CEST and PARACEST MR contrast agents

In this review we describe the status of development for a new class of magnetic resonance (MR) contrast agents, based on chemical exchange saturation transfer (CEST). The mathematics and physics relevant to the description of the CEST effect in MR are presented in an appendix published in the online version only. We discuss the issues arising when translating in vitro results obtained with CEST agents to using these MR agents in in vivo model studies and in humans. Examples are given on how these agents are imaged in vivo. We summarize the status of development of these CEST agents, and speculate about the next steps that may be taken towards the demonstration of CEST MR imaging in clinical applications.

A concentration-independent method to measure exchange rates in PARACEST agents

The efficiency of chemical exchange dependent saturation transfer (CEST) agents is largely determined by their water or proton exchange kinetics, yet methods to measure such exchange rates are variable and many are not applicable to in vivo measurements. In this work, the water exchange kinetics of two prototype paramagnetic agents (PARACEST) are compared by using data from classic NMR line-width measurements, by fitting CEST spectra to the Bloch equations modified for chemical exchange, and by a method where CEST intensity is measured as a function of applied amplitude of radiofrequency field. A relationship is derived that provides the water exchange rate from the X-intercept of a plot of steady-state CEST intensity divided by reduction in signal caused by CEST irradiation versus 1/omega(1)(2), referred to here as an omega plot. Furthermore, it is shown that this relationship is independent of agent concentration. Exchange rates derived from omega plots using either high-resolution CEST NMR data or CEST data obtained by imaging agree favorably with exchange rates measured by the more commonly used Bloch fitting and line-width methods. Thus, this new method potentially allows access to a direct measure of exchange rates in vivo, where the agent concentration is typically unknown.

Hyperpolarized long-lived states in solution NMR: three-spin case study in low field

Recent work has shown that singlet states in two-spin systems can possess lifetimes exceeding the T(1) relaxation time, provided that the system is kept under conditions that minimize the effects of the chemical shift Hamiltonian (for instance under low magnetic field or RF irradiation). Similar observations have been made in hyperpolarized states of multi-spin systems prepared via parahydrogen-induced polarization (PHIP). However, lifetime prolongation mechanisms in multi-spin systems are still under investigation. Here we present experimental observations of a long-lived state in a three-spin system prepared by PHIP and stored at low field. The observed lifetime of the long-lived state is 144s, about twice as long as the longest T(1) measured in the system at high field. The results are analyzed using a recently proposed theory of lifetime prolongation in multi-spin systems in low field. It is shown that quantum mechanical selection rules governing intramolecular dipolar relaxation in low field account for the enhanced lifetime and spectral features of this state.

Long-lived states in solution NMR: theoretical examples in three- and four-spin systems

Long-lived spin states have been observed in a variety of systems. Although the dynamics underlying the long lifetimes of these states are well understood in the case of two-spin systems, the corresponding dynamics in systems containing more spins appear to be more complex. Recently it has been shown that a selection rule for transitions mediated by intramolecular dipolar relaxation may play a role in determining the lifetimes of long-lived states in systems containing arbitrary numbers of spins. Here we present a theory of long-lived states in systems containing three and four spins and demonstrate how it can be used to identify states that have little or no intramolecular dipolar relaxation.

MRI detection of paramagnetic chemical exchange effects in mice kidneys in vivo

In this report, the On resonance PARamagnetic CHemical Exchange Effects (OPARACHEE) method was implemented in vivo using WALTZ-16* as a preparation pulse with a standard spin echo sequence to detect the accumulation and clearance of the TmDOTA-4AmC(-) in mouse kidney. The performance of the technique in vivo is described in terms of the magnitude of the contrast effect versus the bolus agent concentration and signal-to-noise ratio (SNR) levels. The lowest injected concentration of TmDOTA-4AmC(-), 200 microL of a 2-mM stock solution (corresponds to approximately 0.2 mM agent in plasma), reduced the total water signal in the kidney papilla by 45% 3 min after the a bolus injection. The results show that the OPARACHEE methodology employing low-amplitude RF trains can detect paramagnetic exchanging agents in vivo.

