Isotopic replacement

The use of selective isotope replacement of carbon and hydrogen atoms in the structure of xanthophylls in combination with LHCII reconstitution should greatly aid the assignment of multiple v4 twisting bands. This assignment would help localize the areas of distortion within the carotenoid molecule and understand the possible causes of this distortion. 

Let agr be the common value of the reactivity coefficients for both isotopes. Replacing the subscripts liq by SW , in by runoff and evaluating, we get the rate of change of 87Sr/86Sr as... 

The rate constant of a reaction changes when an isotope replaces the atom participating in the reaction. Deuterium and tritium are the most commonly used isotopes, deuterium especially. The quantitative... 

Static dielectric measurements [8] show that all crystals in the family exhibit a very large quantum effect of isotope replacement H D on the critical temperature. This effect can be exemphfied by the fact that Tc = 122 K in KDP and Tc = 229 K in KD2PO4 or DKDP. KDP exhibits a weak first-order phase transition, whereas the first-order character of phase transition in DKDP is more pronounced. The effect of isotope replacement is also observed for the saturated (near T = 0 K) spontaneous polarization, Pg, which has the value Ps = 5.0 xC cm in KDP and Ps = 6.2 xC cm in DKDP. As can be expected for a ferroelectric phase transition, a decrease in the temperature toward Tc in the PE phase causes a critical increase in longitudinal dielectric constant (along the c-axis) in KDP and DKDP. This increase follows the Curie-Weiss law. Sc = C/(T - Ti), and an isotope effect is observed not only for the Curie-Weiss temperature, Ti Tc, but also for the Curie constant C (C = 3000 K in KDP and C = 4000 K in DKDP). Isotope effects on the quantities Tc, P, and C were successfully explained within the proton-tunneling model as a consequence of different tunneling frequencies of H and D atoms. However, this model can hardly reproduce the Curie-Weiss law for Sc-... 

The C=0 stretching frequency of a number of 4-hydroxy- and 4-alkoxy-coumarins, including several anticoagulant drugs, has been identified by isotopic replacement of the carbonyl carbon atom by 3C as the highest frequency band in the 1750-1550 cm-1 region. Introduction of the isotopic atom causes a reduction in the C=0 frequency of ca. 30 cm-1 (82JHC475). 

Optical propagation loss for polymeric electro-optic materials is typically in the order of 1 dB/cm when care is taken to avoid scattering losses associated with processing and poling-induced damage [2, 3, 5, 63, 64, 257]. Lower loss values can be obtained by isotopic replacement of protons with deuterium and with halogens [211, 304, 305]. With effort, electro-optic material losses can be reduced to approximately 0.2 dB/cm for the telecommunication wavelengths of 1.3 and 1.55 microns. 

Microwave spectra (74PMH(6)5) provide a rich source of detailed molecular structures in the gas phase. For molecules with more than four atoms the isotopic replacement technique must be used in order to obtain a complete structure. Not only the ground-state conformation but also rotational barriers... 

Microwave spectra provide a rich source of minute details of molecular structures. Spectra are primarily analyzed in terms of the accurate values of the moments of inertia. This generally gives the molecular conformation and some precise structural features may emerge. To obtain a complete structure, it is necessary to measure changes in moments of inertia that accompany isotopic replacement of each atom <1974PMH(6)53>. From accurate measurements of the Stark effect, when electrostatic fields are applied, information about the electron distribution is also obtained. 

Similarly, optical activity due to isotopic replacement has been observed for oxygen and carbon [12]. 

With the development of the role for prochirality in biology, the need to demonstrate the existence of this phenomenon and to characterize it in precise structural terms became urgent. Along with such developments, chemists were interested in the study of compounds in which isotopic replacement led to chirality which could be expressed as optical activity. In other words, it was necessary to carry out determinations of the configurations of prochiral compounds. 

This is valid for the geometrical arrangement not only of addends, but also of heteroatoms or specific isotopes replacing C-atoms of the fullerene core. For the ease of reading, this will not be stressed each time in the following discussions. 

Methods of isomorphous replacement usually fail to give any reliable structural information as the exchange of counterions may modify the structural appearance of the charged macromolecule considerably. Isotopic replacement of hydrogen by... 

Yasunobi T, Yasumoii I (1971) Pressure jump and isotope replacement studies of acetylene hydrogenation on palladium surface. J Phys Chem 75 880... 

Identity rate constants (/ ) may be studied by use of isotope replacement techniques but need specialised skill and equipment for their accurate measurement. Free energy relationships for quasi-symmetrical reactions (Equation 23) offer a simple approach to calculating identity... 

Chapter 22 The Electron Donor to P680 EPR Spectroscopy II.A. Identification of Z as Tyrosine by Isotope Replacement... 

As an approach to using 2D-IR for the examination of the structures and dynamics of individual residues of extended structures, it is natural to turn to isotopic substitution. The use of isotopic replacements has been essential in the interpretation of vibrational spectra and their relationship to structure. Iso-topomers have frequencies, force fields, and anharmonicities that are different from one another. Of particular interest in applications of 2D-1R is the use of isotopes to shift frequencies into regions where their couplings can be measured, free from interference by other modes of the system. For the amide-I mode, which is mainly a C=0 stretching coordinate, the shifts by and... 

A more quantitative analysis of the extent of phase separation may be obtained by using the technique of isotopic replacement. Two samples are prepared having the same structure except that the atoms of an element found in one sample have been replaced by an isotope in the other. In this work, this is accomplished by hydrating samples with mixtures of H2O and D2O of various proportions. For samples that have two phase structures the ratio of SANS intensities is given by ... 

---