Two-dimensional Ca-43-H-1 correlation solid-state NMR spectroscopy

Calcium-43 (nuclear spin, S = 7/2) is an NMR insensitive low-gamma quadrupolar nucleus and up until recently only one-dimensional solid-state Ca-43 NMR spectra have been reported. Through-space correlation experiments are challenging between spin-1/2 and low-gamma quadrupolar nuclei because of the intrinsically weak dipolar interaction and the often-low natural abundance of the quadrupolar nucleus. Rotary-resonance recoupling (R-3) has recently been used to re-introduce hetero-nuclear dipolar interactions for sensitive high-gamma quadrupolar nuclei, but has not yet been applied in the case of low-gamma half-integer quadrupolar nuclei. Here an effective and robust 2D H-1-Ca-43 NMR correlation experiment combining the R-3 dipole-recoupling scheme with 2D HMQC is presented. It is demonstrated that the weak Ca-43-H-1 dipolar coupling in hydroxyapatite and oxy-hydroxyapatite can be readily re-introduced and that this recoupling scheme is more efficient than conventional cross-polarization transfer. Moreover, three Ca-43-H-1 dipolar coupled calcium environments are clearly resolved in the structurally unknown oxy-hydroxyapatite. This local information is not readily available from other techniques such as powder XRD and high resolution electron microscopy. R-3-HMQC is also a desirable experiment because the set-up is simple and it can be applied using conventional multi-resonance probes. (C) 2008 Elsevier Inc. All rights reserved