Diffusion-tensor MR imaging of the human brain with gradient- and spin-echo readout: technical note.

Diffusion-tensor MR imaging of the brain is an objective method that can measure diffusion of water in tissue noninvasively. Five adult volunteers participated in this study that was performed to evaluate the potential of gradient- and spin-echo readout for diffusion-tensor imaging by comparing it with single-shot spin-echo echo-planar imaging. Gradient- and spin-echo readout provides comparable measures of water diffusion to single-shot spin-echo echo-planar readout with significantly less geometrical distortion at the expense of a longer imaging time.

PRESTO-SENSE: an ultrafast whole-brain fMRI technique.

A new ultrafast MR imaging method is proposed and tested, which enables whole-brain fMRI with a true temporal resolution of 1 sec. The method combines a 3D PRESTO pulse sequence with the concept of sensitivity-encoding with multiple receiver coils (SENSE). The so-called PRESTO-SENSE technique is demonstrated on a set of functional block-type motor and visual experiments and compared with conventional functional imaging techniques, such as PRESTO and EPI. Comparable image quality and activation areas are found with all sequences. The noise characteristics of the proposed method are analyzed in detail and their implications for ultrafast fMRI studies are discussed. Magn Reson Med 43:779-786, 2000.

Middle cerebral artery (MCA) susceptibility sign at susceptibility-based perfusion MR imaging: clinical importance and comparison with hyperdense MCA sign at CT.

PURPOSE: To describe the radiologic findings of susceptibility changes in acute middle cerebral artery (MCA) thromboembolism detected with three-dimensional (3D) susceptibility-based perfusion magnetic resonance (MR) imaging and to compare the detectability and clinical value of this sign with those of the hyperdense MCA sign at computed tomography (CT). MATERIALS AND METHODS: Twenty-three patients (mean age, 55 years) underwent CT and MR imaging within the first 6 hours after the onset of acute MCA stroke. The hyperdense MCA sign at CT and the presence of susceptibility changes in acute thromboembolism as depicted on T2*-weighted 3D perfusion MR images were assessed. The presence of each sign was correlated with clinical presentation. RESULTS: The sensitivity of the hyperdense MCA sign at CT was 54% (negative predictive value, 71%) compared with 82% (negative predictive value, 86%) for the susceptibility changes at MR imaging. There were no false-positive CT or MR readings. The presence of the MCA susceptibility sign correlated positively with the initial clinical presentation (chi(2) = 7.987, P =.009, Spearman rho = 0.589). However, neither of the signs was a predictor for clinical outcome in cases of spontaneous MCA stroke. CONCLUSION: In addition to the information traditionally provided with reconstructed perfusion parameter maps, 3D susceptibility-based perfusion MR images allow the identification of acute MCA thromboembolism with a sensitivity higher than that of CT.

Ultra-fast three-dimensional MR perfusion imaging of the entire brain in acute stroke assessment.

We sought to evaluate a three-dimensional (3D) whole-brain perfusion technique based on echo-shifting (PRESTO) for its performance in evaluation of acute stroke. Twenty-six patients were scanned within 6 hours after onset of hemispheric symptoms, and the results were compared with results of diffusion-weighted imaging (DWI) and digital subtraction angiography (DSA). The signal-to-noise ratio of the images was 61 +/- 3 pre-contrast and 47 +/- 3 at the bolus peak. Brain coverage on perfusion parameter maps was 95% +/- 2% compared with that displayed on T2-weighted images, with only minor artifacts related to susceptibility at the skull base. Measured regional cerebral blood volume (rCBV) reduction closely correlated to lesion size on initial DWI and to final clinical outcome (P = 0.006), consistent with results previously reported for 2D perfusion methods. Mismatches between DWI and perfusion imaging characterized the total extent of tissue at risk, and the contrast timing correlated with the amount of collateral circulation as shown on DSA. In conclusion, 3D imaging using the PRESTO technique permits high-quality perfusion imaging of the entire brain.

Phase navigator correction in 3D fMRI improves detection of brain activation: quantitative assessment with a graded motor activation procedure.

