Anisotropic longitudinal electronic relaxation affects DNP at cryogenic temperatures.

We report the observation of anisotropic longitudinal electronic relaxation in nitroxide radicals under typical dynamic nuclear polarization conditions. This anisotropy affects the efficiency of dynamic nuclear polarization at cryogenic temperatures of 4 K and high magnetic fields of 6.7 T. Under our experimental conditions, the electron paramagnetic resonance spectrum of nitroxides such as TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) is only partly averaged by electronic spectral diffusion, so that the relaxation times T1e(ω) vary across the spectrum. We demonstrate how the anisotropy of T1e(ω) can be taken into account in simple DNP models.

Long-lived states to sustain hyperpolarized magnetization.

Major breakthroughs have recently been reported that can help overcome two inherent drawbacks of NMR: the lack of sensitivity and the limited memory of longitudinal magnetization. Dynamic nuclear polarization (DNP) couples nuclear spins to the large reservoir of electrons, thus making it possible to detect dilute endogenous substances in magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI). We have designed a method to preserve enhanced ("hyperpolarized") magnetization by conversion into long-lived states (LLS). It is shown that these enhanced long-lived states can be generated for proton spins, which afford sensitive detection. Even in complex molecules such as peptides, long-lived proton states can be sustained effectively over time intervals on the order of tens of seconds, thus allowing hyperpolarized substrates to reach target areas and affording access to slow metabolic pathways. The natural abundance carbon-13 polarization has been enhanced ex situ by almost four orders of magnitude in the dipeptide Ala-Gly. The sample was transferred by the dissolution process to a high-resolution magnet where the carbon-13 polarization was converted into a long-lived state associated with a pair of protons. In Ala-Gly, the lifetime T(LLS) associated with the two nonequivalent H(alpha) glycine protons, sustained by suitable radio-frequency irradiation, was found to be seven times longer than their spin-lattice relaxation time constant (T(LLS)/T(1) = 7). At desired intervals, small fractions of the populations of long-lived states were converted into observable magnetization. This opens the way to observing slow chemical reactions and slow transport phenomena such as diffusion by enhanced magnetic resonance.

Hyperpolarizing gases via dynamic nuclear polarization and sublimation.

A high throughput method was designed to produce hyperpolarized gases by combining low-temperature dynamic nuclear polarization with a sublimation procedure. It is illustrated by applications to 129Xe nuclear magnetic resonance in xenon gas, leading to a signal enhancement of 3 to 4 orders of magnitude compared to the room-temperature thermal equilibrium signal at 7.05 T.

Overhauser effects in non-conducting solids at 1.2 K.

Recently, it was observed that protons in non-conducting solids doped with 1,3-bisdiphenylene-2-phenylallyl (BDPA) or its sulfonated derivative (SA-BDPA) can be polarized through Overhauser effects via resonant microwave irradiation. These effects were present under magic angle spinning conditions in magnetic fields between 5 and 18.8 T and at temperatures near 100 K. This communication reports similar effects in static samples at 6.7 T and, more importantly, at temperatures as low as 1.2 K, in a different dynamic regime than in the previous study. Our results provide new information towards understanding the mechanism of the Overhauser effect in non-conducting solids. We discuss possible origins of the fluctuations that can give rise to an Overhauser effect at such low temperatures.

Cubic three-dimensional hybrid silica solids for nuclear hyperpolarization.

Hyperpolarization of metabolites by dissolution dynamic nuclear polarization (D-DNP) for MRI applications often requires fast and efficient removal of the radicals (polarizing agents). Ordered mesoporous SBA-15 silica materials containing homogeneously dispersed radicals, referred to as HYperPolarizing SOlids (HYPSOs), enable high polarization - P(1H) = 50% at 1.2 K - and straightforward separation of the polarizing HYPSO material from the hyperpolarized solution by filtration. However, the one-dimensional tubular pores of SBA-15 type materials are not ideal for nuclear spin diffusion, which may limit efficient polarization. Here, we develop a generation of hyperpolarizing solids based on a SBA-16 structure with a network of pores interconnected in three dimensions, which allows a significant increase of polarization, i.e. P(1H) = 63% at 1.2 K. This result illustrates how one can improve materials by combining a control of the incorporation of radicals with a better design of the porous network structures.

Solid-state NMR for the study of membrane systems: the use of anisotropic interactions.

The use of solid-state nuclear magnetic resonance (NMR) as a tool to determine the structure of membrane molecules is reviewed with a particular emphasis on techniques that provide information on orientation or order. Experiments reported here have been performed in membranes, rather than in micelles or organic solvents. Several ways to prepare and handle the samples are discussed, like sample orientation and magic-angle spinning (MAS). Results concerning lipids, membrane peptides and proteins are included, as well as a discussion regarding the potential of such methods and their pitfalls.

Anisotropy of rotational diffusion, dipole-dipole cross-correlated NMR relaxation and angles between bond vectors in proteins.

