Singlet Vinylcarbenes: Spectroscopy and Photochemistry
摘要:
The first direct spectroscopic characterization of singlet vinylcarbenes is reported. Several vinyl amidines were converted to the corresponding vinylchlorodiazirines through straightforward Graham oxidation. Irradiation of the diazirines at 8 K in N2 matrices gave the corresponding singlet vinylchlorocarbenes, which could be characterized by IR, UV/vis, and calculational modeling. Hence, vinylchlorocarbene, 1-methylvinylchlorocarbene, and 1-cyclopentenylchlorocarbene were all generated and investigated. The spectra are consistent with localized carbene structures compared to the very delocalized triplet vinylcarbenes. In all cases, the carbenes readily cyclize to the corresponding cyclopropenes on visible irradiation, together with H-shift in the parent vinylchlorocarbene. In the cyclopentenyl system, cyclization leads to a highly strained bicyclo[3.1.0]hexene.
A convenient method has been developed for direct conversion of nitriles to amidines in high yields. The method can also be applied to the preparation of guanidines from N-alkyl cyanamides.
Disease-Causing Mutations in the G Protein Gαs Subvert the Roles of GDP and GTP
作者:Qi Hu、Kevan M. Shokat
DOI:10.1016/j.cell.2018.03.018
日期:2018.5
The single most frequent cancer-causing mutation across all heterotrimeric G proteins is R201C in G alpha s. The current model explaining the gain-of-function activity of the R201 mutations is through the loss of GTPase activity and resulting inability to switch off to the GDP state. Here, we find that the R201C mutation can bypass the need for GTP binding by directly activating GDP-bound G alpha s through stabilization of an intramolecular hydrogen bond network. Having found that a gain-of-function mutation can convert GDP into an activator, we postulated that a reciprocal mutation might disrupt the normal role of GTP. Indeed, we found R228C, a loss-of-function mutation in G alpha s that causes pseudohypoparathyroidism type 1a (PHP-Ia), compromised the adenylyl cyclase-activating activity of G alpha s bound to a non-hydrolyzable GTP analog. These findings show that disease-causing mutations in G alpha s can subvert the canonical roles of GDP and GTP, providing new insights into the regulation mechanism of G proteins.
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