Measurements of the stabilities of isolated retinal chromophores

The barrier energies for isomerization and fragmentation were measured for a series of retinal chromophore derivatives using a tandem ion mobility spectrometry approach. These measurements allow us to quantify the effect of charge delocalization on the rigidity of chromophores. We find that the role of the methyl group on the $C13$ position is pivotal regarding the ground state dynamics of the chromophore. Additionally, a correlation between quasi-equilibrium isomer distribution and fragmentation pathways is observed.

Figure 1

Several of the important isomers of the RPSB. In animal vision the native state of the chromophore is the $11-cis$ isomer shown in (a), which photoisomerizes into the $all-trans$ isomer (b). In bacteriorhodopsin the native state is the $all-trans$ one and photoisomerization results in the $13-cis$ isomer (c). The gas phase fragmentation of the RPSB involves the formation of cyclized isomers (d) and (e), eventually resulting in Toluene emission (f).

Figure 2

Source distribution of RPSB. (a) The counts are imaged as a function of drift time and mass over charge. (b) Mass spectrum resulting from summing over the drift times. (c) IMS of the RPSB (solid), resulting from an integration over a narrow mass window around mass $340\mathrm{m}/\mathrm{z}$ indicated by the blue dashed lines in (a). The dashed line represents the measured IMS after selection is applied to a narrow time window indicated by the red dashed lines, corresponding to peak A.

Figure 3

Selection and activation measurement of the RPSB at $V_{\mathrm{act}}=180\mathrm{V}$ with the selection applied to peak A. (a) The number of counts are plotted as a function of drift time and mass. (b) Mass spectrum resulting from summing over the drift times. (c) IMS of the RPSB resulting from an integration over a narrow mass window around mass $340\mathrm{m}/\mathrm{z}$.

Figure 4

(a) IMS of the retinal and the retinal derivatives. (b) The structure of the RPSB derivatives (c) The IMS of the retinal and the retinal derivatives where the $x$ axis is the cross section divided by mass.

Figure 5

Selection and activation measurements where selection is applied to peak X of the dimethylated RPSB, for various activation voltages $V_{\mathrm{act}}$. The dashed black line corresponds to source distribution (when no selection and actibation is applied).

Figure 6

Selection and activation measurements where selection is applied to peak A of the dimethylated RPSB, for various activation voltages $V_{\mathrm{act}}$. The dashed black line corresponds to source distribution (when no selection and activation is applied).

Figure 7

Thresholds plots for several of the transitions of the dimethylated RPSB.

Figure 8

Schematic illustration of the ground-state energies and the energy barrier for isomerization of the RPSB and the RPSB derivatives measured here.

Figure 9

Quasi-equilibrium distribution of the RPSB and the RPSB derivatives where at the $x$ axis the cross section is dividing by the mass of the molecule, measured at $V_{\mathrm{act}}=180\mathrm{V}$.

Figure 10

IMS-IMS distribution as a function of activation voltage when selection applied on peak A and X of the 9-des and 13-des RPSB derivatives.