Cross correlations between the fluctuations of dipolar (13)C(α)-(1)H(α) interactions yield information about the relative orientation of successive (13)C(α)-(1)H(α) bond vectors in proteins, in turn providing a direct handle on their structure and dynamics in solution. However, overall anisotropic reorientation must be taken into account in the interpretation of cross-correlation rates. The protein shown, human ubiquitin, has amino acid residues in white where the cross-correlation rates deviate from those predicted for a rigid structure.

Single-transition coherence transfer by adiabatic cross polarization in NMR.

Spin-state selective coherence transfer between single-transition coherences in a scalar-coupled two-spin system may be achieved by employing highly selective, constant amplitude radio frequency fields. We extend this method by employing amplitude-modulated pulses to achieve an adiabatic transfer, as shown in the picture (I=intensity). Different limiting cases are considered. The imperfect reversibility of the transfer reveals deviations from adiabaticity.

Mapping the B1 field distribution with nonideal gradients in a high-resolution NMR spectrometer.

To understand the behavior of many NMR experiments, it is important to determine the spatial distribution of the B1 field. In this paper, we show how this distribution can be mapped independently of spin density, coil responsiveness, and nonlinearities of the B0 field gradients. As a by-product we obtain a map of the (possibly nonlinear) spatial variation of the B0 field gradients used in the imaging procedure.

Attenuation of cross-peak intensities in QUIET-BIRD-NOESY experiments.

The buildup curves in QUIET-BIRD-NOESY experiments, which are designed to isolate two-spin subsystems within macromolecules, are attenuated by transverse relaxation and evolution under homonuclear couplings during the bilinear rotation decoupling (BIRD) pulse sandwich. If the signals of both source and target spins are attenuated equally (uniform damping), this is readily accounted for by normalizing the cross peaks with respect to the diagonal peaks. However, unequal attenuation of source and target spins (differential damping) affects the initial buildup slopes and hence leads to apparent cross-relaxation rates that are significantly distorted from their true values. A simple method for recognizing this situation and extracting accurate cross-relaxation rates is presented.

Tilt angle dependence of cross-relaxation in off-resonance ROESY.

We present an efficient experimental method to evaluate whether the effective cross-relaxation rate between a pair of spins vanishes when applying an off-resonance spin-lock field. It is shown that the cross-relaxation rate can be made to vanish even when the two spins concerned resonate at different offsets and experience significantly different tilt angles of their respective spin-lock fields. This is verified experimentally using a sample of 15N-labeled human ubiquitin, through selective excitation of chosen amide protons. The results are relevant for the quantitative interpretation of off-resonance ROESY experiments.

Offset profiles of selective pulses in isotopically labeled macromolecules.

The experimental verification of offset profiles and calibration of selective pulses in NMR is usually carried out with doped water samples but not under conditions typical of macromolecules with short T2, long T1, and possibly homo- and heteronuclear couplings. A new method for selective excitation in isotopically labeled macromolecules is shown to be particularly suited to this purpose. This is illustrated for a backbone amide resonance in a sample of 15N-labeled human ubiquitin.

Identification of spin diffusion pathways in isotopically labeled biomolecules.

One-dimensional NOE experiments applicable to labeled macromolecules are presented which allow the manipulation of specific spin diffusion pathways and thus unambiguously identify clandestine spins through which the direct NOE is mediated. A treatment of spin diffusion using average Liouvillian theory is shown to describe adequately these phenomena. Experiments are carried out on an 15N-labeled sample of human ubiquitin.

Measurement of relaxation rates of N(H) and H(alpha) backbone protons in proteins with tailored initial conditions.

Several methods are presented for the selective determination of spin-lattice and spin-spin relaxation rates of backbone protons in labeled proteins. The relaxation rates of amide protons in (15)N labeled proteins can be measured by using two-way selective cross-polarization (SCP). The measurement of H(alpha) relaxation rates can be achieved by combining this method with homonuclear Hartmann-Hahn transfer using doubly selective irradiation. Various schemes for selective or nonselective inversion of the longitudinal proton magnetization lead to different initial recovery rates. The methods have been applied to lysine K6 in (15)N-labeled human ubiquitin and to leucine L5 in (15)N- and (13)C-labeled octapeptide YG*G*F*LRRI (GFL) in which the marked residues are (15)N- and (13)C-labeled.

Stereotypic biases in social decision making and memory: testing process models of stereotype use.

Two information-processing mechanisms that could potentially contribute to judgmental discrimination against the members of stereotyped social groups were examined in two experiments, using a mock juror decision-making task. Both postulated mechanisms involve biased processing of judgment-relevant evidence. The interpretation hypothesis asserts that the activation of stereotypic concepts influences the perceived probative implications of other evidence. The selective processing hypothesis asserts that stereotype-consistent evidence is processed more extensively than is inconsistent evidence. Judgment and memory data from the first experiment supported the general notion that stereotype-based discrimination emerges from biased evidence processing. The specific pattern of results supported selective processing rather than interpretation biases as the critical process underlying observed judgmental discrimination. The second experiment corroborated this conclusion by showing that a manipulation that prevents selective processing of the evidence effectively eliminated biases in judgments and recall pertaining to stereotyped targets. Implications for a general understanding of stereotyping and discrimination are discussed.

