Real-time swelling-series method improves the accuracy of lamellar neutron-diffraction data

Neutron-diffraction data were collected from stacked bilayers of 1,2-dioleoyl-sn-glycero-phosphocholine under conditions of increasing relative humidity at both 0 and 8.06% ²H₂O. Over the period of data collection, the d-repeat of both swelling-series samples increased. Each family of structure factors, representing each of the five orders of diffraction, are shown to lie on smooth curves, allowing structure factors of intermediate d-repeat to be determined. In the case of the 8.06% ²H₂O data, but not the 0% ²H₂O data, all observed structure factors lie on a single continuous transform. 8.06% ²H₂O has a net neutron-scattering density of zero; its use in neutron-diffraction experiments presents a novel application of the so-called 'minus fluid' approach, without mathematical manipulation. The data are used to demonstrate the increased accuracy inherent in this real-time swelling-series approach. A quantitative analysis of errors caused by differences in d-repeat in difference subtractions is presented.