5' Guanylylimidodiphosphate, a potent activator of adenylate cyclase systems in eukaryotic cells

5' Guanylylimidodiphosphate (Gpp(NH)p) stimulates adenylate cyclase [ATP pyrophosphate lyase (cyclizing), EC 4.6.1.1] activity in plasma membranes isolated from frog and salmon erythrocytes, from rat adrenal, hepatic, and fat cells, and from bovine thyroid cells. The nucleotide acts cooperatively with the various hormones (glucagon, secretin, ACTH, thyrotropin, and catecholamines) that stimulate these adenylate cyclase systems with resultant activities that equal or exceed those obtained with hormone plus GTP or with fluoride ion. In the absence of hormones, Gpp(NH)p is a considerably more effective activator than GTP, and, under certain conditions of incubation, stimulates rat fat cell adenylate cyclase to levels of activity (about 20 nmoles of 3',5' adenosine monophosphate mg protein per min) far higher than reported hitherto for any adenylate cyclase system examined. The nucleotide activates frog erythrocyte adenylate cyclase when the catecholamine receptor is blocked by the competitive antagonist, propranolol, and activates the enzyme from an adrenal tumor cell line which lacks functional ACTH receptors. In contrast, Gpp(NH)p does not stimulate adenylate cyclase in extracts from Escherichia coli B. Gpp(NH)p appears to be a useful probe for investigating the mechanism of hormone and nucleotide action on adenylate cyclase systems in eukaryotic cells.published_or_final_versio

The CiCs(SiI)n defect in silicon from a density functional theory perspective

Carbon is an important defect in silicon (Si) as it can interact with intrinsic point defects and affect the operation of devices. In heavily irradiated Si containing carbon the initially produced carbon interstitial - carbon substitutional (CiCs) defect can associate with self-interstitials (SiI’s) to form, in the course of irradiation, the CiCs(SiI) defect and further to form larger complexes namely CiCs(SiI)n defects by the sequential trapping of self-interstitials defects. In the present study, we use density functional theory to clarify the structure and energetics of the CiCs(SiI)n defects. Here we report that the lowest energy CiCs(SiI) and CiCs(SiI)2 defects are strongly bound with -2.77 eV and -5.30 eV, respectively

Carbon related defects in irradiated silicon revisited

Electronic structure calculations employing hybrid functionals are used to gain insight into the interaction of carbon (C) atoms, oxygen (O) interstitials, and self-interstitials in silicon (Si). We calculate the formation energies of the C related defects C(i)(Si(I)), C(i)O(i), C(i)C(s), and C(i)O(i)(Si(I)) with respect to the Fermi energy for all possible charge states. The C(i)(Si(I))(2+) state dominates in almost the whole Fermi energy range. The unpaired electron in the C(i)O(i)(+) state is mainly localized on the C interstitial so that spin polarization is able to lower the total energy. The three known atomic configurations of the C(i)C(s) pair are reproduced and it is demonstrated that hybrid functionals yield an improved energetic order for both the A and B-types as compared to previous theoretical studies. Different structures of the C(i)O(i)(Si(I)) cluster result for positive charge states in dramatically distinct electronic states around the Fermi energy and formation energies