Laboratory rotational spectroscopy and interstellar search for the protein precursor 4-oxobutanenitrile (HCOCH$_2$CH$_2$CN)
Understanding the presence and distribution of prebiotic precursors in the interstellar medium (ISM) is key to tracing the chemical origins of life. Among them, 4-oxobutanenitrile (\ch{HCOCH2CH2CN}) has been identified in laboratory simulations as a plausible intermediate in the formation of glutamic acid, a proteinogenic amino acid. Here, we report its gas-phase rotational spectrum, measured using two complementary techniques: chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy (2$-$18 GHz) and free-jet millimeter-wave (FJ$-$AMMW) absorption spectroscopy (59.6$-$80 GHz). Quantum chemical calculations revealed nine low-energy conformers, of which the TC conformer was assigned based on the measured spectra. The resulting spectroscopic parameters were used to search for the molecule in the ultradeep spectral survey of the G+0.693-0.027 molecular cloud, located in the Galactic Center. No signal attributable to 4$-$oxobutanenitrile was detected. A stringent upper limit to its column density was derived ($N<$ 4 $\times$10$^{12}$ cm$^{-2}$), corresponding to a molecular abundance of $<$ 2.9 $\times$10$^{-11}$ relative to H$_{2}$. This upper limit lies well below the observed abundances of simpler structurally related species containing $-$HCO and $-$CN groups, underscoring the challenge of detecting increasingly complex prebiotic molecules in the ISM and the need for future, more sensitive astronomical facilities.