  We report a method to control the conformation of a weak polyampholyte (the
protein $\beta$-casein) in Langmuir monolayers by light, even though the
protein is not photosensitive. Our approach is to couple the monolayer state to
a photochemical reaction excited in the liquid subphase. The conformational
transition of the protein molecule is triggered through its sensitivity to a
subphase bulk field (pH in this study), changing in the course of the
photochemical process. Thus, reaction of photoaquation of the ferrocyanide ion,
which increases the subphase pH from 7.0 to about 8.3, produces a change in the
surface monolayer pressure, $\Delta\Pi$, between -0.5 and +1.5 ${\rm mN/m}$
(depending on the surface concentration), signalling a conformational switch.
The approach proposed here can be used to selectively target and influence
different interfacial properties by light, without embedding photosensitisers
in the matrix.
