Herein, we present the first experimental observation of isolated progesterone, an endogenous steroid, placed in the gas phase by laser ablation and characterized in a supersonic expansion by Fourier transform microwave techniques. Guided by quantum-chemical calculations, we assigned the rotational spectrum of the most stable structure. The internal rotation of the acetyl methyl group led to the observation of A-E doublets in the spectrum, which were analyzed, resulting in a V3 barrier of 2.4425 ± 0.0025 kJ mol-1. By fitting over 250 transitions, we determined accurate rotational constants that enabled us to compare the gas phase geometrical parameters with those of crystalline forms and complexes with progesterone receptors. Our results indicate that the A ring of progesterone that contains the ketone group is surprisingly flexible, despite its rigid appearance. This finding is particularly significant, since this ring is an active biological site that is involved in strong intermolecular interactions. Notably, progesterone C21H30O2 is the largest molecule investigated using laser ablation rotational spectroscopy.

Departamento de Química Física Facultad de Ciencia y Tecnología Universidad del País Vasco 48940 Leioa Spain

Department of Analytical Chemistry University of Chemistry and Technology Technická 5 166 28 Prague 6 Czech Republic

Grupo de Espectrocopía Molecular Edificio Quifima Área de Química Física Laboratorios de Espectroscopia y Bioespectroscopia Parque Científico UVa Unidad Asociada CSIC Universidad de Valladolid 47011 Valladolid Spain

Instituto Biofisika University of the Basque Country 48940 Leioa Spain

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