On the chemistry of 1-pyrroline in solution and in the gas phase.

1-Pyrroline has a highly characteristic odor, which is employed by living organisms for chemical signaling and other purposes, but the mechanism whereby this odor is formed remains poorly understood. Here we used a combination of ambient mass spectrometry (AMS) and nuclear magnetic resonance (NMR) spectroscopy to experimentally address the mechanistic aspects of 1-pyrroline volatility and other controversies regarding the chemistry of this compound. Our results indicate that in solution the volatility of the monomer species is significantly higher than that of the trimer species, and 1-pyrroline is evaporated mainly in its monomer state. Neat 1-pyrroline is essentially the pure trimer and displays ca. 100-fold lower evaporation rate than the monomer state in solution. In the gas-phase the trimer species is irreversibly decomposed into monomer species. Under equilibrium conditions the vapor of 1-pyrroline entirely consists of monomer species. The evaporation rate of 1-pyrroline in water has a step-wise dependence on the solution pH, the abrupt increase in volatility (>1,000-fold) occurring around the pKa value of 1-pyrroline (6.8). The pronounced step-wise dependence of 1-pyrroline volatility around neutral pH may also be an important evolutionary factor allowing living systems to regulate the odor strength from very weak to very strong with minimal efforts.