Solvent Dependence Of Chemical Shifts Research Articles

Investigation of solvated calcium dication structure in pure water, methanol, and ethanol solutions by means of K and L2,3-edges X-ray absorption spectroscopy

The interaction of an ion with surrounding solvent molecules is revealed by X-ray absorption spectroscopy in transmission mode for the case of calcium dication dissolved in pure water, methanol and ethanol at the Ca K and L2,3-edges. The near-threshold K-shell excitation leads to two distinct solvent-dependent features, the Ca 1s to 3d and 4p electronic transitions, clearly indicating the ligand field decreases as water is changed to methanol and methanol to ethanol. On the other hand, the Ca L2,3-edge excitation leads to small changes in the absorption spectra and the peak assignment – mostly in terms of Ca 2p to eg and t2g transitions for each of the two spin orbit components – is strongly depending on the geometry of the first solvation shell around the dication and on the magnitude of the ligand-field multiplet effects. Our interpretation is supported by ab-initio quantum chemical calculations of the solvent dependence of chemical shifts for the Ca 3d and 4p unoccupied levels of the 1s and 2p excited states, performed on a small cluster including the nearest neighbor solvent molecules surrounding the Ca dication assuming a cubic (8-fold) solvent coordination.

The helical conformations of 14- and 16-residue fragments of suzukacillin, a membrane channel-forming polypeptide

The suzukacillin fragments, Boc-Ala-Aib-Aib-Gln-Aib-Leu-Aib-Gly-Leu-Aib-Pro-Val-Aib-Aib-OMe (14), Boc-Ala-Aib-Ala-Aib-Aib-Gln-Aib-Leu-Aib-Gly-Leu-Aib-Pro-Val-Aib-Aib-OMe (16G) and the completely apolar 16-residue peptide in which the glutamine residue has been replaced by alanine (16A) have been studied by 270 MHz 1H-HMR, in C 2HCl 3 and (C 2H 3) 2SO solution. Intramolecularly hydrogen-bonded NH groups have been identified by temperature and solvent dependence of chemical shifts. Peptides 14 and 16A adopt folded 3 10 helical conformations stabilized by 11 and 13 hydrogen bonds, respectively. In peptide 16G there are 12 intramolecular hydrogen bonds, with the glycine NH being solvent-exposed, in contrast to 14 and 16A

The 310 helical conformation of the amino terminal decapeptide of suzukacillin. 270 MHz hydrogen-1 NMR evidence for eight intramolecular hydrogen bonds

The amino terminal suzukacillin decapeptide fragment, Boc-Aib-Pro-Val-Aib-Val-Ala-Aib-Ala-Aib-Aitbh-eO Me, two pentapeptides Boc-AibPrc-Val-AibVal-OMe and Boc-Ala-AibAla-AibAibOMe, and the tripeptide Boc-Ala-AibAibOMe have been studied by 270-MHz 'H NMR spectroscopy. By use of solvent dependence of chemical shifts in a CDC13-(CD3),S0 system and temperature dependence of amide NH chemical shifts in (CD3),S0, the intramolecularly hydrogen bonded NH groups in these peptides have been identified. The tripeptide possesses one hydrogen bond, both pentapeptides show evidence for three intramolecular hydrogen bonds, and the decapeptide has eight NH groups participating in hydrogen bonding. An Ala( 1)-Aib(2) @ turn is proposed for the tripeptide. Both pentapeptides favor 310 helical conformations composed of three consecutive B turns. The decapeptide adopts a 310 helical conformation with some flexibility at the Va1(5)-Ala(6) segment. The proposed conformations are consistent with the known stereochemical preferences of Aib residues.

Investigations on the conformation of the cyclic hexapeptide cyclo-glycyl-L-prolyl-glycyl-glycyl-L-prolyl-glycyl by means of proton magnetic resonance and parallel studies on the cyclodecapeptide gramacidin S

AbstractThe first proton magnetic resonance observations made with cyclic hexapeptides suggested the presence of two types of amide protons: one participating in intramolecular hydrogen bonds and more strongly shielded, the other exposed to the solvent and less shielded (SCHWYZER, et al. 1964 [2]). This was taken as evidence for the β‐type structure proposed for homodetic cyclic peptides with 2 · (2n + 1) amino acid residues in the ring (SCHWYZER [4]), n = 1, 2, 3 … These conclusions have now been confirmed by studies on the solvent dependence of chemical shifts in cyclo‐glycyl‐prolyl, cyclo‐glycyl‐prolyl‐glycyl‐glycyl‐prolyl‐glycyl (III) and gramicidin S. On passing from a typical amide proton acceptor solvent (hexadeuterodimethylsulfoxide or tetradeuteromethanol) to trifluoroacetic acid the solvent‐exposed protons become more shielded and the intramolecularly hydrogen bonded ones less so (Figure 5). The proposed β‐structure of the cyclodecapeptide gramicidin S (n = 2 in the above formula; HODGKIN & OUGHTON [8], crystalline state; SCHWYZER [9], in solution) which has recently been supported by side‐chain interaction studies (using a nuclear magnetic resonance marker) on the biologically active derivative, diphthalylgramicidin S (SCHWYZER & LUDESCHER [1]) as well as by other methods [6] [7], is thus additionally supported. Our experiments, as well as those of KOPPLE [11], strongly favor a β‐structure for simple cyclohexapeptides in solution; this is particularly true for compound III which is, with respect to the two proline residues, a model of gramicidin S.