Two-dimensional infrared correlation spectroscopy of synthetic and biological apatites

Two-dimensional (2D) infrared (IR) correlation spectroscopy was used to monitor the v₁, v₃ phosphate contour (900-1200 cm^-1) of maturing poorly crystalline hydroxyapatite in synthetic (synthesized at constant and variable pH) and biological (calcified turkey leg tendon) systems. The 2D IR plots of the mineral prepared at variable pH exhibit peaks at 961, 999, 1018, 1036, 1095, 1126, and 1150 cm^-1. The peaks at 961, 999, and 1095 cm^-1 represent vibrations of PO₄^3- in an apatitic/stoichiometric environment of poorly crystalline HA, while those at 1018, 1036, and 1126 cm^-1 arise from PO₄^3- in a nonstoichiometric/acid phosphate environment of poorly crystalline HA. The 2D IR analysis suggests that the intensities of peaks associated with PO₄^3- in a nonstoichiometric/acid phosphate environment decrease as the reaction progresses. The 2D IR plots of the mineral formed at constant pH showed only bands characteristic of PO₄^3- in a stoichiometric/acid phosphate environment. Analysis of the 2D IR plots of the mineral from calcified turkey leg tendon reveals peaks at 1019, 1039, 1075, 1126, and 1147 cm^-1. The peaks at 1019, 1039, and 1126 cm^-1 are characteristic of PO₄^3- in a nonstoichiometric/acid phosphate environment of poorly crystalline HA, while the band at 1075 cm^-1 is characteristic of PO₄^3- in an apatitic/stoichiometric environment of poorly crystalline HA. Thus, the in vitro experiment in which the mineral is formed at variable pH is a better model of the mineral phase in calcified turkey leg tendon. In addition, the asynchronous plots from both the synthetic and biological minerals revealed those peaks which were noncorrelated. Also, this method of data analysis provided enhanced resolution of the highly overlapped v₁, v₃ phosphate contour commonly seen in Fourier transform-IR spectra of calcified tissue.