Latex Fractionation by Sedimentation and Colloidal Crystallization: The Case of Poly(styrene-co-acrylamide)

Poly(styrene-co-acrylamide) (PS-AAM) latex was prepared, fractionated by sedimentation under gravity, and characterized by PCS, infrared spectra, secondary and backscattered electron imaging in the scanning electron microscope, and electron spectroscopy imaging in an analytical transmission electron microscope. Three latex fractions were obtained. The lower fraction was opalescent and its particles were the more uniform, concerning size, chemical composition, and topochemical features. This lower fraction was still further fractionated by zonal centrifugation in a density gradient, yielding two fractions with similar macrocrystal-forming abilities but different sizes and chemical compositions. These results confirm those previously obtained for the PS-HEMA latex. Copyright 2000 Academic Press.     

ELECTRON SPIN ECHO ENVELOPE MODULATION OF THE TRANSIENT EPR SIGNALS OBSERVED IN PHOTOSYNTHETIC ALGAE AND CHLOROPLASTS*

Phase memory decays were obtained from the transient signals sampled within 50 ns of laser excitation by time resolved electron spin echo (ese) spectroscopy in normal protonated, deuterated, deuterated ¹⁵N photosynthetic algae and broken chloroplasts. Previously, it has been shown that application of time resolved ese to study these systems, in particular, gives rise to two kinds of signals (standard and special ese). The standard ese signal at g = 2.0023 and the special ese signal exhibit similar electron spin echo envelope modulation (eseem). The modulation frequency and pattern can be identified with that obtained from stable oxidized P₇₀₀ in the same system. The two lower field resonances of the standard ese signal do not show eseem. The results support the proposed mechanism for formation of special ese and the notion that the standard ese is due to at least two radicals. It is also demonstrated that we can observe eseem of P⁺₇₀₀ under ambient temperature conditions.     

Electron spin echo envelope modulation of trapped radicals in disordered systems: nitrogen modulation from isotopically substituted chlorophyll a cations

The electron spin echo envelope modulation of the chlorophyll a radical cation has been examined for radicals containing ¹⁴N and ¹⁵N. The modulation is found to be due primarily to the nitrogen nuclei in the heterocycle and the nuclear quadrupole interaction plays a large part in determining the modulation from ¹⁴N. The modulation from ¹⁵N allows limits to be set on the isotropic and anisotropic hyperfine interactions.     

Characterization of free radicals from vitamin K1 and menadione by 2 mm-band EPR,ENDOR and ESEEM

The anion and cation radicals of vitamin K₁ and its analog menadione were characterized using the magnetic resonance techniques of Electron Nuclear Double Resonance (ENDOR), Electron Spin Echo Envelope Modulation (ESEEM), and Electron Paramagnetic Resonance (EPR) at X-band and 2 mm-band. Theg-factor anisotropy of the radicals at 2 mm-band allow them to be distinguished from each other in the solid state. Theg-factor matrix of the radical anion of vitamin K₁ is virtually identical with that reported for the reduced A₁ acceptor in green plant photosystem I thus demonstrating that reduced A₁ is the anion radical of vitamin K₁.     

Nuclear coherences in photosynthetic reaction centers following light excitation

Transient electron paramagnetic resonance is used to study the secondary radical pair in plant photosystem I. Nuclear coherences are observed in the transverse magnetization at lower temperatures following light excitation. Comparative studies of deuterated and deuterated¹⁵N-substituted cyanobacteriaS. lividus indicate assignment of these coherences to nitrogen nuclei in the primary donor and deuterons in the secondary acceptor. The modulation amplitude of a deuteron matrix line, as a function of the microwave power, reveals a distinct resonance behavior. The maximum amplitude is obtained when the Rabi frequency equals the nuclear Zeeman frequency.