Solid-state NMR for the characterization of retinal chromophore and Schiff base in TAT rhodopsin embedded in membranes under weakly acidic conditions

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TAT rhodopsin extracted from the marine bacterium SAR11 HIMB114 has a characteristic Thr-Ala-Thr motif and contains both protonated and deprotonated states of Schiff base at physiological pH conditions due to the low pK_a. Here, using solid-state NMR spectroscopy, we investigated the ¹³C and ¹⁵N NMR signals of retinal in only the protonated state of TAT in the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho (1'-rac-glycerol) (POPE/POPG) membrane at weakly acidic conditions. In the ¹³C NMR spectrum of ¹³C retinal-labeled TAT rhodopsin, the isolated 14-¹³C signals of 13-trans/15-anti and 13-cis/15-syn isomers were observed at a ratio of 7:3. ¹⁵N retinal protonated Schiff base (RPSB) had a significantly higher magnetic field resonance at 160 ppm. In ¹⁵N RPSB/λ_max analysis, the plot of TAT largely deviated from the trend based on the retinylidene-halide model compounds and microbial rhodopsins. Our findings indicate that the RPSB of TAT forms a very weak interaction with the counterion.