Abstract
Hydrated calcium pyrophosphates (CPP, Ca2P2O7·nH2O) are a fundamental family of materials among osteoarticular pathologic calcifications. In this contribution, a comprehensive multinuclear NMR (Nuclear Magnetic Resonance) study of four crystalline and two amorphous phases of this family is presented. 1H, 31P and 43Ca MAS (Magic Angle Spinning) NMR spectra were recorded, leading to informative fingerprints characterizing each compound. In particular, different 1H and 43Ca solid state NMR signatures were observed for the amorphous phases, depending on the synthetic procedure used. The NMR parameters of the crystalline phases were determined using the GIPAW (Gauge Including Projected Augmented Wave) DFT approach, based on first-principles calculations. In some cases, relaxed structures were found to improve the agreement between experimental and calculated values, demonstrating the importance of proton positions and pyrophosphate local geometry in this particular NMR crystallography approach. Such calculations serve as a basis for the future ab initio modeling of the amorphous CPP phases. Statement of significanceThe general concept of NMR crystallography is applied to the detailed study of calcium pyrophosphates (CPP), whether hydrated or not, and whether crystalline or amorphous. CPP are a fundamental family of materials among osteoarticular pathologic calcifications. Their prevalence increases with age, impacting on 17.5% of the population after the age of 80. They are frequently involved or associated with acute articular arthritis such as pseudogout. Current treatments are mainly directed at relieving the symptoms of joint inflammation but not at inhibiting CPP formation nor at dissolving these crystals. The combination of advanced NMR techniques, modeling and DFT based calculation of NMR parameters allows new original insights in the detailed structural description of this important class of biomaterials.
Highlights
Crystalline calcium pyrophosphate dihydrates (CPPD, Ca2P2O7·2H2O) are among the most common forms of pathologic articular minerals: their prevalence increases with age, impacting on 17.5% of the population after the age of 80.1 often asymptomatic, they are frequently involved or associated with acute articular arthritis such as pseudogout, and, more rarely, with chronic polyarthritis and destructive arthropathy; current treatments are mainly directed at relieving the symptoms of joint inflammation but not at inhibiting calcium pyrophosphate (CPP) formation nor at dissolving these crystals.[2,3,4]
Gras et al.[7] performed a systematic investigation of the synthesis of pure hydrated calcium pyrophosphates. They described the pH and temperature conditions leading to the formation of m-CPPT β, t-CPPD and m-CPPD,[10] as well as the identification of a new monohydrated calcium pyrophosphate phase exhibiting monoclinic symmetry, referred to as m-CPPM (CPPM: Ca2P2O7·H2O),[11] and an unreferenced highly metastable trihydrated monoclinic calcium pyrophosphate phase derived from the structure of m-CPPT β
The purpose of this study is to demonstrate, using a combined experimental-computational approach, how solid state NMR can be used for the structural investigation of calcium pyrophosphate phases, whether hydrated or anhydrous, and whether crystalline or amorphous
Summary
Crystalline calcium pyrophosphate dihydrates (CPPD, Ca2P2O7·2H2O) are among the most common forms of pathologic articular minerals: their prevalence increases with age, impacting on 17.5% of the population after the age of 80.1 often asymptomatic, they are frequently involved or associated with acute articular arthritis such as pseudogout, and, more rarely, with chronic polyarthritis and destructive arthropathy; current treatments are mainly directed at relieving the symptoms of joint inflammation but not at inhibiting calcium pyrophosphate (CPP) formation nor at dissolving these crystals.[2,3,4]CPP have been identified in vivo as two polymorphs of CPPD:[5] a triclinic form with a known structure,[6] and a monoclinic form with a recently solved structure[7] (respectively denoted as t-CPPD and m-CPPD). Other crystalline forms of hydrated calcium pyrophosphates have been synthesized in vitro and characterized, including a dimorphic monoclinic tetrahydrate (CPPT: Ca2P2O7·4H2O), referred to as m-CPPT α and m-CPPT β.8,9. Gras et al.[7] performed a systematic investigation of the synthesis of pure hydrated calcium pyrophosphates. They described the pH and temperature conditions leading to the formation of m-CPPT β, t-CPPD and m-CPPD,[10] as well as the identification of a new monohydrated calcium pyrophosphate phase exhibiting monoclinic symmetry, referred to as m-CPPM (CPPM: Ca2P2O7·H2O),[11] and an unreferenced highly metastable trihydrated monoclinic calcium pyrophosphate phase derived from the structure of m-CPPT β.12. They described the pH and temperature conditions leading to the formation of m-CPPT β, t-CPPD and m-CPPD,[10] as well as the identification of a new monohydrated calcium pyrophosphate phase exhibiting monoclinic symmetry, referred to as m-CPPM (CPPM: Ca2P2O7·H2O),[11] and an unreferenced highly metastable trihydrated monoclinic calcium pyrophosphate phase derived from the structure of m-CPPT β.12 The preparation of amorphous phases of biological interest, noted a-CPP (Ca2P2O7·nH2O), has been reported by Slater et al.[13] and Gras et al.,[10] and it was found that these phases are stable compared to amorphous calcium orthophosphate and amorphous calcium carbonate
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