David S. Kliger

Multiphoton absorption spectra using thermal blooming: I. Theory

Abstract A theoretical treatment is presented which describes the formation of a thermal lens produced by pulsed laser excitation through multiphoton absorption processes. The effect of the thermal lens on a monitoring laser beam is discussed. Estimates are given showing that it is possible to obtain two-photon absorption spectra by this technique using commercially available pulsed dye lasers.

Multiphoton absorption spectra using thermal blooming: II. Two-photon spectrum of benzene

Abstract The two-photon absorption spectrum of benzene obtained from a thermal blooming experiment is presented. The feasibility of using thermal blooming to obtain two-photon spectra, discussed in the preceding paper, is thus verified. The dependence of the thermal blooming signal on laser energy and the time behavior of the thermal lens are also shown to agree with theoretical predictions. The two-photon spectrum of benzene enables us to locate the 1B2u. 1B1u, and 1E2g states. We thus definitively establish the state ordering in benzene as 1B2u

Retinal rod GTPase turnover rate increases with concentration: a key to the control of visual excitation?

Guanosine triphosphate (GTP) binding proteins mediate cellular responses to hormones, neurotransmitters, growth factors and light. Activated GTP binding proteins are shut off by GTPase mediated hydrolysis of GTP. Photoreceptor GTPase rates are reported to be 10-50 times too slow to account for electrophysiological recovery time after light stimulus. Recovery rates of other parts of the system, however, appear fast enough. We present evidence that the GTPase rate increases markedly with photoreceptor membrane concentration implying the existence of a diffusible factor controlling the GTPase. When extrapolated to physiological concentrations, the GTPase turnover rate is fast enough (0.25-1.5 sec) to account for the recovery rate of the light stimulated signal of the photoreceptor cells.

Water and ligand entry in myoglobin: Assessing the speed and extent of heme pocket hydration after CO photodissociation

A previously undescribed spectrokinetic assay for the entry of water into the distal heme pocket of wild-type and mutant myoglobins is presented. Nanosecond photolysis difference spectra were measured in the visible bands of sperm whale myoglobin as a function of distal pocket mutation and temperature. A small blue shift in the 560-nm deoxy absorption peak marked water entry several hundred nanoseconds after CO photodissociation. The observed rate suggests that water entry is rate-limited by the escape of internal dissociated CO. The heme pocket hydration and geminate recombination yields were found to be the primary factors controlling the overall bimolecular association rate constants for CO binding to the mutants studied. The kinetic analysis provides estimates of 84%, 60%, 40%, 0%, and 99% for the steady-state hydrations of wild-type, H64Q, H64A, H64L, and V68F deoxymyoglobin, respectively. The second-order rate constants for CO and H2O entry into the empty distal pocket of myoglobin are markedly different, 8 × 107 and 2 × 105 M–1·s–1, respectively, suggesting that hydrophobic partitioning of the apolar gas from the aqueous phase into the relatively apolar protein interior lowers the free energy barrier for CO entry.

Unusual excitation intensity dependence of fluorescence of CdTe nanoparticles

The fluorescence of thiol-capped CdTe nanoparticles have been measured using nanosecond laser spectroscopy. While the bandedge fluorescence intensity of CdTe nanoparticles has been found to increase linearly with excitation intensity at low intensities, an unusual decrease of fluorescence intensity with increasing excitation intensity has been observed at higher intensities. This unusual excitation intensity dependence has been tentatively explained by nonlinear exciton–exciton annihilation or Auger photoionization effect that takes place within the 7 ns excitation laser pulse. In contrast, the fluorescence intensity of CdS nanoparticles increases monotonically with excitation intensity in the same intensity range studied. The results indicate that CdTe nanoparticles show stronger nonlinear optical properties due possibly to stronger quantum confinement effect and could be potentially useful for nonlinear optical applications.