Long-lived states in solution NMR: selection rules for intramolecular dipolar relaxation in low magnetic fields

Recent work has shown that singlet states of two-spin systems in low magnetic fields can have lifetimes up to an order of magnitude longer than the usual spin-lattice relaxation time. This result may enable new applications of NMR, and in particular hyperpolarized NMR via parahydrogen-induced polarization, to the study of slow processes that take place over previously inaccessible timescales. At present it is unclear whether similar results apply to multi-spin systems, or if these long lifetimes are a peculiarity of the two-spin case. Moderately long-lived states have been observed in systems containing more than two spins, although the mechanisms that prolong their lifetimes are not well understood. Here we present formalism for the study of relaxation in multi-spin systems in low magnetic fields. This approach is used to derive a family of quantum-mechanical selection rules governing intramolecular dipolar relaxation at low field that may account for the extended lifetimes observed in multi-spin systems.

Structure and serological analysis of the Hafnia alvei 481-L O-specific polysaccharide containing phosphate in the backbone chain

The lipopolysaccharide was extracted from cells of Hafnia alvei 481-L bacterial strain and, after mild acid hydrolysis, the O-specific polysaccharide was isolated and characterised. On the basis of chemical analyses and NMR spectroscopic studies of the polysaccharide and oligosaccharides obtained after Smith degradation, or hydrogen fluoride treatment, it was found that the repeating unit of the O-specific polysaccharide is a phosphorylated hexasaccharide: [see text]. The biological repeating unit of the H. alvei 481-L O-antigen has galactose phosphate at the nonreducing terminus. Serological tests indicate that this strain represents an individual serotype in the H. alvei genus.

High-resolution anatomic, diffusion tensor, and magnetization transfer magnetic resonance imaging of the optic chiasm at 3T

PURPOSE

To evaluate techniques for anatomical and physiological imaging of the intracranial optic nerve (ON), optic chiasm (OC), and optic tract (OT) at 3T with the aim of visualizing axonal damage in multiple sclerosis (MS).

MATERIALS AND METHODS

Imaging was performed on a 3T scanner employing a custom-designed head coil that consisted of a coil array with four coils (30 x 30 cm(2)). Oblique fast spin echo (FSE) images, magnetization transfer (MT)-enhanced 3D gradient-echo (GRE) time-of-flight (TOF) images, and line scan diffusion images (LSDI) were obtained. Full diffusion tensor (DT) analysis was performed, and apparent diffusion coefficient (ADC), fractional anisotropy (FA), and fiber direction maps were obtained.

RESULTS

FSE anatomic images were obtained with an in-plane resolution of 0.39 x 0.52 mm(2). The in-plane resolution of the MT and LSDI images was 0.78 x 0.78 mm(2). The OC, intracranial ON, and OT can be seen on these images. The dominant fiber orientations in the OC, ON, and OT, as derived from the DT images, are displayed.

CONCLUSION

This study shows that by using 3T and a custom-designed, four-channel head coil, it is possible to acquire high-resolution anatomical and physiological images of the OC, ON, and OT. The pilot results presented here pave the way for imaging the anterior visual pathway in patients with MS.

On-resonance low B1 pulses for imaging of the effects of PARACEST agents

Application of the exchange-sensitive, low-power RF pulses positioned on the bulk water resonance for imaging of the effects of PARACEST agents is proposed as an alternative to the standard CW off-resonance irradiation. Specifically, we applied a low-power WALTZ-16 RF train, with the 90 degrees pulse unit replaced by a pulse of the fixed length (WALTZ-16*). Using this sequence, the bulk water signal was found to be sensitive to exchange lifetimes with PARACEST complex bound protons, the transverse relaxation time of bulk water, and longitudinal relaxation time of bound protons. In this report, the concept of using WALTZ-16* to "activate" a PARACEST effect is introduced and some of the salient features of this technique with respect to experimental conditions and performance levels are discussed. Computational predictions are verified and explored by comparison with experimental spectroscopic and imaging data. It is shown that WALTZ-16* can be used to detect PARACEST agents with an RF intensity as low as 200 Hz for concentrations as low as a few tens of microM for lanthanide chelates having appropriate water-exchange rates (Tm,Dy).