Motion poses severe problems for BOLD fMRI, particularly in clinical studies, as patients exhibit more involuntary movements than controls. This study focuses on the merits of a motion correction technique incorporated in multishot fMRI scans, so-called phase navigator correction. The technique entails real-time assessment and off-line elimination of signal fluctuations caused by subject motion. The purpose of this study was to quantify and characterize the effect of this type of improvement on 3D fMRI brain activity maps. For imaging, the 3D PRESTO method was used, with a relatively simple finger opposition task. The followed strategy was guided by the notion that application of any fMRI imaging tool in clinical studies requires several qualities, such as high and spatially homogeneous sensitivity to brain activity, and low sensitivity to motion. A graded motor activation protocol in 10 healthy subjects revealed that image stability was improved by approximately 20%, by the use of phase navigator correction. As a result, sensitivity for task-related BOLD signal change was enhanced considerably in the brain activity maps. Implications for use of this fMRI technique in patient studies are discussed.

Differentiation of gray matter and white matter perfusion in patients with unilateral internal carotid artery occlusion.

In this study, we investigated differences between gray matter and white matter perfusion in patients with a unilateral occlusion of the internal carotid artery (ICA) with dynamic susceptibility contrast. Seventeen patients and 17 control subjects were studied, using T2*-weighted gradient echo acquisition. Gray and white matter regions were obtained by segmentation of inversion recovery MRI. Lesions were excluded by segmentation of T2-weighted MRI. In the symptomatic hemisphere, cerebral blood volume was increased in white matter (P < .05) but not in gray matter. No cerebral blood flow changes were found. All timing parameters (mean transit time [MTT], time of appearance, and time to peak) showed a significant delay for both white and gray matter (P < .05), but the MTT increase of white matter was significantly larger than for gray matter (P < .05). These findings indicate that differentiation between gray and white matter is essential to determine the hemodynamic effects of an ICA occlusion.

The solution structure of the synthetic circular peptide CGVSRQGKPYC. NMR studies of the folding of a synthetic model for the DNA-binding loop of the ssDNA-binding protein encoded by gene V of phage M13.

The cyclic disulfide peptide CGVSRQGKPYC was prepared to obtain a constrained analogue of residues 17-27 of the DNA-binding loop of the gene-V-encoded sDNA-binding protein of filamentous bacteriophage M13. Amino acid sequences very similar to that of the beta-loop have been found in various phage-encoded ssDNA-binding proteins, and it has been proposed that such a loop may occur as a common motif in this class of proteins. The conformation, in aqueous solution, of the synthetic gene-V-protein binding-loop analogue has been investigated by means of two-dimensional-1H-NMR techniques. Subsequent structure calculations show that the molecule forms a beta-loop that includes a turn formed by three residues. This structure, very unusually for a cyclic disulfide peptide, is highly similar to that of the analogous part of the binding loop of the native protein. Comparison with experiments on other cyclic disulfide peptides indicates that the formation, of the beta-sheet (beta-hairpin) secondary structure is essentially governed by the amino acid composition of the 11-residue sequence. The disulfide bridge in the 11-residue sequence is essential for conformational stability, as indicated by the finding that the open peptide analogue that encompasses residues Ser17-Ser27 does not adopt a detectable secondary structure in water. The bridge replaces the role of the loop formed by residues 49-58 in the protein, which act as a scaffold to hold the N-terminal and C-terminal ends of the DNA-binding loop together.

Refined solution structure of the Tyr41-->His mutant of the M13 gene V protein. A comparison with the crystal structure.

