Under these circumstances, the visible absorption of K is expected to be more red-shifted than is observed and this suggests torsion around single bonds of the retinylidene chromophore. This is in contrast to the development of a strong counterion interaction and double bond torsion in L. Thus, photon energy is stored in electrostatic modes in K and is transferred to torsional modes in L. This transfer is facilitated by the reduction in bond alternation that occurs with the initial loss of the counterion interaction, and is driven by the attraction of the Schiff base to a new counterion. Nevertheless, the process appears to be difficult, as judged by the multiple L substates, with weaker counterion interactions, that are trapped at lower temperatures. The doublebond torsion ultimately developed in the first half of the photocycle is probably responsible for enforcing vectoriality in the pump by causing a decisive switch in the connectivity of the active site once the Schiff base and its counterion are neutralized by proton transfer.energy transduction ͉ photocycle intermediates ͉ dynamic nuclear polarization ͉ ion transport ͉ retinal protein T he light-driven ion pump, bacteriorhodopsin (bR), has been studied extensively since it was discovered in the 1970s. Its availability and stability have made it the prototypical transmembrane protein, ion pump, retinal pigment, and model for G protein-coupled receptors. As such, it has been the target of a wide variety of biophysical techniques that have garnered a great deal of information about the structure of the protein and the changes that it undergoes during its functional photocycle. Nevertheless, it remains unclear how the protein stores and channels energy to translocate ions and prevent backflow.An important feature of the pump cycle ( Fig. 1) is that the change in connectivity of the active site between the two sides of the membrane occurs midway through the photocycle (in the transition from the early M state to the late M state), long after the initial photoisomerization of the retinylidene chromophore from all-trans to 13-cis (Fig. 2), and long before the thermal reisomerization of the chromophore at the end of the photocycle. Because the change in connectivity is divorced from the major isomerization events, much attention has been directed to the process(es) that might be responsible. However, in the fuller context, the more interesting question is how the active site remains connected to the extracellular surface for so long after the photoisomerization event, and what finally releases it from that set of interactions. In this light, it is not surprising that vibrational spectroscopy finds indications of a strained chromophore in the K (1-4) and L (5-8) intermediates, and a relaxed chromophore in the N intermediate (9). Evidence of strain is also seen in magic angle spinning (MAS) NMR spectra. Furthermore, MAS NMR has pinpointed the release of this strain to the transition from early M to late M (i.e., coincident with the connectivity change) and determin...