High resolution heteronuclear correlation NMR spectroscopy between quadrupolar nuclei and protons in the solid state

A high resolution two-dimensional solid state NMR experiment is presented that correlates half-integer quadrupolar spins with protons. In this experiment the quadrupolar nuclei evolve during t1 under a split-t1, FAM-enhanced MQMAS pulse scheme. After each t1 period ending at the MQMAS echo position, single quantum magnetization is transferred, via a cross polarization process in the mixing time, from the quadrupolar nuclei to the protons. High-resolution proton signals are then detected in the t2 time domain during wPMLG5* homonuclear decoupling. The experiment has been demonstrated on a powder sample of sodium citrate and 23Na-1H 2D correlation spectra have been obtained. From the HETCOR spectra and the regular MQMAS spectrum, the three crystallographically inequivalent Na+ sites in the asymmetric unit were assigned. This MQMAS-wPMLG HETCOR pulse sequence can be used for spectral editing of half-integer quadrupolar nuclei coupled to protons.

Improvement of homonuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance utilising radiofrequency imperfections

The often annoying imperfections in the phases and pulses of typical radiofrequency multiple-pulse irradiation schemes for homonuclear dipolar decoupling are revisited and analysed here. The analysis is with respect to one such multiple-pulse sequence, namely, the windowed phase-modulated Lee-Goldburg sequence. The error terms in the Hamiltonian due to pulse imperfections may lead to effective rotation of the spins around the z-axis giving rise to image free and high-resolution 1H spectra. Certain precautions to be taken with regard to scale factor estimation are also detailed. The analysis also points out the range of off-set values where the best homonuclear dipolar decoupling performance of a particular pulse scheme may be obtained.

Structural and genetic characterization of glycosylation of type a flagellin in Pseudomonas aeruginosa

Type a flagellins from two strains of Pseudomonas aeruginosa, strains PAK and JJ692, were found to be glycosylated with unique glycan structures. In both cases, two sites of O-linked glycosylation were identified on each monomer, and these sites were localized to the central, surface-exposed domain of the monomer in the assembled filament. The PAK flagellin was modified with a heterogeneous glycan comprising up to 11 monosaccharide units that were O linked through a rhamnose residue to the protein backbone. The flagellin of JJ692 was less complex and had a single rhamnose substitution at each site. The role of the glycosylation island gene cluster in the production of each of these glycosyl moieties was investigated. These studies revealed that the orfA and orfN genes were required for attachment of the heterologous glycan and the proximal rhamnose residue, respectively.

Selective polarization inversion of protons in rotating solids

The selective inversion of lines under phase modulated Lee-Goldburg (PMLG) decoupling in MAS proton spectroscopy is demonstrated. Short pulses inserted between consecutive PMLG irradiation intervals selectively invert the polarization of an on-resonance line while sustaining a high resolution proton evolution. The pulse scheme is combined with windowed-PMLG detection to obtain a one-dimensional high resolution spectrum with one of the proton lines inverted. Initial preparation of the protons in selectively inverted states can be used to follow the flow of polarization during spin diffusion. Examples of proton-proton spin exchange in alanine and histidine are demonstrated. Selective inversion is also used in conjunction with proton carbon LG-cross-polarization to achieve carbon spectra with lines characterized by different polarization states.