The three-dimensional solution structure of mutant Tyr41-->His of the single-stranded DNA binding protein encoded by gene V of the filamentous bacteriophage M13 has been refined in two stages. The first stage involved the collection of additional NOE-based distance constraints, which were then used in eight cycles of back-calculations and structure calculations. The structures of the gene V protein dimers were calculated using simulated annealing, employing restrained molecular dynamics with a geometric force field. In the second stage of the refinement procedure, solvent was explicitly included during the dynamic calculations. A total of 30 structures was calculated for the protein, representing its solution structure in water. The first calculation step significantly improved the convergence of the structures, whereas the subsequent simulations in water made the structures physically more realistic. This is, for instance, illustrated by the number of hydrogen bonds formed in the molecule, which increased considerably upon going to aqueous solution. It is shown that the solution structure of the mutant gene V protein is nearly identical to the crystal structure of the wild-type molecule, except for the DNA-binding loop (residues 16-28). This antiparallel beta-hairpin is twisted and partially folded back towards the core of the protein in the NMR structure, whereas it is more extended and points away from the rest of the molecule in the X-ray structure. Unrestrained molecular dynamics calculations suggest that this latter conformation is energetically unstable in solution.

Secondary structure of the single-stranded DNA binding protein encoded by filamentous phage Pf3 as determined by NMR.

Nuclear magnetic resonance spectroscopy was employed to study the single-stranded DNA binding protein encoded by the filamentous Pseudomonas bacteriophage Pf3. The protein is 78 amino acids long and occurs in solution predominantly as a homodimer with a molecular mass of 18 kDa. Sequence-specific 1H and 15N resonance assignments have been obtained using homo- and heteronuclear two- and three-dimensional experiments. The secondary structure of the protein monomer was determined from a qualitative interpretation of nuclear Overhauser enhancement spectra and amide exchange data. It consists of a five-stranded antiparallel beta-sheet and three beta-hairpins. Problems caused by the protein's tendency to aggregate at concentrations needed for NMR spectroscopy were largely overcome by designing a mutant (Phe36-->His) which exhibits significantly improved solubility characteristics over the wild-type protein. It is shown that this mutation only locally affects the structure of the protein; the chemical shifts of the wild-type and mutant species differ only for a few residues near the site of the mutation, and the secondary structures of the proteins are identical. The secondary structure of the Pf3 single-stranded DNA binding protein is compared to that of the Ff gene V protein, the only single-stranded DNA binding protein for which the complete three-dimensional structure is known to date [Folkers, P. J. M., Nilges, M., Folmer, R. H. A., Konings, R. N. H. & Hilbers, C. W. (1994) J. Mol. Biol. 236, 229-246; Skinner, M. M., Zhang, H., Leschnitzer, D. H., Guan, Y., Bellamy, H., Sweet, R. M., Gray, C. W., Konings, R. N. H., Wang, A. H.-J. & Terwilliger, T. C. (1994) Proc. Natl Acad. Sci. USA 91, 2071-2075]. It is found that the secondary structures of the two proteins are very similar which supports the hypothesis that a five-stranded antiparallel beta-sheet with protruding beta-hairpins is a common motif in a certain class of single-stranded DNA binding proteins. In addition, the sequence and folding predicted earlier for the DNA binding wing in the single-stranded DNA binding protein of phage Pf3 [de Jong, E. A. M., van Duynhoven, J. P. M., Harmsen, B. J. M., Tesser, G. I., Konings, R. N. H. & Hilbers, C. W. (1989) J. Mol. Biol. 206, 133-156] is borne out by the present study. It closely resembles that in the single-stranded DNA binding protein of phage Ff, which may indicate that such a wing is a recurrent motif as well.

A model of the complex between single-stranded DNA and the single-stranded DNA binding protein encoded by gene V of filamentous bacteriophage M13.

A contact analysis and a series of restrained molecular dynamics simulations were employed to derive a model of the complex between single-stranded DNA and the single-stranded DNA-binding protein encoded by gene V of the filamentous phage M13. The study is based on the recently elucidated solution structure of the Tyr41-->His mutant of the protein. Electron microscopy studies, indicating that the complex forms a flexible, left-handed helical coil with a diameter of 8 to 9 nm and an average pitch of 9 nm, were taken into consideration. The contact analysis served to determine the helix parameters that permit the energetically most favourable packing of protein molecules. Then a protein super-helix was built, into which two extended strands of DNA were modelled using restrained molecular dynamics. Specific constraints were included to ensure that the DNA would position itself into the binding groove of the protein. These constraints are based on recent NMR spin label experiments which offered a direct identification of the amino acids of the protein present in the DNA-binding domain. We present a model for the complex which is in full agreement with the existing reliable biophysical and biochemical data. A description of the protein-protein interface is given and the protein-DNA interaction is discussed in view of the derived model. In addition, we demonstrate that, on the basis of the available experimental data, and not imposing the left-handedness of the nucleoprotein complex, it is feasible to build also a plausible model for the complex which exhibits the opposite, i.e. right-handed, helical sense. This nucleoprotein structure features characteristics highly similar to those of the left-handed helix.

