THE PHOTOCHEMICAL QUANTUM YIELD FOR THE SELF-SENSITIZED PHOTOOXIDATION OF BILIRUBIN. EVIDENCE FOR OXYGEN-INDUCED INTERSYSTEM CROSSING AT HIGH OXYGEN CONCENTRATIONS

Abstract— The in vitro self-sensitized photooxygenation of bilirubin has been studied as a function of oxygen and bilirubin concentrations in a variety of solvents. The results which have been interpreted in terms of a singlet oxygen reaction indicate bilirubin triplet yields of 0.05–0.08 and further show that oxygen induced intersystem crossing is appreciable at high oxygen concentrations. The results also show that the quantum efficiency of the photochemical reaction would be of the order of 1% at the bilirubin and oxygen concentrations typical for the in vivo treatment of neonatal jaundice.     

Intensity stabilization of dye laser radiation by saturated amplification

A novel scheme is proposed for stabilizing the intensity of a fluctuating laser source, using a partly saturated high gain traveling wave laser amplifier with a nonsaturable absorber distributed in the amplifying medium. Calculations for homogeneously broadened laser gain band in an adiabatic approximation are presented, which indicate that the stabilization factor can be increased by several orders of magnitude with the help of the absorber. An experiment is reported in which a two-stage dye laser amplifier with inserted attenuator, pumped by a nitrogen laser, is used to amplify and stabilize the intensity of an ultra-narrowband repetitively pulsed dye laser. The output intensity changes by less than a factor of four when the input is changed over three orders of magnitude. Work supported by the U.S. Office of Naval Research, Contract No. N00014-67-A-0112-0071, and by a Grant from the Research Development Fund, Stanford University. On leave from the Laboratorio de Elettronica Quantistica del CNR di Milano, Milano, Italy.     

Fine-structure measurements of two-photon transitions in atomic cesium with a tunable dye laser

Two-photon transitions have been observed from the 6²S ground state of atomic cesium to selected higher lying n²D levels up to n = 19. The transitions were excited by a narrowband dye laser pumped by a nitrogen laser and detected by a space charge limited cesium vapor photodiode. The fine structure intervals of the n²D levels for n = 15, 17 and 19 were measured by direct comparison with the ground-state hyperfine interval, and the line strength ratios of the fine-structure components were measured and compared with the predictions of a simple theoretical model.     

Discrete Morse Theory for Computing Cellular Sheaf Cohomology

Sheaves and sheaf cohomology are powerful tools in computational topology, greatly generalizing persistent homology. We develop an algorithm for simplifying the computation of cellular sheaf cohomology via (discrete) Morse theoretic techniques. As a consequence, we derive efficient techniques for distributed computation of (ordinary) cohomology of a cell complex.     

Direct determination of diffusion properties of random media from speckle contrast

We present a simple scheme to determine the diffusion properties of a thin slab of strongly scattering material by measuring the speckle contrast resulting from the transmission of a femtosecond pulse with controlled bandwidth. In contrast with previous methods, our scheme does not require time measurements nor interferometry. It is well adapted to the characterization of samples for pulse shaping, nonlinear excitation through scattering media, and biological imaging.     

Genetic engineering of the heme pocket in human serum albumin: modulation of O2 binding of iron protoporphyrin IX by variation of distal amino acids

Complexing an iron protoporphyrin IX into a genetically engineered heme pocket of recombinant human serum albumin (rHSA) generates an artificial hemoprotein, which can bind O2 in much the same way as hemoglobin (Hb). We previously demonstrated a pair of mutations that are required to enable the prosthetic heme group to bind O2 reversibly: (i) Ile-142-->His, which is axially coordinated to the central Fe2+ ion of the heme, and (ii) Tyr-161-->Phe or Leu, which makes the sixth coordinate position available for ligand interactions [I142H/Y161F (HF) or I142H/Y161L (HL)]. Here we describe additional new mutations designed to manipulate the architecture of the heme pocket in rHSA-heme complexes by specifically altering distal amino acids. We show that introduction of a third mutation on the distal side of the heme (at position Leu-185, Leu-182, or Arg-186) can modulate the O2 binding equilibrium. The coordination structures and ligand (O2 and CO) binding properties of nine rHSA(triple mutant)-heme complexes have been physicochemically and kinetically characterized. Several substitutions were severely detrimental to O2 binding: for example, Gln-185, His-185, and His-182 all generated a weak six-coordinate heme, while the rHSA(HF/R186H)-heme complex possessed a typical bis-histidyl hemochrome that was immediately autoxidized by O2. In marked contrast, HSA(HL/L185N)-heme showed very high O2 binding affinity (P1/2O2 1 Torr, 22 degrees C), which is 18-fold greater than that of the original double mutant rHSA(HL)-heme and very close to the affinities exhibited by myoglobin and the high-affinity form of Hb. Introduction of Asn at position 185 enhances O2 binding primarily by reducing the O2 dissociation rate constant. Replacement of polar Arg-186 with Leu or Phe increased the hydrophobicity of the distal environment, yielded a complex with reduced O2 binding affinity (P1/2O2 9-10 Torr, 22 degrees C), which nevertheless is almost the same as that of human red blood cells and therefore better tuned to a role in O2 transport.     

A note on the integration of competing psychophysical theories

Virtually from its inception psychophysics has been caught on the horns of the dilemma as to the proper form of the fundamental psychophysical law. This paper illustrates an integration of two competing theories.     

Manganese(III)-mediated oxidative free-radical cyclisations of allenyl malonates

The regioselective synthesis of alkenylcyclanones by the oxidative radical cyclisation of malonyl radicals onto both allenes and an allylsilane is reported, along with the diastereoselective formation of [3.3.0]-bicyclic γ-lactones from two of these alkenylcyclanones. Furthermore, the cyclisation of 3-, 4- and 5-allenyl malonates on exposure to manganese(III) acetate under both oxidative and reductive termination conditions is reported.