Effects of stereotypes on decision making and information-processing strategies.

In two experiments we investigated the effects of stereotyping on (a) reactions to a behavioral transgression and (b) the recall of information bearing on it. Subjects read a case file describing a transgression committed by a target (in Experiment 1, a job-related infraction; in Experiment 2, a criminal act). In some cases, the target's transgression was stereotypic of the target's ethnic group (conveyed through his name), and in other cases it was not. After reading the case file, subjects judged the likelihood that the transgression would recur and recommended punishment for the offense. These judgment data supported the hypothesis that stereotypes function as judgmental heuristics. Specifically, subjects used a stereotype of the target to infer the reasons for his transgression, and then based their punishment decisions on the implications of these inferences, considering other relevant information only when a stereotype-based explanation of the behavior was not available. However, recall data suggested that once a stereotype-based impression of the crime and its determinants was formed, subjects reviewed other available information in an attempt to confirm the implications of this impression. This led to differential recall of presented information, depending on whether its implications were consistent with, inconsistent with, or irrelevant to those of the stereotype.

Event memory: the effects of processing objectives and time delay on memory for action sequences.

The memorial representations of events that result from different types of goal-directed cognition are conceptualized on the basis of the general model of information processing proposed by Wyer and Srull (1980, 1984). In a test of this conceptualization, subjects read a passage describing the events that took place at a cocktail party. They were told either (a) to form an impression of the party and the events that occurred, (b) to empathize with the person from whose perspective the passage was written, or (c) to remember the information presented in a way that would allow them to reproduce it. The stimulus passage contained two target events, each consisting of actions that were either described chronologically or in reverse order, and were either presented together or were separated by other unrelated material. After either a short or a long delay, subjects recalled the information they read in the order it came to mind. Finally, subjects were given the individual event actions and told to place them in the order they were presented. The actions comprising target events were generally more likely to be recalled together and in chronological order when subjects had learned about them with either an impression formation or an empathy objective than when they had read about them with the goal of remembering them. However, orderings of these actions were affected by task objectives only after a long delay. The effect of task objectives on the order of recalling the events themselves showed a quite different pattern; for example, subjects with an empathy objective were most likely to recall the last target event presented before the first one after a long delay, whereas subjects with an impression objective were least likely to do so. The proposed model provided a reasonable account of these and other effects of task objectives on memory for events and the actions comprising them.

Social stereotypes and information-processing strategies: the impact of task complexity.

Subjects read information about a defendant in a criminal trial with initial instructions to judge either his guilt (guilt judgment objective) or his aggressiveness (trait judgment objective). The defendant was either Hispanic or ethnically nondescript. After considering the evidence, subjects made both guilt and aggressiveness judgments (regardless of which type of judgment they were instructed to make at the time they read the information) and then recalled as much of the information they read as they could. Results favored the hypothesis that when subjects face a complex judgmental situation, they use stereotypes (when available and relevant) as a way of simplifying the judgment. Specifically, they use the stereotype as a central theme around which they organize presented evidence that is consistent with it, and they neglect inconsistent information. Subjects with a (complex) guilt judgment objective judged the defendant to be relatively more guilty and aggressive and recalled more negative information about him if he was Hispanic than if he was ethnically nondescript. In contrast, subjects with a (simple) trait judgment objective did not perceive either the guilt or aggressiveness of the two defendants to be appreciably different, and did not display any significant bias in their recall of the evidence. These and other results are discussed in terms of the information-processing strategies subjects are likely to use when they expect to make different types of judgments.

Emotional communication in close relationships.

Emotional communication patterns characterizing interactions between partners in close relationships were investigated by asking 29 couples who were married or living together to engage in a videotaped discussion of a problem they were having in their relationship. In a later experimental session, partners identified specific communications that they believed had an important influence on the discussion and then rated the communications in terms of the feelings the communicator intended to convey and the recipient's reactions. Partners attempted to reciprocate both the positive and negative feelings that they perceived their partner to express toward them. However, only negative feelings were actually reciprocated. This was because subjects were sensitive to differences in the negative feelings their partners reported expressing and interpreted those feelings correctly, but they were inaccurate in perceiving their partners' expressions of positive feelings. Men (but not women) interpreted their partners' failures to express love as an indication of hostility, whereas women (but not men) interpreted their partners' lack of hostility as an indication of love. These and other results were conceptualized in terms of a general model of emotional communication. Parameters of the model pertaining to the hostility of partners' communications were often related to women's satisfaction with their relationship and their beliefs about relationships in general. However, they were unrelated to men's satisfaction and general beliefs. This suggested that women are generally more adversely affected by overt expression of hostility than are men.