Halogen bonding is a recently rediscovered secondary interaction that shows potential to become a complementary molecular tool to hydrogen bonding in rational drug design and in material sciences. Whereas hydrogen bond symmetry has been the subject of systematic studies for decades, the understanding of the analogous three-center halogen bonds is yet in its infancy. The isotopic perturbation of equilibrium (IPE) technique with (13)C NMR detection was applied to regioselectively deuterated pyridine complexes to investigate the symmetry of [N-I-N](+) and [N-Br-N](+) halogen bonding in solution. Preference for a symmetric arrangement was observed for both a freely adjustable and for a conformationally restricted [N-X-N](+) model system, as also confirmed by computation on the DFT level. A closely attached counterion is shown to be compatible with the preferred symmetric arrangement. The experimental observations and computational predictions reveal a high energetic gain upon formation of symmetric, three-center four-electron halogen bonding. Whereas hydrogen bonds are generally asymmetric in solution and symmetric in the crystalline state, the analogous bromine and iodine centered halogen bonds prefer symmetric arrangement in solution.

We have investigated the influence of electron density on the three-center [N–I–N]+ halogen bond. A series of [bis(pyridine)iodine]+ and [1,2-bis((pyridine-2-ylethynyl)benzene)iodine]+ BF4– complexes substituted with electron withdrawing and donating functionalities in the para-position of their pyridine nitrogen were synthesized and studied by spectroscopic and computational methods. The systematic change of electron density of the pyridine nitrogens upon alteration of the para-substituent (NO2, CF3, H, F, Me, OMe, NMe2) was confirmed by 15N NMR and by computation of the natural atomic population and the π electron population of the nitrogen atoms. Formation of the [N–I–N]+ halogen bond resulted in >100 ppm 15N NMR coordination shifts. Substituent effects on the 15N NMR chemical shift are governed by the π population rather than the total electron population at the nitrogens. Isotopic perturbation of equilibrium NMR studies along with computation on the DFT level indicate that all studied systems possess static, symmetric [N–I–N]+ halogen bonds, independent of their electron density. This was further confirmed by single crystal X-ray diffraction data of 4-substituted [bis(pyridine)iodine]+ complexes. An increased electron density of the halogen bond acceptor stabilizes the [N···I···N]+ bond, whereas electron deficiency reduces the stability of the complexes, as demonstrated by UV-kinetics and computation. In contrast, the N–I bond length is virtually unaffected by changes of the electron density. The understanding of electronic effects on the [N–X–N]+ halogen bond is expected to provide a useful handle for the modulation of the reactivity of [bis(pyridine)halogen]+-type synthetic reagents.

Counterions influence three-center halogen bonds differently than coordination bonds of transition metals.

The solution symmetry of [N–Cl–N]+and [N–F–N]+halogen bonds is discussed, in comparison to the iodine and bromine-centered bonds as well as to the corresponding three-center [N–H–N]+hydrogen bond.

Mucosal and systemic immune responses to a new oral cholera vaccine, consisting of the B subunit plus killed vibrios, were studied in Bangladeshi volunteers and compared with those to clinical cholera. A single peroral dose of vaccine induced a local IgA antitoxin response in intestinal-lavage fluid of seven of eight vaccinees; the response closely mimicked that of patients convalescing from cholera, and evidence of the induction of local immunologic memory was found as well. Two peroral doses were needed for stimulation of an intestinal IgA immune response to the lipopolysaccharide of Vibrio cholerae that was comparable to the response obtained after clinical cholera. This response to peroral immunization was considerably stronger than that to parenteral vaccination, although the intramuscular route gave rise to the strongest IgG antitoxin and antilipolysaccharide responses in serum. The results suggest that B subunit-whole cell vaccine, when given in at least two oral doses, may be a good candidate for use in cholera prophylaxis.

