A time resolved photo-CIDEP study of electron transfer in durosemiquinone radicals

The time dependent E.S.R. spectra of the duroquinone anion radical, DQ^-, produced by laser flash photolysis, are studied in detail and shown to be quantitatively consistent with previous theoretical predictions. It is established that the degenerate electron transfer reaction between DQ^- and its parent molecule is an important spin relaxation mechanism in this radical and that it has a profound effect on the form of the experimental E.S.R. time profiles.     

Surfactant headgroup orientation at the air/water interface

We have used vibrational sum-frequency spectroscopy to provide the first measurement of the spectrum and orientation of the polar headgroup of a charged alkyl surfactant at the air/water interface. Sum-frequency spectra of sodium dodecyl sulfate (SDS) are used to arrive at all participating elements of the second-order susceptibility tensor. We use these chi(2) elements, together with calculated values of the hyperpolarizability, to determine the tilt of the S-O bond attached to the alkyl chain and the twist of the S-O-C plane. Thus, a full characterization of the orientation of the surfactant headgroup has been achieved. This is the first demonstration of the feasibility of sum-frequency measurements of sulfate modes in the 1100 cm-1 region, opening possibilities for future investigations of surfactant behavior in this spectral region at aqueous and solid interfaces.     

How important is the back reaction of electrons via the substrate in dye-sensitized nanocrystalline solar cells?

The role of the conducting glass substrate (fluorine-doped tin oxide, FTO) in the back reaction of electrons with tri-iodide ions in dye-sensitized nanocrystalline solar cells (DSCs) has been investigated using thin-layer electrochemical cells that are analogues of the DSCs. The rate of back reaction is dependent on the type of FTO and the thermal treatment. The results show that this back-reaction route cannot be neglected in DSCs, particularly at lower light intensities, where it is the dominant route for the back transfer of electrons to tri-iodide. This conclusion is confirmed by measurements of the intensity dependence of the photovoltages of DSCs with and without blocking layers. It follows that blocking layers should be used to prevent the back reaction in mechanistic studies in which the light intensity is varied over a wide range. Conclusions based on studies of the intensity dependence of the parameters of DSCs such as photovoltage and electron lifetime in cells without blocking layers, must be critically re-examined.     

Hydrogenation of

The course of the hydrogenation of [5]- and [6]metacyclophane (1b and 1c) and their thermochemistry is described. Both compounds are hydrogenated rapidly (within 10 s) to furnish the bridgehead olefins 13b and 12c. The accompanying hydrogenation enthalpies are -220 and -141 kJmol(-1), respectively. Strain energies (SE) and olefinic strains (OS) of a number of bridgehead olefins have been evaluated by DFT calculations; it was concluded that 13b belongs to the class of hyperstable olefins which correlates nicely with its reluctance to undergo hydrogenation. By combining experimental hydrogenation enthalpies and DFT calculations, SE of 187 and 121 kJmol(-1) were derived for 1b and 1c.     

Probing the exposure of tyrosine and tryptophan residues in partially folded proteins and folding intermediates by CIDNP pulse-labeling

A nuclear magnetic resonance (NMR) technique has been devised to probe the structures of disordered, partially folded states of proteins at the level of individual amino acid residues. Chemically induced dynamic nuclear polarization (CIDNP) is first generated in exposed aromatic side-chains of the denatured state and then transferred to the high-resolution NMR spectrum of the native state by stimulating rapid refolding of the protein. Crucial improvements in sensitivity were achieved by carrying out the polarization-producing photochemistry in a deoxygenated sample of the disordered state of the protein in a magnetic field of 4.0 T and recording the (1)H NMR spectrum of the refolded native state at 9.4 T (400 MHz). Application of this method to the low pH molten-globule state of alpha-lactalbumin reveals remarkably nativelike environments for the aromatic residues in the primary hydrophobic core of the protein. This result provides compelling evidence that the detailed fold of a protein can be established prior to the formation of the cooperative close-packed native structure.     

Increasing the sensitivity of time-resolved photo-CIDNP experiments by multiple laser flashes and temporary storage in the rotating frame

Pulse sequences have been developed that add up time-resolved photo-CIDNP signals from n successive laser flashes not in the acquisition computer of the NMR spectrometer but in the experiment itself, resulting in a greatly improved signal-to-noise ratio. For this accumulation, CIDNP is first stored in the transverse plane and then on the z axis, and finally superimposed on CIDNP produced by the next flash. These storage cycles also result in a very efficient background suppression. Because only one free induction decay is acquired for n flashes, the noise is digitized only once. The signal gain is demonstrated experimentally and analyzed theoretically. Losses are mostly due to nuclear spin relaxation, and to a small extent to instrument imperfections. With 10 laser flashes, a signal increase by a factor of about 7.5 was realized. As their main advantage compared to signal averaging in the usual way, these sequences yield the same signal-to-noise ratio with fewer laser flashes; the theoretical improvement is by a factor of n.