4 research outputs found
Effect of pretreatment temperature on the surface modification of diatomite with trimethylchlorosilane
Artículo científicoDiatomite samples from Costa Rica were purified
using acidic treatments with hydrochloric acid, thermally
treated (400–1000 C) and then silylated with
trimethylchlorosilane in toluene under inert atmosphere.
The purification process allows to decrease the concentration
of metals presented in the crude diatomite, as is confirmed
by X-ray Fluorescence (XRF) Analysis. The
silylated materials were analyzed by using Hyperpolarized
129Xe Nuclear Magnetic Resonance Spectroscopy (HP
129Xe NMR), Fourier Transform Infrared Spectroscopy
(FTIR), X-ray Diffraction (XRD), Scanning Electron
Microscopy (SEM), Thermogravimetric Analysis (TGA),
rehydration tests, and contact angle measurements. XRD
measurements indicate that diatomite is mainly amorphous,
but presents several crystalline phases (kaolinite, cristobalite,
and quartz). Pretreatments at high temperatures
cause changes in those crystalline phases, resulting in more
amorphous materials. However, there is no difference in
the overall structure of purified and thermally treated diatomite
samples with respect to the silylation products. In
addition, SEM measurements show no effect over the pore
structure of the materials. On the other hand, TGA measurements
and rehydration tests show lower losses of water
for silylated materials prepared using higher pretreatment
temperatures. Moreover, HP 129Xe NMR, FTIR, and contact
angle measurements evidence a modification due to
covalent attachment of Si(CH3)3-groups to the surface,
which increases for higher pretreatment temperatures. The
results provide valuable information about external factors
that influence the surface modification of diatomite. This
can be useful to control modifications that can be achieved
in a similar way
Theory of Solid-State Photo-CIDNP in the Earth's Magnetic Field
To date, solid-state photo-CIDNP experiments have been performed only using magic angle spinning NMR in a high-field regime, which is not associated with physiologically relevant spin dynamics. Here, we predict that nuclear spin polarization up to 10%, almost 9 orders of magnitude larger than thermal equilibrium polarization, can arise in cyclic photoreactions at the earth field due to a coherent three-spin mixing mechanism in the S-T- or S-T+ manifold. The effect is maximal at a distance of about 30 angstrom between the two radicals, which nearly coincides with the separation between the donor and secondary acceptor in natural photosynthetic reaction centers. Analytical expressions are given for a simple limiting case. Numerical computations for photosynthetic reaction centers show that many nuclei in the chromophores and their vicinity are likely to become polarized. The theory predicts that only modest hyperfine couplings of a few hundred kilohertz are required to generate polarization of more than 1% for radical-radical distances between 20 and 50 angstrom, that is, for a large number of radical pairs in electron-transfer proteins.</p
A set-up to study photochemically induced dynamic nuclear polarization in phtosynthetic reaction centres by solid-state NMR
418-423Recently, solid-state NMR spectroscopy became a viable method to investigate
photosynthetic reaction centres (RCs) on t e atomic level. To study the
electronic structure of the radical cation state of the RC, occurring after the
electron
emission, solid-state NMR using an illumination set-up can be
exploited. This
paper describes the illumination set-up we designed for
a standard Bruker wide-bore MAS NMR probe. In addition we demonstrate its application
to get information from the active site
in photosynthetic reaction centres of Rhodobacter spaeroides R-26 by
photochemically induced dynamic nuclear polarization (photo-CIDNP). Solid-state
NMR spectra of natural abundance 13C in detergent solubilized
quinone depleted photosynthetic reaction centres under continuous illumination
showed exceptionally strong nuclear spin polarization in NMR lines. Both enhanced-absorptive
and emissive polarization were seen in the carbon spectrum which could be
assigned to a bacteriochlorophyll a (BChl a) cofactor, presumably
the special pair BChl a. The sign and intensities of the 13C
NMR signals provide information about the electron spin density distribution of
the transiently formed radical P.+
on the atomic level
Electron Spin Density Distribution in the Special Pair Triplet of <em>Rhodobacter sphaeroides</em> R26 Revealed by Magnetic Field Dependence of the Solid-State Photo-CIDNP Effect
Photo-CIDNP (photochemically induced dynamic nuclear polarization) can be observed in frozen and quinone-blocked photosynthetic reaction centers (RCs) as modification of magic-angle spinning (MAS) NMR signal intensity under illumination. Studying the carotenoidless mutant strain R26 of Rhodobacter sphaeroides, we demonstrate by experiment and theory that contributions to the nuclear spin polarization from the three-spin mixing and differential decay mechanism can be separated from polarization generated by the radical pair mechanism, which is partially maintained due to differential relaxation (DR) in the singlet and triplet branch. At a magnetic field of 1.4 T, the latter contribution leads to dramatic signal enhancement of about 80 000 and dominates over the two other mechanisms. The DR mechanism encodes information on the spin density distribution in the donor triplet state. Relative peak intensities in the photo-CIDNP spectra provide a critical test for triplet spin densities computed for different model chemistries and conformations. The unpaired electrons are distributed almost evenly over the two moieties of the special pair of bacteriochlorophylls, with only slight excess in the L branch.</p