Practical Aspects of Carbon-13 Double Quantum NMR
We want to address an erroneous view that has recently been advanced on 13C double quantum NMR. A recent publication (I) introduced a modified version (I) of the original double quantum NMR experiments; this new version claims to offer an attractive increase in sensitivity by a factor of three, or a decrease in the size of the required data matrix by a factor of four. To avoid disappointment among spectroscopists we wish to make it clear that this modified version offers no gain in sensitivity whatsoever in comparison with the other double quantum experiments (2-6), provided they are properly optimized. It is shown here that the sensitivity of this modified experiment can be even slightly lower. The claim of a reduction of the size of the required data matrix is also shown to be incorrect. In the original two-dimensional double quantum “C experiments, the acquisition time t2 starts immediately after the final “read pulse” (Fig. 1), and the time during which the double quantum coherence evolves is labeled ti. The detected 13C-13C satellites (7) are modulated in amplitude as a function of tl with the double quantum frequencies (2). In the quadrature-Fi-detection version of this experiment (3), results are combined with those of a second experiment which contains a composite 45: pulse (8) in the evolution period. This allows the sign of the modulation frequency to be determined, which simplifies the interpretation of the results. A simpler and more sensitive alternative to obtain quadrature F, detection has been proposed more recently (6); an increase of the flip angle of the final “read pulse” to 135” provides directly satellite signals which are modulated in phase as a function oft,. In either case, data acquisition is started after the final read pulse and the coherence transfer echo (9) is detected (4, 6). The time domain signal of a carbon A, directly coupled to carbon X, is then given by sA(tl, t2) = c exp[-i(nA + &)t,] exp[i& + d&2]
Separation of the different orders of NMR multiple-quantum transitions by the use of pulsed field gradients
Abstract Any order of transition can be detected separately in a two-dimensional multiple-quantum spin-echo experiment by applying a matching pair of pulsed field gradients. The method is based on the fact that the rate of defocusing and refocusing in an inhomogeneous magnetic field is proportional to the order of coherence. Natural line widths can be obtained for each order of transition.
Vertebral Fusion in Two Early Jurassic Sauropodomorph Dinosaurs from the Lufeng Formation of Yunnan, China
Here we describe two instances of pathological vertebral fusion in two genera of sauropodomorph dinosaurs from the Early Jurassic Lufeng Formation in Yunnan, China. The first is a specimen, of Lufengosaurus huenei with two fused cervical vertebrae, and the other is a specimen of the Lufeng basal sauropod, with two fused caudal vertebrae. Both pathologies are consistent with spondyloarthropathy and represent the earliest known occurrence of that disease in dinosaurs. The two specimens affirm that early dinosaurs suffered from the same bone diseases as living vertebrates. Spondyloarthropathy in these dinosaurs may have been induced by long-term mechanical stress, such as weight bearing, and/or limited motion at the joint that would otherwise have inhibited such remodeling. In both cases, surface remodeling suggests that the animals survived well beyond the initiation of spondyloarthropathy.