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Guanosine 5'-[y-thioltriphosphate (GTP[yS])forms a stable ternary complex with polypeptide chain elongation factor Tu (EF-Tu) and aminoacyl-tRNA, and this complex binds rapidly and tightly to a properly programed ribosome. However, the rate constant for the subsequent hydrolysis of the f-y pyrophosphate bond (3.9 x 10-3 sl at 50C) is less than 1/2,500th of that for the analogous reaction of GTP. We have taken advantage ofthis low rate to determine the rate constant for dissociation of the complex of poly(U)-programed ribosomes, EF-Tu, PhetRNAPhe, and GTP['yS] (2.7 x 10-3 s-') and the second-order rate constant for formation ofthis complex (3.3 x 106 M-' s-1). Therefore, the Kd of the complex may be calculated to be 8.2 x 10-10M. An analogous near-cognate complex-with Leu-tRNAI" in place of Phe-tRNA~Ie has been determined by equilibrium methods to have a Kd >1.7 x 10-6 M. These results indicate that under equilibrium conditions the ribosome can distinguish cognate and nearcognate ternary complexes with great accuracy. Therefore, its failure to showthis high specificitywith the physiological ternary complexes containing GTP isdue to the speed of GTP hydrolysis being similar to the speed of dissociation of the near-cognate complex. The low specificity of the physiological reaction is corrected by subsequent proofreading. The results reported here suggest that proofreading is necessary not simply for high accuracy but for the combination ofspeed and accuracy required in protein biosynthesis.The selection of an aminoacyl-tRNA (AA-tRNA) by a mRNAprogramed ribosome is one ofthe key reactions determining the fidelity of protein biosynthesis which, by current estimates (1, 2), exceeds 99.95%. The mechanism of this selection process has attracted a good deal ofinterest since Hopfield (3) proposed that it exemplified a form ofsubstrate selection known as kinetic proofreading. The selection is thought to occur according to the following scheme: the mRNA-programed ribosome binds a ternary complex ofEFTu, GTP, and the AA-tRNA with rate constant ki, and it either dissociates this ternary complex with rate constant k-1 or hydrolyzes its GTP with rate constant k2. Second, the products EF-Tu-GDP and Pi dissociate from the ribosome, and the AAtRNA may either enter the ribosomal A (acceptor) site with rate constant k3 or dissociate from the ribosome with rate constant k4, depending on the strength of the codon-anticodon interaction (4-6). We have shown previously (6) that errors are made quite frequently in the first recognition step but that they are usually corrected by the second recognition step, which is therefore known as proofreading (3). This paper addresses the question ofwhy the first recognition step-selection and hydrolysis of a ternary complex-is insufficiently accurate to achieve the required fidelity by itself and hence requires subsequent proofreading. Failure to obtain high specificity in the GTPase reaction has generally been attributed to there being only a small difference in free energy between the cog...

(15)N NMR chemical shift became a broadly utilized tool for characterization of complex structures and comparison of their properties. Despite the lack of systematic studies, the influence of solvent on the nitrogen coordination shift, Δ(15)N(coord), was hitherto claimed to be negligible. Herein, we report the dramatic impact of the local environment and in particular that of the interplay between solvent and substituents on Δ(15)N(coord). The comparative study of CDCl(3) and CD(3)CN solutions of silver(I)-bis(pyridine) and silver(I)-bis(pyridylethynyl)benzene complexes revealed the strong solvent dependence of their (15)N NMR chemical shift, with a solvent dependent variation of up to 40 ppm for one and the same complex. The primary influence of the effect of substituent and counter ion on the (15)N NMR chemical shifts is rationalized by corroborating Density-Functional Theory (nor discrete Fourier transform) calculations on the B3LYP/6-311 + G(2d,p)//B3LYP/6-31G(d) level. Cooperative effects have to be taken into account for a comprehensive description of the coordination shift and thus the structure of silver complexes in solution. Our results demonstrate that interpretation of Δ(15)N(coord) in terms of coordination strength must always consider the solvent and counter ion. The comparable magnitude of Δ(15)N(coord) for reported transition metal complexes makes the principal findings most likely general for a broad scale of complexes of nitrogen donor ligands, which are in frequent use in modern organometallic chemistry.