The structures of the supercooled α and β phases of p-dichlorobenzene are determined at 100 °K and a detailed comparison made with the γ-phase structure previously reported at that temperature [Acta. Crystallogr. B 31, 911 (1975)]. The effects of temperature are investigated by a redetermination of the β-phase structure at room temperature and by measurement of the thermal expansion coefficients for the α- and β-phase crystals. The average atomic thermal displacements are found to be in the expected thermodynamic order. For chlorine we find Ū (β) =0.184 Å, Ū (α) =0.169 Å, and Ū (γ) =0.157 Å at 100 °K. Rigid body analysis of our data gives good agreement between calculated and observed librational lattice frequencies. New limits are placed on the density changes associated with the phase transitions [ΔV (α→β) =0.1±0.4% and ΔV (γ→α) =0.5±0.4%]. The effects of crystal field on the molecular geometry are limited to a slight out-of-plane displacement (0.045 Å) of the chlorine atoms in the γ phase. The possibility of such distortions in the high temperature α and β phases is masked by slight positional disorder (∼0.02 Å). The chlorine–chlorine interactions are found to be attractive and anisotropic. This indicates the necessity of including anisotropic terms in semiempirical atom–atom potential functions. All presently available functions, including Coulomb–Coulomb terms, are found to be inadequate for this reason.

We have been studying the mechanism by which light and nucleoside triphosphates activate the discmembrane phosphodiesterase (3ligonucleate 5'-nucleotidohydrolase; EC 3.1.4.1) in frog rod outer segments. GTP is orders of magnitude more effective than ATP as a cofactor in the light-dependont activation step. GTP and the analogue guanylyl-imidodiphosphate function equally as allosteric activators of photoreceptor phosphodiesterase rather than participating in the fornation of a phosphorylated activator. Moreover, we have found a light-activated (5-fold) GTPase which participates in the modulation of photoreceptor phosphodiesterase. This GTPase activity appears necessary for the reversal of phosphodiesterase activation in vitro and may play a critical role in the in vivo regulation of light-sensitive phosphodiesterase.The K. for GTP in the light-activated GTPase reaction is <1 AM. The light sensitivity of this GTPase (number of photons required for half-maximal activation) is identical to that of light-activated phosphodiesterase. The GTPase action spectrum corresponds to the absorption spectrum of rhodopsin. There is, in addition, a light-insensitive GTPase activity with a Km for GTP of 90 SM. At GTP concentrations above 5 ;sM, there is no appreciable activation of GTPase activity by lighLt. The substrate Km values for guanylate cyclase, light-activated GTPase, and light-activated phosphodiesterase order an enzyme array that might ermit light to simultaneously cause the hydrolysis of both the substrate and product of guanylate cyclase. These findings reveal yet another facet oflight regulation of photoreceptor/cyclic GMP levels and also provide a striking analogy to the GTP regulation of nonphotoreceptor, hormone-sensitive adenylate cyclase. In 1971 we found that light could regulate the levels of cyclic nucleotides in membrane suspensions prepared from vertebrate photoreceptors (1). Subsequent studies revealed that the same process was occurring in the intact retina and in vlvo (2,3). We later found that the level of cyclic nucleotides was regulated by a light-activated cyclic GMP (cGMP) phosphodiesterase (dGTP triphosphohydrolase; EC 3.1.5.1) and that activation of phosphodiesterase had an additional requirement for a nucleoside triphosphate (4). It was not known whether the nucleoside triphosphate could participate in the formation of a stable phosphorylated protein (which acts as a regulator) or whether the triphosphate serves as an allosteric modifier in the light-dependent activation of phosphodiesterase.Here, we report that GTP is orders of magnitude more effective than ATP in the light-dependent activation of photoreceptor phosphodiesterase (oligonucleate 5'-nucleotidohydrolase; EC 3.1.4.1). Our data reveal that GTP functions as an allosteric modifier which gains access to its receptor as a consequence of illumination.We also report a light-activated outer-segment GTPase which exhibits an identical light dependence and action spectrum as the phosphodiesterase (5). We find that this GTPase regulates ...

We report experiments which involve a light sensitive GTPase in the light dependent activation of retinal rod 3'5'-cyclic guanosine monophosphate (cGMP) phosphodiesterase (PDE). The data suggest that the light activated GTPase is intermediate between rhodopsin and PDE in the light-dependent activation sequence. We list the many striking similarities between hormone sensitive adenylate cyclase and light activated PDE in order to emphasize that the findings presented herein may have predictive value for ongoing studies of the hormone sensitive adenylate cyclase specifically regarding the role of the hormone activated GTPase in the activation sequence.

Five GTP binding proteins in rat cerebral cortex synaptic membranes were identified by photoaffinity labelling with [3H] or [32P](P3-azido-anilido)-P1-5' GTP (AAGTP). When AAGTP-treated membranes were incubated with colchicine or vinblastine and subsequently washed, a single AAGTP-labelled protein of 42 kD was released into the supernatant. About 30% of the total labelled 42-kD protein was released into supernatants from membranes pretreated with colchicine or vinblastine compared with 15% released from control membranes. The amount of adenylate cyclase regulatory subunit (G unit) remaining in these membranes was assessed with reconstitution studies after inactivating the adenylate cyclase catalytic moiety with N-ethylmaleimide (NEM). Forty to fifty percent of functional G units were lost from membranes treated with colchicine prior to washing. This 40-50% loss of functional G unit after colchicine treatment corresponds to the previously observed 42% loss of NaF and guanylyl-5'-imidodiphosphate [Gpp(NH)p]-activated adenylate cyclase. Release of the AAGTP-labelled 42-kD protein from colchicine-treated synaptic membranes is double that from lumicolchicine-treated membranes. This colchicine-mediated release of 42-kD protein correlates with a doubling of functional G unit released from synaptic membranes after colchicine treatment. These findings suggest multiple populations of the G unit within the synaptic plasma membrane, some of which may interact with cytoskeletal components.

The crystal structure of triphenylphosphonium cyclopentadienylide has been determined from an X-ray crystallographic investigation. Mo Ka radiation was used and the X-ray intensities were measured with a computer-controlled diffractometer. The space group is monoclinic, Plfn; the lattice constants are a = 10.564, b = 8.986, c = 18.708 Á, ß = 93.410; there are four molecules per cell. The amount of ylene-ylide character (ca. 20-80%) is estimated from the P-C bond length, the -electron delocalization in the five ring evaluated from the C-C distances, and from the plus charge on P obtained from the P photoelectron spectrum. These data are compared with the results from a previously reported MO analysis. The conformations about the P-C (five ring) bond in the compound and about the P-Cbonds in a number of other ylides can be explained in terms of the orientations necessary for C p-orbital overlap with the two P d orbitals of symmetry.