Patent Application: US-22015894-A

Abstract:
it is shown that is is possible to inhibit the transfer of magnetization in a chemically exchanging system with several sites in dynamic equilibrium , so that the forward and the backward reaction rates involving two selected species can be studied without being significantly perturbed by other exchange processes . this can be achieved either by selective inversion of the magnetization of the two chosen sites in the course of the reaction interval , or alternatively by inversion of all other sites in the network of exchanging nuclei that one wishes to &# 34 ; decouple &# 34 ; from the selected pair of interest .

Description:
in the schematic pulse sequence embodiment of fig2 the invention displays a first irradiation of a 180 ° selective gaussian pulse , irradiated to selectively invert the source spin component . for this experiment the mixing time interval is defined as τ m . after waiting a time interval τ m / 2 a second selective 180 ° gaussian pulse is irradiated to selectively invert either the perturbing spin system or the source and target spin systems simultaneously and thereby to effect a &# 34 ; decoupling &# 34 ; of the magnetization transfer . a subsequent time interval τ m / 2 is then waited before irradiating a hard non - selective pulse to excite all spins including the target and / or the one or several perturbing spins to be observed . following this excitation , a signal is observed in which the magnetization transfer has been controlled . fig1 a - 1f show the time dependence of proton - decoupled 31 p resonances of source spins trans - zrcl 4 l 2 , target spins cis - zrcl 4 l 2 , and perturbing spins of ligand l =( ch 3 o 3 ) 3 po in cdcl 3 , prepared as described by frey et al . 10 and observed in two complementary series of experiments at 250 k with a bruker msl 300 spectrometer . fig1 a through 1c are difference spectra obtained by inverting the source resonance selectively in odd transients only , using a 180 ° gaussian pulse of 21 ms duration . the magnetization is allowed to migrate freely from source to target spin components and from source to perturbing spin components during a mixing time τ m which is incremented from 0 to 1 . 5 s . the three resonances are observed after a non - selective 90 ° pulse of 22 microseconds , and only narrow windows ( 20 hz wide ) are shown . one notices build - up curves for both conversion processes , and the rates are estimated to be k source → target = 5 . 3 s - 1 and k source → perturbing = 4 . 5 s - 1 . in contrast thereto fig1 d through 1f of fig1 are similar experiments , but with the inventive selective inversion pulse applied to the perturbing resonance in the middle of the τ m interval . the conversion of the magnetization from the source spin component to the perturbing spin component is completely inhibited , although the chemical reaction continues to occur unhindered . the conversion of source to target is easily measured by the remaining build - up curve . the vertical scales of the plots in fig1 a through 1f were amplified with ratios of 1 : 5 : 2 in fig1 a through 1c , and 1 : 3 : 1 . 2 in fig1 d through 1f . the proton - decoupled 31 p spectrum at 250 k and 121 . 5 mhz of fig1 a through 1f , at atmospheric pressure , consists of three singlets at relative offsets of 0 . 83 and 791 hz , which correspond to the phosphate groups of the source , target , and perturbing spins respectively , with approximate relative amplitudes of 4 . 7 : 1 : 14 . 8 . the source resonance can be easily inverted selectively with the 180 ° gaussian pulse . the inversion profile of such a g 1 pulse truncated at 2 . 5 % is very sharp 12 . if multiplets with a finite width must be inverted , rather than singlets as in this experimental example of fig1 it is advisable to use shaped pulses with a &# 34 ; top hat &# 34 ; profile such as g 3 , i - burp or q 3 pulses 13 - 15 . unfortunately , such sophisticated pulses tend to have a longer duration and they are therefore less suitable for studying fast exchange processes . after a mixing time τ m , a non - selective 90 ° monitoring pulse is applied to excite and observe the entire spectrum . the resulting spectra are subtracted from a normal unperturbed spectrum , so that the signals asymptotically decay to zero for large τ m . as shown in fig1 a through 1c , one can readily observe the migration of magnetization from source to target and from source to perturbing spin components during the mixing time τ m . in the inventive quiet - exsy experiments of fig1 d through 1f , the longitudinal magnetization of the perturbing component is selectively inverted in the middle of τ m . as a result , the transfer of magnetization from source component to perturbing component is reversed during the second half of τ m so that the net transfer is vanishingly small at the end of the mixing period . thus , the magnetization behaves as if there were no reaction from source to perturbing nor , for that matter , from target to perturbing . the time - dependence of the signals reflects a simplified effective kinetic matrix where only the interconversion of the source and target isomers seems to occur . one might say loosely that the conversion to and from the perturbing form has been &# 34 ; decoupled &# 34 ; by selective inversion . this is illustrated in fig1 d through 1f . if the exchange rates are fast on the time - scale over which the mixing interval is sampled , the &# 34 ; decoupling &# 34 ; is incomplete , because it is only effective to first order . in such cases , several consecutive inversion pulses can be used , for example at ( 1 / 4 ) τ m and ( 3 / 4 ) τ m or more generally at [( 2k + 1 )/ 2n ] τ m with k and n integer , in analogy to the quiet - noesy and related experiments 11 , 16 , 17 . further insight can be obtained by initially inverting the magnetization of various &# 34 ; source &# 34 ; spins other than that of the source resonance . this would amount to comparing cross - sections taken from two - dimensional exchange matrices at different resonances in the ω 1 domain . note , however , that the quiet - exsy method is inherently one - dimensional . in systems with more than three sites , it is generally not possible to &# 34 ; decouple &# 34 ; all undesirable sites with a simple monochromatic inversion pulse , unless they happen to be degenerate because of fast exchange . one has the option between simultaneously inverting all sites that should be excluded , or inverting the two sites that one wishes to investigate . the latter approach can easily be implemented in practice by using modulated pulses 11 . the following list of references describes prior art relevant to the present invention and the contents of the listed articles are hereby incorporated by reference into the present specification : ( 1 ) jackman , l . m . ; cotton , f . a . ed ., &# 34 ; dynamic nmr spectroscopy &# 34 ;, academic press , n . y ., 1975 . 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