Formamide conformational studies

Bremard C, Buntinx G, Ginestet G (1997) Vibrational studies and Monte Carlo simulations of the sorption of aromatic carbonyls in faujasitic zeolites. J Mol Struct 410 379-382 Breneman CM, Wiberg KB (1990) Determining atom-centered monopoles from molecular electrostatic potentials The need for high samphng density in formamide conformational analysis. J Comput Chem... 

Further, for studying the role of pH and salt concentrations on bulk-electrostatic and non-bulk electrostatic contributions the same approach was made to experiments on the influence of the alcohols mentioned above on the oxygen affinity at various KC1 concentrations and pH-values 144,146). The results obtained indicate that at a low alcohol concentration the bulk-electrostatic contributions are dominant and that with increasing size of the alkyl group, alcohol and KC1 concentration, the nonbulk electrostatic, hydrophobic contributions increase. Recent results of kinetic measurements of 02 release show that cosolvents such as alcohols and formamide influence mainly the allosteric parameter L, i.e. -the equilibrium between T and R conformation and that the separation of the alcohol effects into bulk-electrostatic and hydrophobic (non-bulk electrostatic) contributions is justified. 

Hydration-free enthalpies of different conformers of formamide and allyl vinyl ether were studied using the ellipsoidal cavity model and the BLYP functional170,177. A very good agreement between the MP2/SCRF and the DFT(BLYP)/SCRF results was shown for the... 

The conformation of cyclodipeptides containing two nonidentical l-aromatic amino acid residues has been addressed recently [90JCS(P2)127]. In such cases, it may be possible to assess the relative strength of the attractive interaction between the piperazinedione ring and the different aromatic groups by NMR studies. On the basis of detailed analyses, the author has concluded that in the case of cyclo[L-5(MeO)Trp-L-Tyr(Me)], a fast conformational equilibrium exists between the two folded-extended conformers (Scheme 10) above room temperature in DMSO and N,N-dimethyl formamide (DMF) solutions. 

Recently, studies of the conformation of oligomers were extended to peptides derived from /3-methyl-L-aspartates. Their synthesis (n = 2 up to 14) was described by Goodman and Boardman (82), and later the specific rotations of their solutions in dimethyl formamide, dichloro-acetic acid and in chloroform were determined (83). The oligomers exist in a random-coil form in the first two solvents, but helices become stable in chloroform for n — 11 and 14. These peptides are unusual since their L-amino-acid residues produce a left-hand helix (84, 85) whereas most of the investigated polyamino acids crystallise as a right-hand helix (86). 

Studies on the dilute solution behavior of sulfonated ionomers have shown these polymers to exhibit unusual viscosity behavior in solvents of low polarity. These results have been interpreted as arising from strong ion pair associations in low polarity diluents. Solvents of higher polarity, such as dimethyl sulfoxide and dimethyl formamide induce classic polyelectrolyte behavior in sulfonate ionomers even at very low sulfonate levels. To a first approximation these two behaviors, ion pair interactions or polyelectrolyte behavior, are a consequence of solvent polarity. Intramolecular association of Lightly Sulfonated Polystyrene (S-PS) results in a reduced viscosity for the ionomer less than that of polystyrene precursor at low polymer levels. Inter-association enhances the reduced viscosity of the ionomer at higher polymer concentrations. Isolation of the intra- and inter-associated species of S-PS has been attempted (via freeze drying). A comparison of selected properties reveals significant differences for these two conformations. 

Proton NMR studies of N-methyl formamide (NMF) and NMA at high dilution in deuterated solvents have shown that the level of cis isomer of NMF is 8% in water, 10.3% in chloroform, 8.8% in benzene, and 9.2% in cyclohexane, while the level of cis-NMA (a model for the secondary peptide bond) is 1.5% in water and does not change very much in nonpolar solvents [18]. Ab initio molecular calculations suggest that the small difference in dipole moments in cis and trans forms explain the relative insensitivity of amides to solvent change, unlike esters [22,41], This may be explained by nearly identical free energies of solvation for the two isomers [18]. The energy difference between cis and trans isomers in aqueous solution (AG° = 2.5 kcal mol-1) accounts for the preferential trans conformation adopted by most peptide bonds. Similar results were obtained with nonproline tertiary amides [22]. 

An interesting study (15) has been carried out with formamide derived imi-date salts which have two different alkyl groups on the nitrogen atom. The unsymmetrical imidate 137 was obtained as a 3 1 mixture of isomers 137A and 137B and the imidate 138 as isomers 138A and 1388 which were obtained pure on separation. It has already been mentioned that imidate salts derived from formamide exist in the syn conformation and that salt 1388 has been analyzed by X-ray (cf. 52). 

Time-resolved resonance Raman studies of the detailed course of cytochrome interaction with carbon monoxide gives an idea of relaxation and conformational changes in the vicinity of the actual reaction site/ A brief conference report lists rate constants for forward and reverse rate constants for interaction of deoxy-hemerythrin with dioxygen, nitrogen monoxide, hydrazoic acid, formamide, and fluoride/ ... 

PI-TPMs are soluble in dimethyl formamide (DNff), so they are amenable to be studied by solution NMR in that solvent, hi this paper we rqxirt a study of the solution spectra of a model compound and PtTPMs obtamed by the one-step route, in DMF-d7. NOE measurements sided by molecular modeling proved to be a very valuable tool to understand the conformational details of dns class of polymers in solution. It was found th a pentamer model satisfactorily explains the NOE values measured experimentally. The structure of the model ccmipound and the polymers studied are shown in Figure 1. 

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