Rotational correlation time of the

For Gdin-based agents, the relaxivity response is most often related to a change in water accessibility or to the variation of the size and consequently of the rotational correlation time of the complex. In addition to Gdm complexes, PARACEST agents are... 

Fig. 11. HN(CO)CANH-TROSY experiment for establishing sequential 13Ca(t—1), 15N(/), Hn(0 connectivity in 13C/15N/2H labelled proteins. Delay durations A = l/ (4JHn) 2Tn = 23 33 ms, 2Ta = 22-28 ms, depending on rotational correlation time of the protein Tc= 1/(47C/C ) S = gradient + field recovery delay 0 < k < Ta/t2jmax-Phase cycling i = x = x, — x + States-TPPI 03 = x 4>rec. = x, — x.

In a protein-ligand complex the correlation time rc of the vector connecting the electron and nuclear spins, depends on the rotational correlation time of the protein-ligand complex, Tr, on the electron relaxation time, rs, and on the lifetime of the complex, rm [6, 9] ... 

Fig. 3 Cross sections through the peaks corresponding to in Fig. 2A and Fig. 4B,C respectively and T in Fig. 6C. CCR-rates can be deduced from the intensities of the peaks (see text). Intensities of the single lines in the absence of CCR (unbound peptide) are equal, because of the short rotational correlation time of the peptide...

Fig. 9. NMRD profiles (25° C) of [GdD0TA(B0M)3(H20)] (open circles), its inclusion complex with P-cyclodextrin (filled circles) and of [GdD0TA(B0M)3(H20)]-HSA adduct (squares). The different shapes and amplitudes of the profiles are primarily due to the different rotational correlation times of the paramagnetic complexes.

The main difficulty of this method is the unknown value of the term X (l + p /3) and therefore the value for pure water is often used. On the other hand, it has the advantage that the measurement is directly performed on a solution of the Gd(III) complex and that the rotational correlation time of the Gd-coordinated water oxygen vector is actually determined 12)-,... 

The broadening of the heme proton-resonances is mainly due to the increase in the rotational correlation time of the heme inside micelle. The increase in linewidth, Aoobs of a paramagnetic complex inside micelles compared to that in simple aqueous or non-aqueous solution is given as [20-22] ... 

Thus, we have determined the distances between the adenylyl moiety and the two divalent metal ion binding sites on glutamine synthetase by 13C and 3 P NMR, spin-labeled EPR, and fluorescence energy transfer methods. The results obtained from each method are in good agreement. The data show that the adenylyl regulatory site is close to the catalytic site (12-20 A). Additional data on the rotational correlation time of the adenyl derivatives reveal that the adenylyl site is located on the surface of the enzyme. 

Fig. 1. A Gd(III) complex with one inner sphere water molecule, surrounded by bulk water. Inner sphere proton relaxivity is due to interactions between the Gd electron spin and the water protons on the inner sphere water. Outer sphere relaxivity arises from interactions between the Gd electron spin and bulk water protons. rR stands for the rotational correlation time of the molecule, kex for the water/proton exchange rate and 1/T, 2c for the electron spin relaxation rates of the Gd(III)...

Gd(III) chelates, the relatively low relaxivity is the consequence of the flexibility of the linker group between the Gd(III) chelate and the rigid dendrimer molecule (slow water exchange is also limitative). Internal flexibility has been also proved for certain non-covalently bound Gd(III) chelate - protein adducts. The tr value determined for MP-2269 bound to bovine serum albumin is 1.0 ns, one order of magnitude lower than the rotational correlation time of the protein molecule [50]. 

Provided the interactions between a contrast agent and an enzyme are sufficiently strong, a large increase in relaxivity will be observed due to the increased rotational correlation time of the adduct. This behavior was observed for a Gd-DTPA derivative substituted by an arylsulfonamide pendant arm [116]. 

The rotational correlation times of the largest aggregates of Gd(CPw3) and Gd(CPwl7) are 649 and 1360 ps, respectively, and their radii are 1.2 and 1.6 nm. The opposite was expected, because the methylation of the amide functions precludes... 

In this section, some of the approaches described above for enhancing the sensitivity and information content of protein-ligand NOEs are demonstrated for relatively large protein-inhibitor complexes. In addition, we demonstrate that a medium-quality 3D X-filtered NOESY spectrum can be obtained for a large protein-inhibitor complex by using a stabilized, uniformly 13C/15N-labeled protein sample in conjunction with an elevated experimental temperature to increase the rotational correlation time of the protein-ligand complex. 

These values are too short to be influenced significantly by Tr, the rotational correlation time of the enzyme-Gd3+ complex, or Tm, the mean residence time of water molecules in the first coordination sphere of the metal. Moreover, the minima in the plots of Tj p vs. Wj2 indicate that Tc must be dominated by Ts, the electron spin relaxation time. The Ts values for Gd + in this system are longer than most of those determined previously for Gd3+. The electron spin relaxation time for aqueous Gd3+ is (4-7) x 10 10s at 30 MHz (42), while values for Ts of (2-7) x 10 10s have been reported for complexes of Gd3+ with pyruvate kinase (37) and a value of 2.2 x 10- s has been found for a Gd 1"-lysozyme complex (36). Moreover, we have estimated a Tc of 6,8 x 10 10s for Gd + bound to parvalbumin.3 The long Gd3+ correlation times found in the present study are consistent with a poor accessibility of these Gd3+ sites to solvent water molecules. 

Some systems cannot be well described by translational motion, so instead they require a model based on rotational diffusion. The commonly used model is one where the nuclei and radical form a bound complex, then this complex rotates to modulate dipolar coupling.74 Here, the overall correlation time consists of the rotational correlation time of the solvent complex, xT, and the exchange rate of molecules in and out of the complex, tm, where 1 /tc 1 /tr I 1 /tm. The form of this spectral density function is simpler4,25 ... 

---