Structure of the Lipopolysaccharide Core Region of the Bacteria of the Genus Proteus

The lipopolysaccharide (LPS) core structure was studied in seven rough strains of Proteus and 26 smooth strains belonging to various Proteus O-serogroups. All LPSs share a common heptasaccharide fragment, which includes two Kdo, three Hep, one Glc, and one GalA residue. Core structures differ between strains and within each strain in the presence of a variety of additional monosaccharides and non-sugar substituents. In many strains, the LPS includes a cyclic acetal of GalNAc in the open-chain form, which builds up a new type of linkage between monosaccharides. The covalent linkage of aliphatic polyamines, e.g. putrescine and spermidine, to the LPS was confirmed for the first time and the location of the amines at the carboxyl group of a GalA residue established. Analyses revealed peculiar features of the core structure, which are characteristic of P. mirabilis on one hand and P. vulgaris and P. penneri on the other hand.

Structural analysis of the core region of the lipopolysaccharides from eight serotypes of Klebsiella pneumoniae

The core regions of the lipopolysaccharides (LPS) from Klebsiella pneumoniae serotypes O1, O2a, O2a,c, O3, O4, O5, O8, and O12 were analysed using NMR spectroscopy, ESI-MS spectroscopy, and chemical methods. All the LPSs had similar core structures, as shown below, differing only in the number and position of beta-D-galacturonic acid substituents: [carbohydrate structure: see text] where P is H or alpha-Hep, J, K is H or beta-GalA. LPS from all serotypes contained varying proportions of structures having additional or missing phosphate substituents. The core from serotype O1 contained a minor amount of a previously described variant with alpha-DD-Hep-(1-->2)-alpha-DD-Hep-(1-->6)-alpha-GlcN-(1--> replacing the alpha-Hep-(1-->4)-alpha-Kdo-(2-->6)-alpha-GlcN-(1--> component.

Structural analysis of the carbohydrate components of the outer membrane of the lipopolysaccharide-lacking cellulolytic ruminal bacterium Fibrobacter succinogenes S85

The polysaccharides from the outer membrane of the Gram-negative ruminal bacterium Fibrobacter succinogenes were isolated by phenol/water extraction and separated by size-exclusion chromatography in the presence of deoxycholate detergent into a lower-molecular-mass fraction designated 'glycolipid' and a high-molecular-mass 'capsular polysaccharide' fraction. Both fractions lacked typical lipopolysaccharide components including 2-keto-3-deoxyoctulosonic acid and 3-hydroxy fatty acids. Carbohydrate components of these fractions were represented by two polysaccharides and one oligosaccharide (possibly glycolipid) with the following structures: : : where HEAEP is N-(2-hydroxyethyl)-2-aminoethylphosphonic acid, found for the first time in natural compounds. The polysaccharides contained pentadecanoic acid and anteisopentadecanoic acid, possibly present as the acyl components. All constituent monosaccharides except L-rhamnose had a D-configuration. In addition to having a structural role in the outer membrane, these polysaccharides may provide protection for this lipopolysaccharide-less bacterium in the highly competitive ruminal environment, as phosphonic acids covalently linked to membrane polymers have in the past been attributed the function of stabilizing membranes in the presence of phosphatases and lipases.

The structure of the lipopolysaccharide O-antigen produced by Flavobacterium psychrophilum (259-93)

Flavobacterium psychrophilum, a Gram-negative bacterium, is the etiological agent of rainbow trout fry syndrome and bacterial cold water disease, septicemic infections in reared salmonids. In humans Flavobacterium spp. have been associated with neonatal meningitis and septicemia, catheter-associated bacteremia, and pneumonia. Recently, several F. psychrophilum surface molecules, including lipopolysaccharide (LPS), have been implicated in its pathogenesis and identified as potential vaccine and diagnostic candidate macromolecules. Studies on the LPS produced by the bacterium are reported herein. The structure of the antigenic O-polysaccharide contained in the LPS of F. psychrophilum was deduced by the application of analytical NMR spectroscopy, mass spectrometry, glycose and methylation analysis, and partial hydrolysis degradations, and was found to be an unbranched polymer of trisaccharide repeating units composed of L-rhamnose (L-Rhap), 2-acetamido-2-deoxy-L-fucose (L-FucpNAc) and 2-acetamido-4-((3S,5S)-3,5-dihydroxyhexanamido)-2,4-dideoxy-D-quinovose (D-Quip2NAc4NR, 2-N-acetyl-4-N-((3S,5S)-3,5-dihydroxyhexanoyl)-D-bacillosamine) (1 : 1 : 1) and having the structure: -->4)-alpha-L-FucpNAc-(1-->3)-alpha-D-Quip2NAc4NR-(1-->2)- alpha-L-Rhap-(1--> where R is (3S,5S)-CH3CH(OH)CH2CH(OH)CH2CO-.