Photointermediates of visual pigments

Much progress has been made in recent years toward understanding the interactions between various proteins responsible for visual transduction which are initiated by an activated state of visual pigments. However, the changes which take place in the visual pigments themselves to convert them to the activated state are more poorly understood. Many spectroscopic techniques have been applied to this problem in recent years and considerable progress has been made. A major goal of these efforts is to understand at which stages protein change occurs and to characterize its structural features. In the visual system evidence is accumulating, for example, that chromophore independent protein change begins immediately prior to lumirhodopsin formation. Considerable insight has been gained recently into the early intermediates of visual transduction and the stage is set to achieve similar understanding of the later intermediates leading to rhodopsins activated state.

Running, swimming and diving modifies neuroprotecting globins in the mammalian brain

The vulnerability of the human brain to injury following just a few minutes of oxygen deprivation with submergence contrasts markedly with diving mammals, such as Weddell seals (Leptonychotes weddellii), which can remain underwater for more than 90 min while exhibiting no neurological or behavioural impairment. This response occurs despite exposure to blood oxygen levels concomitant with human unconsciousness. To determine whether such aquatic lifestyles result in unique adaptations for avoiding ischaemic–hypoxic neural damage, we measured the presence of circulating (haemoglobin) and resident (neuroglobin and cytoglobin) oxygen-carrying globins in the cerebral cortex of 16 mammalian species considered terrestrial, swimming or diving specialists. Here we report a striking difference in globin levels depending on activity lifestyle. A nearly 9.5-fold range in haemoglobin concentration (0.17–1.62 g Hb 100 g brain wet wt−1) occurred between terrestrial and deep-diving mammals; a threefold range in resident globins was evident between terrestrial and swimming specialists. Together, these two globin groups provide complementary mechanisms for facilitating oxygen transfer into neural tissues and the potential for protection against reactive oxygen and nitrogen groups. This enables marine mammals to maintain sensory and locomotor neural functions during prolonged submergence, and suggests new avenues for averting oxygen-mediated neural injury in the mammalian brain.

Deriving reaction mechanisms from kinetic spectroscopy. Application to late rhodopsin intermediates

A general algebraic approach to the kinetic analysis of time-dependent absorption data is presented that allows the calculation of possible kinetic schemes. The kinetic matrices of all possible reaction mechanisms are calculated from experimental eigenvalues and eigenvectors derived from the decay constants and amplitude spectra (b-spectra) of the global exponential fit to the time-dependence of the absorption data. The eigenvalues are directly related to the decay constants, and the eigenvectors are obtained by decomposing the b-spectra into spectral components representing the intermediates. The analysis method is applied to the late intermediates (lumi, meta I, meta I-380, and meta II) of the rhodopsin photoreaction. The b-spectra are decomposed into lumi, meta I, meta-380, and rhodopsin spectra. The meta-380 component is partitioned into isospectral meta I-380 and meta II components based on physical criteria. The calculated kinetic matrices yield a number of reaction mechanisms (linear scheme with back reactions, branched schemes with equilibrium steps, and a variety of square models) consistent with the photolysis data at 25 degrees C. The problems associated with isospectral intermediates (meta I-380 and meta II) are treated successfully with this method.

PICOSECOND AND NANOSECOND TSOMERIZATION KINETICS OF PROTONATED 11-CIS RETINYLIDENE SCHIFF BASES

Abstract— Picosecond and nonosecond spectroscopy has been used to study the isomerization mechanism of protonated 11‐cis retinylidene Schiff bases. The formation and bleaching of absorption bands within 10 ps and corresponding decay and recovery within 11 ns indicate that the isomerization mechanism of the protonated Schiff bases is not identical to rhodopsin in which the primary photophysical event is probably due to electron transfer or partial isomerization of the chromophore to a nonplanar conformation.

A search for a low-lying excited1 a state in 1,3,5-hexatriene

Abstract The two-photon absorption spectrum of 1,3,5-hexatriene has been investigated using thermal blooming techniques. The spectrum probes excited states lying 4.1 eV to 6.5 eV above the ground state. The investigation does not reveal the excited 1 A − g state predicted, by PPP calculations with single plus double excitation configuration interaction, to lie in this energy range. Calculations of two-photon cross sections indicate that the two-photon transition to this low-lying 1 A − g state would be weak in spite of the fact that the transition is symmetry allowed.