [K+] produced a slowly developing enhancement of current. 4. Oocytes co-injected with cRNA for mKir4.2 and Kir5.1, a protein that does not form functional homomeric channels, displayed membrane currents with properties distinct from those expressing mKir4.2 alone. Co-injected oocytes displayed larger currents than mKir4.2, with novel kinetic properties and an increased sensitivity to Ba2+ block at negative potentials, suggesting that mKir4.2 forms functional heteromultimeric channels with Kir5.1, as has been shown for Kir4.1 5. These results demonstrate for the first time that a Kir4.2 channel gene product forms functional channels in Xenopus oocytes, that these Kir channels display novel properties, and that Kir4.2 subunits may be responsible for physiological modulation of functional Kir channels.

Block of the Kir2.1 channel pore by alkylamine analogues of endogenous polyamines.

Inward rectification induced by mono- and diaminoalkane application to inside-out membrane patches was studied in Kir2.1 (IRK1) channels expressed in Xenopus oocytes. Both monoamines and diamines block Kir2.1 channels, with potency increasing as the alkyl chain length increases (from 2 to 12 methylene groups), indicating a strong hydrophobic interaction with the blocking site. For diamines, but not monoamines, increasing the alkyl chain also increases the steepness of the voltage dependence, at any concentration, from a limiting minimal value of approximately 1.5 (n = 2 methylene groups) to approximately 4 (n = 10 methylene groups). These observations lead us to hypothesize that monoamines and diamines block inward rectifier K+ channels by entering deeply into a long, narrow pore, displacing K+ ions to the outside of the membrane, with this displacement of K+ ions contributing to "extra" charge movement. All monoamines are proposed to lie with the "head" amine at a fixed position in the pore, determined by electrostatic interaction, so that zdelta is independent of monoamine alkyl chain length. The head amine of diamines is proposed to lie progressively further into the pore as alkyl chain length increases, thus displacing more K+ ions to the outside, resulting in charge movement (zdelta) increasing with the increase in alkyl chain length.

Protein kinase C inhibition of cloned inward rectifier (HRK1/KIR2.3) K+ channels expressed in Xenopus oocytes.

1. The effect of protein kinase activators on cloned inward rectifier channels expressed in Xenopus oocytes was examined using a two-electrode voltage clamp. PKA activators caused no change in KIR1.1, KIR2.1, or KIR2.3 current. The PKC activators phorbol 12-myristate 14-acetate (PMA) and phorbol 12, 13-dibutyrate (PDBu) inhibited KIR2.3 currents, but not KIR2.1 or KIR1.1 current. This inhibition was blocked by staurosporine. An inactive phorbol ester, 4 alpha-phorbol 12, 13-didecanoate (4 alpha-PDD), had no effect on KIR2.3. 2. Upon changing solution from 2 to 98 microM K+, KIR2.3 but not KIR1.1 or KIR2.1 currents typically 'ran down' over 5 min to 60-80% of maximum amplitude. Rundown occurred even if PMA was applied before changing to high [K+] solution, indicating that rundown was independent of PKC activity. Rundown was evoked by substituting NMG+ for Na+, showing that it results from low [Na+] and not from high [K+]. 3. These results suggest that KIR2.3, but not KIR1.1 or KIR2.1, is subject to regulation, both by PKC activation and as a consequence of low [Na+]o. The difference in secondary regulation may account for specific responses to PKC stimulation of tissues expressing otherwise nearly identical KIR channels.

Inward rectification and implications for cardiac excitability.

Since the cloning of the first inwardly rectifying K+ channel in 1993, a family of related clones has been isolated, with many members being expressed in the heart. Exogenous expression of different clones has demonstrated that between them they encode channels with the essential functional properties of classic inward rectifier channels, ATP-sensitive K+ channels, and muscarinic receptor-activated inward rectifier channels. High-level expression of cloned channels has led to the discovery that classic strong inward, or anomalous, rectification is caused by very steeply voltage-dependent block of the channel by polyamines, with an additional contribution by Mg2+ ions. Knowledge of the primary structures of inward rectifying channels and the ability to mutate them have led to the determination of many of the structural requirements of inward rectification. The implications of these advances for basic understanding and pharmacological manipulation of cardiac excitability may be significant. For example, cellular concentrations of polyamines are altered under different conditions and can be manipulated pharmacologically. Simulations predict that changes in polyamine concentrations or changes in the relative proportions of each polyamine species could have profound effects on cardiac excitability.

Distribution and characterization of pedal peptide immunoreactivity in Aplysia.

Pedal peptide (Pep) is a very abundant neuropeptide in Aplysia. A radioimmunoassay (RIA) was developed to quantify Pep-like immunoreactivity (IR-Pep) in tissue extracts. IR-Pep was present in very high concentrations in the central nervous system (CNS) and two peripheral tissues: the large hermaphroditic duct (LHD) and the foot. RIA of fractions from high-pressure liquid chromatography (HPLC) indicated that Pep itself was the predominant immunoreactive species in each of these tissues. Lower concentrations of Pep were found in a number of other peripheral tissues. Incorporation of labelled amino acid indicated that Pep was synthesized in the LHD, whereas Pep in the foot was synthesized primarily in central neurons and transported to the foot. IR-Pep was further localized by immunocytology. All peripheral IR-Pep appeared to be associated with neuronal fibers, most commonly varicose axons. Immunoreactive innervation of the LHD and foot was particularly dense but positive staining was also observed in other tissues including tegument, gill, gut, and heart, IR-Pep innervation in all tissues including the LHD appeared to be localized predominantly in muscular portions of the tissue. Spontaneous contractions of isolated LHD were accelerated by the application of Pep. Pep appears to act as a transmitter or neuromodulator at a number of different sites in Aplysia.

Immunocytological localization of pedal peptide in the central nervous system and periphery of Aplysia.

Immunocytology using antisera raised to conjugated pedal peptide (Pep) was used to localize the peptide in the CNS and periphery of Aplysia. A total of over 200 neurons in the CNS exhibited Pep-like immunoreactivity. As expected from results presented in the previous paper, immunoreactive neurons were heavily concentrated in the pedal ganglia, primarily in a broad ribbon comprised of about 60 large contiguous neurons on the dorsal side of each ganglion. Smaller and less numerous immunoreactive neurons were found in the other ganglia. A number of neurons primarily located in the abdominal ganglia had dense networks of immunoreactive varicose fibers surrounding their cell bodies. Many immunoreactive axons were observed in peripheral nerves, particularly those nerves leaving the pedal ganglia. Analyses of sections of body wall indicated that Pep-like immunoreactivity was localized to a series of varicose axons that appeared to be associated with vascular spaces, muscle fibers, and other large cells. These axons likely arise from pedal ganglion nerves that were shown to transport large amounts of 35S-labeled Pep to the periphery. These results suggest that Pep is a transmitter-like neuropeptide that is likely to have a number of important physiological actions in Aplysia.