A new series of surfactant-active complexes of the form [(bpy)Re(CO),NC(CH,).CH,I+, n = 0-17, have been synthesized and characterized. These complexes exhibit a unique intramolecular perturbation of their excited-state manifold by the normally passive alkyl chain. Intramolecular fold back, a strong function of chain length, alters the solvent environment around the excited portion of the molecule with a concomitant change in the state energies and decay paths. Molecular models, cyclodextrin binding studies, absorption and emission spectroscopy, excited-state lifetime measurements, and oxygen-quenching studies support this model. The alkyl chain can function as a molecular switch and invert the lowest excited states in the molecules at low temperature. Energy level diagrams are developed that explain both room-temperature and 77 K results.Luminescent transition-metal complexes have been utilized as photosensitizers in such areas as solar energy conversion,' electron-transfer studies,2 chemi-and electroluminescent system^,^,^ binding dynamics of heterogeneous media,s and probes of macromolecular structure.6 R U " ( L )~~+ complexes (L = 2,2'-bipyridine, 1,lO-phenanthroline, or substituted derivatives) have been the most frequently used probes in these applications due to strong visible absorption, high photochemical stability, efficient luminescence, and relatively long-lived metal to ligand chargetransfer (MLCT) excited states7Further, the emitting-state energies and excited-state redox properties of the sensitizers can be very sensitive to the nature of the metal, coordinating ligands, and solvent environment. Solvent and substituents have been used to control the relative positions of MLCT excited states and effectively "tune" their photophysical and photochemical properties.8-'0 Many of these sensitizers exhibit a wide variety of energetically accessible charge-transfer (CT), ligand-field (LF), and intraligand (IL) excited states. These excited states are of differing orbital parentage and, therefore, can have quite different excited-state characteristics. Since the photophysical properties of the metal complexes are determined predominantly by the lowest energy excited states," it is important to understand thoroughly their excited-state energetics and dynamics in order to be able to design rationally new and more useful photosensitizers and probes.For those sensitizers having lowest lying MLCT states, models are based predominantly on the photophysics and photochemistry of the 'Ru(bpy)32+ (bpy = 2,2'-bipyridine) and related sensitizers, leading to a general picture of metal complex excited-state behavior. Recently, OS(II),'~ Ir(III),I3 Mo(0) and W(O),I4 and R~( I ) " s '~ complexes have been studied with increasing interest.In some cases, these complexes exhibit even more desirable excited-state properties than the Ru(I1) sensitizers. An additional level of richness in the form of multiple luminescences has been observed for a number of transition-metal compounds with a-diimine ligands. As we will show, this ...

The interactions of gadolinium ion, lithium, and two substrate analogues, beta,gamma-imido-ATP (AMP-PNP) and tridentate CrATP, with the calcium ion transport adenosine triphosphatase (Ca2+-ATPase) of rabbit muscle sarcoplasmic reticulum have been examined by using 7Li+ NMR, water proton NMR, and Gd3+ EPR studies. Steady-state phosphorylation studies indicate that Gd3+ binds to the Ca2+ activator sites on the enzyme with an affinity which is approximately 10 times greater than that of Ca2+. 7Li+, which activates the Ca2+-ATPase in place of K+, has been found to be a suitable nucleus for probing the active sites of monovalent cation-requiring enzymes. 7Li+ nuclear relaxation studies demonstrate that the binding of Gd3+ ion to the two Ca2+ sites on Ca2+-ATPase increases the longitudinal relaxation rate (1/T1) of enzyme-bound Li+. The increase in 1/T1 was not observed in the absence of enzyme, indicating that the ATPase enhances the parmagnetic effect of Gd3+ on 1/T1 of 7Li+. Water proton relaxation studies also show that the ATPase binds Gd3+ at two tight-binding sites. Titrations of Gd3+ solutions with Ca2+-ATPase indicate that the tighter of the two Gd3+-binding sites (site 1) provides a ghigher enhancement of water relaxation than the other, weaker Gd3+ site (site 2) and also indicate that the average of the enhancements at the two sites is 7.4. These data, together with a titration of the ATPase with Gd3+ ion, yield enhancements, epsilonB, of 9.4 at site 1 and 5.4 at site 2. Analysis of the frequency dependence of 1/T1 of water indicates that the electron spin relaxation taus of Gd3+ is unusually long (2 X 10(-9) s) and suggests that the Ca2+-binding sites on the ATPase experience a reduced accessiblity of solvent water. This may indicate that the Ca2+ sites on the Ca2+-ATPase are buried or occluded within a cleft or channel in the enzyme. The analysis of the frequency dependence is also consistent with three exchangeable water protons on Gd3+ at site 1 and two fast exchanging water protons at site 2. Addition of the nonhydrolyzing substrate analogues, AMP-PNP and tridenate CrATP, to the enzyme-Gd3+ complex results in a decrease in the observed enhancement, with little change in the dipolar correlation time for Gd3+, consistent with a substrate-induced decrease in the number of fast-exchanging water protons on enzyme-bound Gd3+. From the effect of Gd3+ on 1/T1 of enzyme-bound Li+, Gd3+-Li+ separations of 7.0 and 9.1 A are calculated. On the assumption of a single Li+ site on the enzyme, these distances set an upper limit on the separation between Ca2+ sites on the enzyme of 16.1 A.

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