Elemental fingerprinting of mineral species in iron-fortified milk: anomalous small-angle X-ray scattering and resonant soft X-ray scattering studies

Abstract

Anomalous small-angle X-ray scattering (ASAXS) and resonant soft X-ray scattering (RSoXS) are two related techniques that can enable element-specific structural information to be obtained. The development of iron-fortified milk products can greatly benefit from such techniques, allowing the structure of iron and other minerals (such as native calcium) within the casein micelle to be determined. Each method has advantages and disadvantages: for ASAXS, the sample preparation is straightforward, but the signal is relatively low and information about the structure of Ca is difficult to access. RSoXS can be used to study both Ca and Fe, and the element-specific signals observed are proportionally much higher; however, the measurements are challenging due to the difficulty of precise control of the solution thickness using currently available vacuum-compatible liquid cells. Nevertheless, complementary results from both techniques indicate Fe is co-located with Ca, i.e. within the colloidal calcium phosphate nanoclusters that are present within native casein micelles in milk.

Resonant X-ray scattering of biological assemblies

“…Previous RSoXS measurements from biomolecules in solutions have relied on liquid samples that are loaded into a sandwiched silicon nitride cell with a spacer between two silicon nitride thin films [127][128][129][130]133 Resonant X-ray scattering with polarized soft X-rays, also called P-SoXS, can probe molecular orientation in soft matter without relying on crystallinity, such as within polymer systems 30,150 . NEXAFS is sensitive to polarization of incident X-rays and it can characterize orientational states of peptides, proteins, and nucleic acids [151][152][153] ; however, these studies were carried out on films immobilized on substrates.…”

“…NEXAFS is sensitive to polarization of incident X-rays and it can characterize orientational states of peptides, proteins, and nucleic acids [151][152][153] ; however, these studies were carried out on films immobilized on substrates. Recent RSoXS studies of biomolecules in solution, including casein micelles [127][128][129][130] and BSA 133 , suggest feasibility of P-SoXS of biomolecules in near physiological states that will enable the determination of orientational correlations between domains.…”

“…RSoXS at the Ca L-edge and the Fe LII, III -edge (~710 eV) has been combined with ASAXS at the Fe K-edge (~7.107 keV) to obtain structural information from iron-fortified cow milk 130 .…”

Past, present and future—sample environments for materials research studies in scattering and spectroscopy; a UK perspective

“…Developments in single pixel photon counting technologies including energy discrimination (Martiel et al 2020), integrating detectors will allow new areas to develop. Anomalous SAXS as applied to soft matter has a few devotees (Goerigk et al 2004, Haas et al 2010, Sztucki et al 2012, Ingham et al 2018 but is not currently mainstream. Advances in energy discrimination, especially in 'windowed' mode would open up new areas for pink-beam SAXS instruments where spectroscopy could be combined with scattering.…”

ASAXS measurements on ferritin and apoferritin at the bioSAXS beamline P12 (PETRA III, DESY)

“…The most abundant elements in soft matter and biological samples (like carbon, nitrogen, oxygen, sodium, potassium and calcium) have relatively low energy absorption edges, limiting the practical application of ASAXS for biological macromolecules. Some applications exploit the use of soft X-rays (E < 4 keV) at resonant scattering conditions for soft-matter samples (Ingham et al, 2018;Liu et al, 2016;Salamoń czyk et al, 2017). We note that at low energies resonant scattering is sometimes called RoSXS or ASAXS.…”