Abstract
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 (13)C and (15)N 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 (13)C NMR spectrum of (13)C retinal-labeled TAT rhodopsin, the isolated 14-(13)C signals of 13-trans/15-anti and 13-cis/15-syn isomers were observed at a ratio of 7:3. (15)N retinal protonated Schiff base (RPSB) had a significantly higher magnetic field resonance at 160 ppm. In (15)N 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.