The solution structure of the Tyr41-->His mutant of the single-stranded DNA binding protein encoded by gene V of the filamentous bacteriophage M13.

The solution structure of mutant Tyr41-->His of the single-stranded DNA binding protein encoded by gene V of the filamentous bacteriophage M13 has been investigated by nuclear magnetic resonance spectroscopy. Two- and three-dimensional NMR experiments have been employed with a variety of NMR samples of gene V protein, some of which were uniformly enriched with either 15N or 13C. The structure of mutant Tyr41-->His of the M13 gene V protein which occurs in solution as a symmetric dimer was calculated using a two-stage procedure. The first step of the procedure involved the calculation of a set of individual monomer structures using the distance geometry program DIANA. This was then followed by the calculation of dimer structures employing "simulated annealing" protocols with the program X-PLOR. Hereby, the problem of assignment of intra- and inter-subunit NOEs of the symmetric dimer was circumvented through use of a target function that correctly deals with the intra- and inter-subunit contributions to the NOE peaks. Furthermore, a pseudo energy term was employed to restrain the symmetry of the dimer. In addition to this novel calculation strategy, we have incorporated distance information for a set of NOEs which were unambiguously identified as inter-subunit NOEs using an NMR strategy based on asymmetric labelling. A total of 20 structures were calculated for the M13 gene V protein mutant Tyr41-->His based on approximately 1000 experimental restraints derived from the NMR data. The structure of residues 1 to 15 and 29 to 87 of both monomers is reasonably well determined with an average atomic r.m.s. difference between the individual structures and the respective mean structure of approximately 0.9 A for the backbone atoms and approximately 1.4 A for all atoms. The orientation of the exposed anti-parallel beta-loop (residues 16 to 28) with respect to the core could not be determined. The molecular architecture of each of the monomers includes a five-stranded beta-barrel enclosing a hydrophobic core and two-antiparallel beta-loops. The dimer structure is stabilized predominantly by hydrophobic residues primarily involving the symmetry-related dyad domains (residues 64 to 82) of the monomers. Residues which are close to bound single-stranded DNA were identified previously from binding experiments with spin-labelled oligonucleotides. The solution structure of mutant Tyr41-->His of the M13 gene V protein is consistent with these binding data and provides a clear view of the protein's single-stranded DNA binding path.

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.

Exploring the DNA binding domain of gene V protein encoded by bacteriophage M13 with the aid of spin-labeled oligonucleotides in combination with 1H-NMR.

The DNA binding domain of the single-stranded DNA binding protein gene V protein encoded by the bacteriophage M13 was studied by means of 1H nuclear magnetic resonance, through use of a spin-labeled deoxytrinucleotide. The paramagnetic relaxation effects observed in the 1H-NMR spectrum of M13 GVP upon binding of the spin-labeled ligand were made manifest by means of 2D difference spectroscopy. In this way, a vast data reduction was accomplished which enabled us to check and extend the analysis of the 2D spectra carried out previously as well as to probe the DNA binding domain and its surroundings. The DNA binding domain is principally situated on two beta-loops. The major loop of the two is the so-called DNA binding loop (residues 16-28) of the protein where the residues which constitute one side of the beta-ladder (in particular, residues Ser20, Tyr26, and Leu28) are closest to the DNA spin-label. The other loop is part of the so-called dyad domain of the protein (residues 68-78), and mainly its residues at the tip are affected by the spin-label (in particular, Phe73). In addition, a part of the so-called complex domain of the protein (residues 44-51) which runs contiguous to the DNA binding loop is in close vicinity to the DNA. The NMR data imply that the DNA binding domain is divided over two monomeric units of the GVP dimer in which the DNA binding loop and the tip of the dyad loop are part of opposite monomers. The view of the GVP-ssDNA binding interaction which emerges from our data differs from previous molecular modeling proposals which were based on the GVP crystal structure (Brayer & McPherson, 1984; Hutchinson et al., 1990). These models implicate the involvement of one or two tyrosines (Tyr34, Tyr41) of the complex loop of the protein to participate in complex formation with ssDNA. In the NMR studies with the spin-labeled oligonucleotides, no indication of such interactions has been found. Other differences between the models and our NMR data are related to the structural differences found when solution and crystal structures are compared.

