The distant shielding effect in trinucleoside diposphates
A comparative study on the proton magnetic resonance of the dinucleoside monophosphates TpT, dApT, TpdA, the trinucleoside diphosphates dApTpT, TpTpdA, and poly T has been made. The purpose of this investigation was to establish whether in a trimer the proton chemical shifts of one terminal residue are influenced by the magnetic anistropy of the other terminal residue. The conformation of the trimers was first studied and shown to the similar to that of the corresponding dimer, except that of the percentage of the lefthanded conformers in the population of the trimers is probably lower than in the population of the dimers. The methylprotons, H‐6, and H‐1′ of the 3′‐terminal T residue in TpTpdA (or the 5′‐terminal T residue in dApTpT) are found to be more upfield than the same protons on the dimer TpT up to a magnitude of about 0.1 ppm. The shielding of these protons of the terminal T(s) in these trimers is even larger than those corresponding protons in the poly(T) at the same condition. From these results, it is concluded that a distant shielding effect is exerted by the 5′‐terminal. A residue (or by the 3′‐terminal A residue) on the other terminal residue in the trimers studied.
Conformations of oligonucleotides in solution as determined by sequence-specific antibodies.
The conformations of some related oligoribonucleotides in solution were investigated immunochemically with antisera specific for two synthetic oligonucleotide sequences, A-A-U and A-A-U-U. Radioimmunoassay showed differences of as much as three or more orders of magnitude in binding among oligonucleotides with commonly shared sequences. These large differences, which reflect the loss of many points of contact with antibody because of changes in overall conformation, allow the following conclusions: (1) A-A-U and A-A-U-U have conformations distinct from any present in the Un family of oligomers. (2) Conformations of A-A-U and A-A-U-U differ markedly from those of oligomers of A. The dinucleotide A-A, in particular, bears little resemblance in conformation to the A-A sequence in A-A-U and A-A-U-U. (3) The recognizable conformational unit appears to be the triplet A-A-U, which binds as well as A-A-U-U and far better than its component dimers. Interactions between non-adjacent bases may be a factor here, as well as in codon recognition. The immunological data support the conclusion that, in oligonucleotides, as in polypeptides, primary sequence can determine conformation in solution.
Some calculations on the proton magnetic resonance shift due to hydrogen bonding of ammonia molecules
SummaryThe effect of hydrogen bonding on the proton magnetic shielding constants in the ammonia molecule has been calculated from gauge invariant atomic orbitals. FollowingSchneider, Bernstein andPople, the effect is regarded as made up of two contributions:a) the inter-molecular shift, which is caused by the induced electronic currents in the adjacent molecule, andb) the polarization shift, which is caused by the polarization of the N-H bond due to the electrostatic field of the adjacent molecule. The computed intermolecular shift is −1.02·10−6, the computed polarization shift is −0.45·10−6. The experimental value of the total effect is −1.05·10−6.RiassuntoL’effetto del legame idrogeno sulle costanti dello schermo magnetico dei protoni nella molecola dell’ammoniaca è stato calcolato in base agli orbitali atomici invarianti rispetto al gauge. SecondoSchneider, Bernstein ePople, si ritiene che l’effetto sia la somma di due contributi:a) lo sfasamento intermolecolare, causato dalle correnti elettroniche indotte nella molecola adiacente, eb) lo sfasamento della polarizzazione causato dalla polarizzazione del legame N-H dovuta al campo elettrostatico della molecola adiacente. Lo sfasamento intermolecolare calcolato è − 1.02·10−6, lo sfasamento della polarizzazione calcolato è − 0.45·10−6. Il valore sperimentale dello sfasamento totale è − 1.05·10−6.