Zwitter Ionization of Glycine at Outer Space Conditions due to Microhydration by Six Water Molecules

We investigate glycine microsolvation with water molecules, mimicking astrophysical conditions, in our laboratory by embedding these clusters in helium nanodroplets at 0.37 K. We recorded mass selective infrared spectra in the frequency range $1500–1800{\mathrm{cm}}^{-1}$ where two bands centered at 1630 and $1724{\mathrm{cm}}^{-1}$ were observed. By comparison with the extensive accompanying calculations, the band at $1630{\mathrm{cm}}^{-1}$ was assigned to the ${\mathrm{COO}}^{-}$ asymmetric stretching mode of the zwitter ion and the band at $1724{\mathrm{cm}}^{-1}$ was assigned to redshifted $\mathrm{C}=\mathrm{O}$ stretch within neutral clusters. We show that zwitter ion formation of amino acids readily occurs with only few water molecules available even under extreme conditions.

Figure 1

IR depletion spectra obtained for helium droplets doped with ${\mathrm{D}}_2\mathrm{O}$ clusters, glycine- $d_5$ monomer, and a combination of glycine-$d_5$ and ${\mathrm{D}}_2\mathrm{O}$, measured at different mass channels in the frequency range between 1500 and $1800{\mathrm{cm}}^{-1}$. (a) IR spectrum of pure ${\mathrm{D}}_2\mathrm{O}$ clusters at mass channel $m/z=162$, yielding no band in the investigated frequency range. (b) IR spectrum of pure glycine-$d_5$ at mass channel $m/z=34$. (c)–(e) IR spectra of $(\text{glycine}\text{−}{d}_5)-{({\mathrm{D}}_2\mathrm{O})}_n$ clusters at mass channels $m/z=162$, 182 and 202 recorded at a pressure of 2 mPa. The yellow, purple, and green shaded regions indicate the bands assigned to the $\mathrm{C}=\mathrm{O}$ stretch of glycine-$d_5$ monomer ($1782{\mathrm{cm}}^{-1}$), the redshifted $\mathrm{C}=\mathrm{O}$ stretch of nonionic $(\text{glycine}\text{−}{d}_5)-{({\mathrm{D}}_2\mathrm{O})}_n$ clusters ($1724{\mathrm{cm}}^{-1}$) and the asymmetric stretch of the ${\mathrm{COO}}^{-}$ group in zwitter ionic $(\text{glycine}\text{−}{d}_5)-{({\mathrm{D}}_2\mathrm{O})}_n$ clusters ($1630{\mathrm{cm}}^{-1}$), respectively. Note that $m/z=202$ corresponds to a microsolvated glycine-$d_5$-cluster containing at least 7 water molecules. The signal-to-noise ratio is reduced compared to the measurements at lower mass channels, because only fewer measurements were carried out and averaged.

Figure 2

Comparison of the IR spectrum of $(\text{glycine}\text{−}{d}_5)-{({\mathrm{D}}_2\mathrm{O})}_n$ clusters recorded at mass channel $m/z=162$ (top panel) with the vibrational density of states of $(\text{glycine}\text{−}{d}_5)-{({\mathrm{D}}_2\mathrm{O})}_6$ according to on-the-fly semiclassical dynamics (bottom panel). The harmonic frequencies of the $\mathrm{C}=\mathrm{O}$ stretch of nonionic $(\text{glycine}\text{−}{d}_5)-{({\mathrm{D}}_2\mathrm{O})}_6$ clusters and the asymmetric stretch of the ${\mathrm{COO}}^{-}$ group within zwitter ionic $(\text{glycine}\text{−}{d}_5)-{({\mathrm{D}}_2\mathrm{O})}_6$ clusters are shown by red and blue sticks, respectively. The anharmonic spectra (solid lines) have been obtained from divide-and-conquer semiclassical simulations of these modes. Frequency scaling with respect to the $\mathrm{C}=\mathrm{O}$ stretch of pure glycine-$d_5$ serving as an intrinsic reference is applied (see Supplemental Material [38], Sec. 6). Please note, that, e.g., dissociated clusters with $n=10$ are predicted to show redshifted bands (see Fig. S9 in Ref. [38]). Thus, higher clusters might contribute to the broad absorbance below $1630{\mathrm{cm}}^{-1}$.

Figure 3

Fit of the pick-up curve at $1630{\mathrm{cm}}^{-1}$ of the zwitter ionic $(\text{glycine}\text{−}{d}_5)-{({\mathrm{D}}_2\mathrm{O})}_6$ clusters. (a) Depletion signal as a function of ${\mathrm{D}}_2\mathrm{O}$ partial pressure at mass channel $m/z=142$ (black dots) in comparison with reference pick-up curves for $k=5$, 6, and 7. The quality of the fit is evaluated based on the weighted residual sum of squares (WRSS). The best fit (smallest WRSS) is obtained for $k=6$ and $k=7$. b) Depletion signal as a function of ${\mathrm{D}}_2\mathrm{O}$ partial pressure at mass channel $m/z=102$ (black dots). The best fit (smallest WRSS) is obtained for $k=5$ and $k=6$. c) Fits of the pick-up curve for a combination of clusters with six to ten water molecules (red line), showing the individual contribution of six (dashed red line) and ten (dashed green line) clusters.