Assignment of the 1H NMR spectrum and secondary structure elucidation of the single-stranded DNA binding protein encoded by the filamentous bacteriophage IKe.

By means of 2D NMR techniques, all backbone resonances in the 1H NMR spectrum of the single-stranded DNA binding protein encoded by gene V of the filamentous phage IKe have been assigned sequence specifically (at pH 4.6, T = 298 K). In addition, a major part of the side chain resonances could be assigned as well. Analysis of NOESY data permitted the elucidation of the secondary structure of IKe gene V protein. The major part of this secondary structure is present as an antiparallel beta-sheet, i.e., as two beta-loops which partly combine into a triple-stranded beta-sheet structure, one beta-loop and one triple-stranded beta-sheet structure. It is shown that a high degree of homology exists with the secondary structure of the single-stranded DNA binding protein encoded by gene V of the distantly related filamentous phage M13.

Sequence-specific 1H-NMR assignment and secondary structure of the Tyr41----His mutant of the single-stranded DNA binding protein, gene V protein, encoded by the filamentous bacteriophage M13.

Sequence-specific 1H-NMR assignments are reported for the Tyr41----His (Y41H) mutant of the single-stranded DNA binding protein, encoded by gene V of the filamentous bacteriophage M13 (GVP). The mutant protein was chosen for this purpose because it exhibits significantly improved solubility characteristics over wild-type GVP [Folkers et al. (1991) Eur. J. Biochem. 200, 139-148]. The secondary structure elements present in the protein are deduced from a qualitative interpretation of the nuclear Overhauser enhancement spectra and amide exchange data. The protein is entirely composed of antiparallel beta-structure. It is shown that identical structural elements are present in wild-type GVP. Previously, we have demonstrated that the secondary structure of the beta-loop, encompassing residues 13-31 which is present in GVP in solution, deviates from that proposed for the same amino acid sequence on the basis of X-ray diffraction data [van Duynhoven et al. (1990) FEBS Lett. 261, 1-4]. Now that we have arrived at a complete description of the secondary structure of the protein in solution, other deviations with respect to the crystallographically determined structure became apparent as well. The N-terminal part of the protein is, in solution, part of a triple-stranded beta-sheet while, in the crystal, it is an extended strand pointing away from the bulk of the protein dimer. One of the antiparallel beta-sheets in the protein which had been designated earlier as the complex loop has, in the solution structure, a different pairwise arrangement of the residues in its respective beta-ladders. Residues 30 and 48 are opposite to one another in the solution structure while in the crystal structure residues 32 and 48 are paired. A similar observation is made for the so-called dyad domain of the protein of which the beta-sheet in the solution structure is shifted by one residue with respect to that of the crystal structure.

Characterization of wild-type and mutant M13 gene V proteins by means of 1H-NMR.

