Relaxivity and water exchange studies of a cationic macrocyclic gadolinium(III) complex

We conducted relaxometric and water exchange studies of the cationic [Gd((S,S,S,S)-THP)(H2O)]3+ complex (THP 1,4,7,10-tetrakis(2-hydroxy-propyl)-1,4,7,10-tetraazacyclododecane). While the NMRD profiles obtained are typical for DOTA-like complexes (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate), variable-temperature 7O NMR investigations revealed a relatively high water exchange rate (k(298)(ex) = 1.89 x 10(7) s(-1)). These results differ from those reported for other cationic tetraamide macrocyclic Gd(III) complexes, which exhibit characteristically low exchange rates. Since the low exchange rates are attributed partially to the geometry of the M isomer (square antiprismatic) in the tetraamide derivatives, the atypical water exchange rate observed in [Gd((S,S,S,S)-THP-(H2O)]3+ may result from a twisted square antiprismatic structure in this complex and from the relatively high steric strain at the water coordination site as a result of the presence of methyl groups at the alpha-position with respect to the Gd(III)-bound O atoms of THP.

Ion-pair interactions of lanthanide(III) complexes in aqueous solutions

The formation of ion-pair adducts between the cationic complex La(THP)3+ (THP = 1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane) and the anionic complexes Tm(DOTA)- (DOTA = 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetate), Tm(DTPA)2- (DTPA = diethylenetriamine-N,N,N',N",N"-pentaacetate), Tm(TTHA)3- (TTHA = triethylenetetraamine-N,N,N',N",N"',N"'-hexaacetate), and Tm(DOTP)5- (DOTP = 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetrakis(methylenephosphonate)) is examined by 13C NMR spectroscopy. The induced 13C shifts of the La(THP)3+ complex are followed by titration of the Tm(III) complexes of DOTA, DTPA, and TTHA at pH 7. From these data, the stability constants are calculated to be beta 1 = 64 M-1 (1:1), beta 1 = 296 M-1 (1:1), and beta 2 = 26,000 M-2 (2:1) for the ion pairs of La(THP)3+, with Tm(DOTA)-, Tm(DTPA)2-, and Tm(TTHA)3-, respectively. The La(THP)3+,Tm(DOTP)5- system elicits chiral resolution of the rapidly interconverting Tm(DOTP)5- isomers.

Gd(DOTP)5-outer-sphere relaxation enhancement promoted by nitrogen bases

The relaxation properties of Gd(DOTP)5- (1, 4, 7, 10-tetra-azacyclododecane- N,N',N'',N'''-tetrakis(methylenephosphonic acid)) have been investigated as a function of pH, temperature, concentration, and magnetic field strength. We have found that the complex has one exchangeable water molecule in its inner coordination sphere, at a distance of 3.26 A from the metal ion, and it does not form oligomers in solution in the concentration range 0.2 to 10 mM. The possible presence of two species in solution with an average fractional hydration number is also taken into accounts. The NMRD profiles were recorded at 5 degrees C, 25 degrees C, and 35 degrees C and quantitatively analyzed in terms of the paramagnetic relaxation equations. Interestingly the addition to a solution of the Gd(III)-complex of nitrogen bases results in a marked relaxation enhancement, which shows a strong pH dependence with a maximum around pH = 9. The relaxivity gain has been shown to depend on outer-sphere effects originating from multiple electrostatic interactions between the anionic complex and the organic cations that bring the exchangeable protons of the substrate molecules in to close proximity with the paramagnetic center. High resolution NMR relaxation data for N-methyl-D(-)-glucamine suggest that the hydroxyl group on the beta-carbon plays a role in stabilizing the interaction, presumably through a hydrogen bond with an uncoordinated oxygen atom of the complex.

Exploration of the single-stranded DNA-binding domains of the gene V proteins encoded by the filamentous bacteriophages IKe and M13 by means of spin-labeled oligonucleotide and lanthanide-chelate complexes

Scrutiny of NOE data available for the protein encoded by gene V of the filamentous phage IKe (IKe GVP), resulted in the elucidation of a beta-sheet structure which is partly five stranded. The DNA-binding domain of IKe GVP was investigated using a spin-labeled deoxytrinucleotide. The paramagnetic-relaxation effects observed in the 1H-NMR spectrum of IKe GVP, upon binding of this DNA fragment, could be visualized using two-dimensional difference spectroscopy. In this way, the residues present in the DNA-binding domain of IKe GVP can be located in the structure of the protein. They exhibit a high degree of identity with residues in the gene V protein encoded by the distantly related phage M13 (M13 GVP), for which similar spectral perturbations are induced by such a spin-labeled oligonucleotide. Binding studies with negatively charged lanthanide-1,4,7,10-tetraazacyclodecanetrayl-1,4,7-10- tetrakis(methylene)tetrakisphosphonic acid (DOTP) complexes, showed that these complexes bind to IKe and M13 GVP at two spatially remote sites whose affinities have different pH dependencies. Above pH 7, there is one high-affinity binding site for Gd(DOTP)5-/M13 GVP monomer, which coincides with the single-stranded DNA-binding domain as mapped with the aid of spin-labeled oligonucleotide fragments. The results show that single-stranded DNA binds to conserved (phosphate binding) electropositive clusters at the surface of M13 and IKe GVP. These positive patches are interspersed with conserved or conservatively replaced hydrophobic residues. At pH 5, a second Gd(DOTP)(5-)-binding site becomes apparent. The corresponding pattern of spectral perturbations indicates the accommodation of patches of conserved, or conservatively replaced, hydrophobic residues in the cores of the M13 and IKe dimers.