Functional and shunt states of bacteriorhodopsin resolved by 250 GHz dynamic nuclear polarization–enhanced solid-state NMR

Abstract

Observation and structural studies of reaction intermediates of proteins are challenging because of the mixtures of states usually present at low concentrations. Here, we use a 250 GHz gyrotron (cyclotron resonance maser) and cryogenic temperatures to perform high-frequency dynamic nuclear polarization (DNP) NMR experiments that enhance sensitivity in magic-angle spinning NMR spectra of cryo-trapped photocycle intermediates of bacteriorhodopsin (bR) by a factor of Ϸ90. Multidimensional spectroscopy of U-13 C, 15 N-labeled samples resolved coexisting states and allowed chemical shift assignments in the retinylidene chromophore for several intermediates not observed previously. The correlation spectra reveal unexpected heterogeneity in dark-adapted bR, distortion in the K state, and, most importantly, 4 discrete L substates. Thermal relaxation of the mixture of L's showed that 3 of these substates revert to bR568 and that only the 1 substate with both the strongest counterion and a fully relaxed 13-cis bond is functional. These definitive observations of functional and shunt states in the bR photocycle provide a preview of the mechanistic insights that will be accessible in membrane proteins via sensitivity-enhanced DNP NMR. These observations would have not been possible absent the signal enhancement available from DNP.ultidimensional magic-angle spinning (MAS) solid-state NMR is a general tool in structural studies of membrane proteins that are inaccessible to crystallography and solutionstate NMR, as demonstrated by recent successful applications (1-3). But the sensitivity of these experiments is low, which becomes a significant problem when multidimensional experiments are needed to characterize systems of higher molecular weight. The sensitivity deficit is even more acute when NMR signals are further divided among multiple states, as is often the case for trapped reaction intermediates. Consequently, a 1-2 order of magnitude enhancement of NMR sensitivity is essential for applications of multidimensional MAS NMR methods to studies of reaction intermediates of membrane proteins.One approach to improving the sensitivity of NMR is dynamic nuclear polarization (DNP), in which the Ϸ660-fold greater spin polarization of unpaired electrons in a paramagnetically doped glassy matrix is transferred to nuclei before an NMR experiment (4). Here, we demonstrate that high-frequency DNP by using a stable, high-power 250 GHz microwave source (5) and an efficient, nonperturbing biradical polarizing agent (6, 7), is a potentially general approach for biological MAS NMR. A 43-fold signal enhancement from DNP, combined with operation at 90 K, yields an overall 90-fold signal enhancement over previous experiments at 183 K (8). The resulting Ϸ8,100-fold savings in acquisition time permits 2-dimensional (2D) resolution of signals from mixtures of reaction intermediates that would be impossible to observe absent the enhancement available from DNP.In bacteriorhodopsin (bR), 7 transmembrane helices surround a transport channel in whic...

Reaction Pathways of Photoexcited Retinal in Proteorhodopsin Studied by Pump−Dump−Probe Spectroscopy

“…Heterogeneity in bacteriorhodopsin has been suggested previously to result from conformational plasticity of the protein that would modify the structure around the chromophore and result in a distribution of rate constants for the decay of the excited state. 29 Recently, the simultaneous existence of several 13-cis, 15-syn states (representing slightly different conformations in the neighborhood of the Schiff-base linkage) in dark-adapted bacteriorhodopsin has been directly observed by solid state NMR experiments, 30 as well as heterogeneity within several of the cryotrapped intermediates. 31 Here, we show that heterogeneity leads to "primed" chromophores that isomerize fast and "not-primed" chromophores that isomerize slowly and with low yield.…”

Coherent Two-Dimensional Terahertz Magnetic Resonance Spectroscopy of Collective Spin Waves

“…The methodology is facilitated by the ease with which the regime of strong coupling can be reached between radio frequency or microwave magnetic fields and nuclear or electron spins, respectively, typified by sequences of magnetic pulses that control the magnetic moment directions [1][2][3]. The capabilities meet a bottleneck, however, for far-infrared magnetic resonances characteristic of correlated electron materials, molecular magnets, and metalloproteins.ESR in the terahertz (THz) frequency region can reveal rich information content in chemistry, biology, and materials science [1,2,[4][5][6][7]. In molecular complexes and metalloproteins, THz-frequency zero-field splittings (ZFS) of high-spin transition-metal and rare-earth ions show exquisite sensitivity to ligand geometries, providing mechanistic insight into molecular magnetic properties [4] and protein catalytic function [5].…”

“…Nonlinear manipulation of spins is the basis for all advanced methods in magnetic resonance including multidimensional nuclear magnetic resonance and electron spin resonance (ESR) spectroscopies [1,2], magnetic resonance imaging, and, in recent years, quantum control over individual spins [3]. The methodology is facilitated by the ease with which the regime of strong coupling can be reached between radio frequency or microwave magnetic fields and nuclear or electron spins, respectively, typified by sequences of magnetic pulses that control the magnetic moment directions [1][2][3].…”

“…The methodology is facilitated by the ease with which the regime of strong coupling can be reached between radio frequency or microwave magnetic fields and nuclear or electron spins, respectively, typified by sequences of magnetic pulses that control the magnetic moment directions [1][2][3]. The capabilities meet a bottleneck, however, for far-infrared magnetic resonances characteristic of correlated electron materials, molecular magnets, and metalloproteins.…”

“…ESR in the terahertz (THz) frequency region can reveal rich information content in chemistry, biology, and materials science [1,2,[4][5][6][7]. In molecular complexes and metalloproteins, THz-frequency zero-field splittings (ZFS) of high-spin transition-metal and rare-earth ions show exquisite sensitivity to ligand geometries, providing mechanistic insight into molecular magnetic properties [4] and protein catalytic function [5].…”