Recording of good quality NMR spectra of the single-stranded DNA binding protein gene V of the bacteriophage M13 is hindered by a specific protein aggregation effect. Conditions are described for which NMR spectra of the protein can best be recorded. The aromatic part of the spectrum has been reinvestigated by means of two-dimensional total correlation spectroscopy. Sequence-specific assignments were obtained for all of the aromatic amino acid residues with the help of a series of single-site mutant proteins. The solution properties of the mutants of the aromatic amino acid residues have been fully investigated. It has been shown that, for these proteins, either none or only local changes occur compared to the wild-type molecule. Spin-labeled oligonucleotide-binding studies of wild-type and mutant gene V proteins indicate that tyrosine 26 and phenylalanine 73 are the only aromatic residues involved in binding to short stretches of single-stranded DNA. The degree of aggregation of wild-type gene V protein is dependent on both the total protein and salt concentration. The data obtained suggest the occurrence of specific protein-protein interactions between dimeric gene V protein molecules in which the tyrosine residue at position 41 is involved. This hypothesis is further strengthened by the observation that the solubility of tyrosine 41 mutants of gene V protein is significantly higher than that of the wild-type protein. The discovery of the so-called 'solubility' mutants of M13 gene V protein has finally made it possible to study the solution structure of gene V protein and its interaction with single-stranded DNA by means of two-dimensional NMR.

Structure of the DNA binding wing of the gene-V encoded single- stranded DNA binding protein of the filamentous bacteriophage M13.

The structure in solution of a beta-loop in mutant Y41H of the single-stranded DNA binding protein encoded by gene-V of the filamentous phage M13 has been elucidated using 2-dimensional 1H-nuclear magnetic resonance techniques. Furthermore, these studies enabled us to demonstrate that an identical structural element is present in wild-type gene-V-protein and that this element intimately is involved in the binding of gene-V-protein to single-stranded DNA. It is shown that the structure of the DNA binding wing deviates from that proposed for the same amino acid sequence on the basis of X-ray diffraction data. The structure is, however, identical to that of the DNA binding wing present in the single-stranded DNA binding protein encoded by the genome of the evolutionary distantly related filamentous phage IKe. The latter observations support our current view that in the binding of these proteins to single-stranded DNA a common structural motif is involved.

Solution structure of recombinant hirudin and the Lys-47----Glu mutant: a nuclear magnetic resonance and hybrid distance geometry-dynamical simulated annealing study.

The solution structure of recombinant wild-type hirudin and of the putative active site mutant Lys-47----Glu has been investigated by nuclear magnetic resonance (NMR) spectroscopy at 600 MHz. The 1H NMR spectra of the two hirudin variants are assigned in a sequential manner with a combination of two-dimensional NMR techniques. Some assignments made in our previous paper [Sukumaran, D. K., Clore, G. M., Preuss, A., Zarbock, J., & Gronenborn, A. M. (1987) Biochemistry 26, 333-338] were found to be incorrect and are now corrected. Analysis of the NOE data indicates that hirudin consists of an N-terminal compact domain (residues 1-49) held together by three disulfide linkages and a disordered C-terminal tail (residues 50-65) which does not fold back on the rest of the protein. This last observation corrects conclusions drawn by us previously on hirudin extracted from its natural source, the leech Hirudo medicinalis. The improved sensitivity of the 600-MHz spectrometer relative to that of our old 500-MHz spectrometer, the availability of two variants with slightly different chemical shifts, and the additional information arising from stereospecific assignments of methylene beta-protons and methyl protons of valine have permitted the determination of the solution structure of hirudin with much greater precision than before. Structure calculations on the N-terminal domain using the hybrid distance geometry-dynamical simulated annealing method were based on 685 and 661 approximate interproton distance restraints derived from nuclear Overhauser enhancement (NOE) data for the wild-type and mutant hirudin, respectively, together with 16 distance restraints for 8 backbone hydrogen bonds identified on the basis of NOE and amide NH exchange data and 26 phi backbone and 18 chi 1 side-chain torsion angle restraints derived from NOE and three-bond coupling constant data. A total of 32 structures were computed for both the wild-type and mutant hirudin. The structure of residues 2-30 and 37-48 which form the core of the N-terminal domain is well determined in both cases with an average atomic rms difference between the individual structures and the respective mean structures of approximately 0.7 A for the backbone atoms and approximately 1 A for all atoms. As found previously, the orientation of the exposed finger of antiparallel beta-sheet (residues 31-36) with respect to the core could not be determined on the basis of the present data due to the absence of any long-range NOEs between the exposed finger and the core.(ABSTRACT TRUNCATED AT 250 WORDS)