  (Abridged) Life is distinctly homochiral. The origins of this homochirality
are under active debate. Recently, propylene-oxide has been detected in the
gas-phase interstellar medium (ISM) (McGuire et al. 2016). The enantiomeric
composition of ISM propylene-oxide may be probed through circular polarization
measurements, but accurate estimates of the circular dichroism properties of
the microwave transitions of propylene-oxide are not available. We develop a
model of the circular dichroic activity in torsion-rotation transitions of
closed-shell chiral molecules, such as propylene-oxide. With this model, we
estimate the viability, and optimize observation strategies, of enantiomeric
excess detection in ISM propylene-oxide. We present estimates for the dichroic
activity of the torsion-rotation transitions of propylene-oxide, where we
predict that the circular polarization fractions of emission lines of
enantiopure propylene-oxide relevant to astronomical detection of
propylene-oxide are on the order of 10^(-6). Due to the low predicted circular
polarization fractions, we conclude that enantiomeric characterization of
propylene-oxide in the gas phase of the ISM is impossible with current
astronomical observation techniques. We suggest that only chiral radical
species may be viably employed for enantiomeric excess detection. We estimate
that laboratory experiments may be successful in detecting the enantiomeric
composition of a mixture of propylene-oxide through microwave dichroism
spectroscopy. The theory we present in this paper provides a solid theoretical
underpinning for such laboratory circular dichroism measurements in microwave
transitions.
