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Carbocation Chemistry. Edited by George A. Olah and G. K. Surya Prakash

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Angewandte
Chemie
RNA Interference in Practice
Principles, Basics,
and Methods for
Gene Silencing in
C. elegans, Drosophils, and Mammals. By Ute
Schepers. WileyVCH, Weinheim
2004. 326 pp.,
hardcover
E 99.00.—ISBN
3-527-31020-7
In recent years, our understanding about
the roles played by RNA molecules in
the cell has grown from that of the
simple translator between DNA and
protein, to include catalytic roles in protein synthesis, guiding the processing of
other RNAs, as well as regulatory roles
in the expression of protein-coding
genes. These nonprotein-coding RNAs
(ncRNAs) function on the level of the
RNA itself, and may be the key to
understanding the increased complexity
of mammals compared to “lower organisms”, despite the fact that the number
of protein-coding genes is only modestly
greater.
The book RNA Interference in Practice, by Ute Schepers, focuses on a subclass of these ncRNAs, namely the
small interfering RNAs (siRNAs).
These RNAs can specifically down-regulate the expression of a target gene in
Eukarya by a mechanism called RNA
interference (RNAi). The common
denominator for siRNAs and the related
class of microRNAs (miRNAs) is their
size of about 21 nucleotides, and their
similar mechanism of generation. Both
are excised from longer doublestranded-RNA precursors by the
RNase-III-like enzyme Dicer. It is commonly believed that the RNAi machinery has evolved as a defence system
against RNA viruses, transposons, and
retroelements. The siRNAs function by
an antisense-based mechanism to
target mRNAs and to direct mRNA
cleavage. The siRNAs do not exert
their function as naked RNA, but in
complexation with proteins, forming a
particle called RISC (RNA-induced
silencing complex). Following its assembly, the RISC guides the RNA degradaAngew. Chem. Int. Ed. 2005, 44, 3002 – 3004
tion machinery to the target RNAs and
cleaves the target mRNA in a
sequence-specific,
siRNA-dependent
manner. Only very recently, the hitherto
elusive endonuclease required for
mRNA cleavage has been identified as
one of the integral components of the
RISC complex, termed argonaute protein, ago2.
The main emphasis of the book
RNA Interference in Practice is clearly
on the practical application of the
RNAi technology for gene knockdowns in some animal model systems.
The principles that form the foundation
for such studies are explained at length
in the introductory chapter. This chapter
is informative, covers most of the recent
publications on the subject, and should
appeal to both the specialist researcher
and the general reader. In the three following chapters, protocols for performing RNAi are described for use in the
worm Caenorhabditis elegans and the
fly Drosophila, as well as in mammalian
cells. The main advantage of RNAi is
that the function of a protein-coding
gene can be investigated without
employing the time- and resource-intensive gene-knock-out technology. The
strength of the protocols described
here lies in the fact that they provide
the reader with practical details about
the methods, and will allow researchers
to fully use the advantage of RNAi without the requirement for thorough RNA
experience. The most detailed chapter
of the book is the fourth one, which
deals with RNAi in mammalian cell culture systems or in mice, and covers various state-of-the-art methods for delivering siRNAs into mammalian cells.
The hype about the RNAi technology was recently challenged by accumulating evidence to indicate that offtarget silencing of other genes can
occur in some cases. These problems,
as well as possible ways to circumvent
them by proper siRNA design, are adequately covered in the book. However,
it is not clear why these siRNA design
tools are included in the chapter
“RNAi in mammals”, since they are
equally important in other model systems. Especially beneficial from a practical point of view are the listed Internet
links to useful sites at the end of each
chapter, and the list of suppliers of
RNAi-related chemicals.
www.angewandte.org
RNA interference in Practice is a valuable addition to any library in a molecular biology or cell biology laboratory.
However, the book is of minor value
for researchers studying plant systems,
since this kingdom has not been touched
at all by the author. Moreover, the work
does not always reflect the latest stateof-the-art literature on every aspect,
although it must be clearly admitted
that this would be difficult to achieve
in a subject that is developing so rapidly
as the ncRNA field. Despite these minor
criticisms, the book held my attention
from the first page, and I recommend
it for everyone aiming to apply the
time-saving and cost-effective RNAi
technology. Additionally, this book can
serve as a sound basis for preparing
method-orientated lectures.
Norbert Polacek
Division of Genomics and RNomics
Innsbruck Biocenter
Innsbruck Medical University (Austria)
Carbocation Chemistry
Edited by George A.
Olah and
G. K. Surya Prakash.
Wiley-Interscience,
New York 2004.
400 pp., hardcover
E 92.90.—ISBN
0-471-28490-4
In 1901/1902, Norris and Wentzel in the
USA and Baeyer in Germany discovered the existence of stable triphenylmethyl cations in concentrated sulfuric
acid. The centenary of that discovery
was marked by the conference “100
Years of Carbocations”, which took
place in January 2001 at George Olahs
Loker Hydrocarbon Research Institute,
Los Angeles, and this book collects
together highlights from that conference. It covers the present state of progress on the chemistry of carbocations
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3003
Books
in 14 chapters, the contents of which
have been chosen according to their topicality. It offers both interested students
and specialists an expert source from
which they can learn about developments in this field during the last two
decades.
After a historical overview by P.
