A. As there is very little difference in the average 1H
per unit volume for proteins, the neutron contrasts calculated at different %
v/v
2H2O for components comprising a protein-protein complex
are almost identical (grey and black linear relations). Consequently, the low
resolution structure restored from a SANS with contrast variation experiment
will reflect the shape of the whole complex (grey surface representation).
B. Isotopic labelling of a component with non-exchangeable
deuterium has a dramatic effect on the contrast relationships and the separation
of match points for the individual components and of the whole complex. In this
example, component 1 is labelled on average with 60% non-exchangeable
2H (grey) while component 2 remains as a native
1H-protein (orange). When the scattering contributions of the native
1H-protein are matched out (~40% v/v
2H2O; Δρ2 = 0;
orange line), the scattering intensities will be derived from
2H-component 1, the magnitude of which will be proportionate to
Δρ12 and
V12 and
P1(q). On increasing the %
v/v
2H2O even further a point is reached when
Δρ for the whole complex limits to zero
(~75% v/v; black line) whereby the scattering signal
will be exceptionally weak (essentially incoherent scattering and scattering
from 1H-2H exchange). Eventually 2H-component 1
will be matched out at high % v/v
2H2O (~91% v/v
2H2O;Δρ1 = 0;
grey line) leaving coherent scattering contributions from the
1H-component 2 (proportionate to
Δρ22 and
V22 and
P2(q)). From a set of contrast
variation data it is possible to determine the shapes of the entire complex, of
the individual components and the orientations of the components within the
complex.