Biological relaxation data

R. M. Kroeker, R. M. Henkelman 1986, (Analysis of biological NMR relaxation data with continuous distributions of relaxation-times),/. Mag. Reson. 69, 218. 

It has long been recognized that NMR cross relaxation data can provide information on the solution conformation of biological macromolecules, mainly through the inverse sixth power dependence of the cross-relaxation rates on internuclear distance. Only recently with the advent of sophisticated two-dimensional acquisition techniques has it become possible to attempt the a-priori determination of the solution conformation... 

Further use of relaxation data, now studying the water and the ligand protons34 36, leads to an estimate of the outer sphere hydration of the lanthanides. We know there are no water molecules in the first coordination sphere of course. These outer sphere relaxation data for the different cations are proportional to susceptibilities and electron relaxation times and become very useful in the study of the inner sphere hydration of other complexes M(dipic) (H20)x and M(dipic)2(H20)y, see below. Note that there is no evidence of further association of the Ln(III) tris-dipicolinate complexes with small cations such as sodium ions. Later we shall show that these anions can bind to biological cationic surfaces and act as shift or relaxation probes. 

This is a typical example of the unique application of the electrochemical sensor for the detection of NO release from nor-motensive and hypertensive rats [6, 29, 30, 55, 60-62]. It has been reported, based on spectroscopic measurements, that the endothelium of hypertensive rats produced more N02 /N03 than the endothelium of WKY rats [63]. However, these reports were in contradiction to the data obtained on smooth muscle relaxation, hindered in hypertensive rats [62]. This means that the endothelium of hypertensive rats should produce less NO. Electrochemical measurements with the porphyrinic sensor clearly show that the biologically active net concentration of NO produced by the endothelium of hypertensive rats is lower than that produced by the endothelium of WKY rats. These results correlate well with previously reported smooth muscle relaxation data. Total production of NO by the endothelium of hypertensive rats is higher... 

Characteristic frequencies may be found from dielectric permittivity data or, even better, from conductivity data. The earlier data by Herrick et al. (6) suggest that there is no apparent difference between the relaxation frequency of tissue water and that of the pure liquid (7). However, these data extend only to 8.5 GHz, one-third the relaxation frequency of pure water at 37°C (25 GHz), so small discrepancies might not have been uncovered. We have recently completed measurements on muscle at 37°C and 1°C (where the pure water relaxation frequency is 9 GHz), up to 17 GHz. The dielectric properties of the tissue above 1 GHz show a Debye relaxation at the expected frequency of 9 GHz (8 ) (Figure 3). The static dielectric constant of tissue water as determined at 100 MHz compares with that of free water if allowance is made for the fraction occupied by biological macromolecules and their small amount of bound water (1, 9). 

Finally before discussing each metal in detail, it is necessary to mention the question of measurement temperature. Transition metals often have very fast relaxation times and, whilst copper and molybdenum can in most cases be studied at room temperature, some haemoproteins and many iron-sulphur proteins have to be cooled to close to liquid helium temperatures (4.2°K) for reasonable spectra to be obtained. This means that the nearest we can get to the biological steady state may be by rapidly freezing our samples and that kinetic measurements will also have to involve rapid freeze techniques. Apparatus to do this has been developed but resolution is at present limited to about 5 msec. Since we are studying properties of d electrons, ESR information is in many ways complementary to data obtained from UV/visible spectra where electronic excita-... 

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