The structure of the core region of the lipopolysaccharide from Klebsiella pneumoniae O3. 3-deoxy-alpha-D-manno-octulosonic acid (alpha-Kdo) residue in the outer part of the core, a common structural element of Klebsiella pneumoniae O1, O2, O3, O4, O5, O8, and O12 lipopolysaccharides

The structure of lipid A-core region of the lipopolysaccharide (LPS) from Klebsiella pneumoniae serotype O3 was determined using NMR, MS and chemical analysis of the oligosaccharides, obtained by mild acid hydrolysis, alkaline deacylation, and deamination of the LPS: [carbohydrate structure see text] where P is H or alpha-Hep; J is H or beta-GalA; R is H or P (in the deacylated oligosaccharides). Screening of the LPS from K. pneumoniae O1, O2, O4, O5, O8, and O12 using deamination showed that they also contain alpha-Hep-(1-->4)-alpha-Kdo-(2-->6)-GlcN and alpha-Kdo-(2-->6)-GlcN fragments.

The structure of the carbohydrate backbone of the core-lipid A region of the lipopolysaccharide from Proteus penneri strain 40: new Proteus strains containing open-chain acetal-linked N-acetylgalactosamine in the core part of the LPS

Analysis of the core part of the LPS from several strains of Proteus revealed that P. penneri strains 2, 11, 19, 107, and P. vulgaris serotypes 04 and 08 have the same structure with a new type of linkage between monosaccharidesan open-chain acetal--that was previously determined for P. vulgaris OX2 and P. penneri 17. The LPS from P. penneri strain 40 contains the same structure substituted with one additional monosaccharide: [molecular structure: see text] where (1S)-GalaNAc1 is a residue of N-acetyl-D-galactosamine in the open-chain form. It is connected as a cyclic acetal to positions 4 and 6 of the galactosamine residue having a free amino group. All other sugars are in the pyranose form.

The structure of the nonreducing terminal groups in the O-specific polysaccharides from two strains of Bordetella bronchiseptica

The structures of the polysaccharide chains of the LPS from Bordetella bronchiseptica strains 110H and Bp512 were analysed by NMR spectroscopy and mass spectrometry. The polysaccharides consist of alpha-(1-4)-linked 2,3-diacetamido-2,3-dideoxy-L-galacturonic acid repeating units. Polysaccharides from both strains have 2,3, 4-triamino-2,3,4-trideoxy-alpha-galacturonamide derivatives at their nonreducing ends, a monosaccharide identified for the first time in nature. The polymers from the two strains differ in the nature of the acylation of the amino groups of this monosaccharide. In the strain 110H, the residue is formylated at positions 3 and 4, and has N-formyl-L-alanyl or L-alanyl substituents at N-2. In the strain Bp512, the amino group at position 2 is acetylated, at position 3 it is formylated, and the amino group at position 4 bears a 2-methoxypropionyl substituent. The distribution of the acyl groups was determined from long range 1H-13C correlation (HMBC) NMR spectra. Measurement of the spectra under different pH conditions showed that carboxyl groups of the inner uronic acid residues of the polymeric chain are free, and that carboxyl groups of the terminal residues are amidated. These conclusions were confirmed by the results of mass spectrometric analysis.