Stang, G. A. Olah describes his groundbreaking studies of stable carbocations
in superacid solvents, which led to the
award of a Nobel Prize in 1994. Olah
gives a very exciting account of how,
using an armory of spectroscopic methods in combination with ab initio calculations, it was possible to prove the existence of both nonclassical and classical
carbocations. G. K. S. Prakash and V. P.
Reddy then present a review of the
latest developments in the chemistry of
carbocations, with special emphasis on
carbodications. An important method
for distinguishing between a static carbocation and a rapid equilibrium
between degenerate cations in superacid
solution is the isotopic perturbation
technique. M. Saunders has developed
the method using 13C NMR spectroscopy, and a review by O. Kronja gives
an informative account of this, including
recent results.
It is important to mention that
research on the persistent carbocations
that are now known is not confined to
the well-known American groups, and
we see that in the results reported by
V. G. Shubin and G. I. Borodkin. In
another article from Russia, V. D.
Shteingarts reports on perfluorinated
carbocations, and describes in detail
the stabilizing and destabilizing effects
of fluorine substituents. Thus, we now
have two English-language contributions from leading Russian chemists.
Two groups in France are working on
the chemistry of persistent carbocations:
in the article by J. Sommer and A. Goeppert we learn about new results on protonation equilibria of alkanes in liquid
and solid superacids, then J. C. Jacquesy
discusses the potential applications of
superacid systems for synthesis. The
article by K. K. Laali discusses a possible relationship between the wellknown carcinogenicity and mutagenicity
of polycyclic aromatic hydrocarbons and
features present in the 13C NMR spectra
of the corresponding dihydroaryl cat-
3004
ions. This is an interesting approach,
although the results up to now have
not provided an unambiguous answer
to the question.
The importance of theory and special ab initio calculations for elucidating
the properties of carbocations is clearly
evident in most of the contributions. In
the article by P. von R. Schleyer, C. S.
Wannere, and Z. Chen, the ionic character of zwitterions of carbocations is estimated by the NBO method. The combination of theory and experiment has
also proved to be important in studies
of the cyclopentadienyl cation, which
has four p electrons. Its anti-aromatic
character was first predicted by Roberts
in 1952, and subsequently confirmed
experimentally by Breslow. T. T. Tidwell
and A. D. Allen contribute a review on
this topic that is well worth reading. Xray crystallographic analysis shows that
the unsubstituted cation has a squarepyramidal structure. Lamberts suggestion in 2002 that the tetramethyl derivative has a planar structure turned out to
be wrong, as it is in fact a derivative of
the cyclopentenyl cation. According to
calculations, the anti-Hckel arene is
31.2 kcal mol 1 less stable than the
cyclopentenyl cation, and has a triplet
ground state. Also, the calculated
NICS values show a definite stabilizing
effect, and consequently derivatives of
the solvolysis-generated cations of this
type have greatly reduced reactivity
(solvolysis rate). The homologues of
the cation (indenyl and fluorenyl cations
and radicals) have now been studied
very thoroughly. Coordination with
metal complexes causes a completely
different type of stabilization or destabilization. In the article by B. N. Hietbrink, D. J. Tantillo, K. N. Houk, and
C. A. Merlic, this principle is illustrated
by a study in which the aromatic
moiety of the cations 2-benzonorbornenyl and 7-benzonorbornenyl is complexed with [Cr(CO)3].
With their contribution on carbocations in the chemistry of gold, H. Schmidbaur and K. A. Porter open up a new
chapter in the area of nonclassical carbocations. Since 1990, Schmidbaurs
group have been preparing various
stable penta- and hexavalent carbocations of the type [C(AuL)5]+ and [C(AuL)6]2+ (where gold is in oxidation
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
state + 1 and the ligand L is usually
phosphane), and studying their structure
by various methods, including 197Au
Mssbauer spectroscopy. These species,
which are isolobal analogues of CH5+
and CH62+, are usually stabilized by
aurophilic interactions.
H. Mayr and A. R. Ofial report on a
systematic study of the use of carbocations in organic synthesis. The results
indicate that in many polar reactions
the reactivity can be described by a
linear free energy (LFE) expression
using electrophilic and nucleophilic
parameters. Increases in second-order
rate constants at room temperature by
as much as 1025 times (!) have been predicted. However, the road up to this
stage has been long and arduous: first
the LFE expression was determined
for 209 reactions of 23 benzhydryl cations with 39 carbon nucleophiles (activated double bonds), then the correlation was carried over and extended to
other nucleophiles (carbanions, amines,
and hydride donors) and to reactive aliphatic carbocations. Electrophilicity and
nucleophilicity are normalized, general,
and inverse quantities in the linear equation.
This is an excellent book, which
should be in every good chemistry
department library. It clearly shows
that the field of carbocations continues
to be highly topical. Specialists will perhaps regret that there are no contributions on single-crystal X-ray analyses
of aliphatic carbocations nor on structural determinations for new aromatic
p and s complexes. I also expected to
find an article on vinyl cations. The
book is completed by a subject index,
but unfortunately there is no author
index. There are few errors, and the
structural formulas are clear and in a
uniform style. Most of the articles
cover the literature up to the middle of
2004, and the authors report on the
latest state of progress in their special
areas.
Dieter Lenoir
Institut fr kologische Chemie
GSF Forschungszentrum
Oberschleißheim (Germany)
DOI: 10.1002/anie.200485277
Angew. Chem. Int. Ed. 2005, 44, 3002 – 3004
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