Hogyu Han

β-Azidoalanine as an IR Probe : Application to Amyloid Aβ(16-22) Aggregation

Beta-azidoalanine dipeptide 1 was synthesized, and its azido stretching vibration in H2O and dimethyl sulfoxide (DMSO) was studied by using Fourier transform (FT) IR spectroscopy. The dipole strength of the azido stretch mode is found to be about 19 and 5 times larger than those of the CN and SCN stretch modes, respectively, which have been used as local environmental IR sensors. The azido stretch band in H2O is blue-shifted by about 14 cm(-1) in comparison to that in DMSO, indicative of its sensitivity to the electrostatic environment. To test the utility of beta-azidoalanine as an IR probe of the local electrostatic environment in proteins, azidopeptide 4 was prepared by its incorporation into Abeta(16-22) peptide of the Alzheimers disease amyloid beta-protein at position Ala21. The amide I IR spectrum of 4 in D2O suggests that the azidopeptide thus modified forms in-register beta-sheets in aggregates as observed for normal Abeta(16-22). The azido peak frequency of 4 in aggregates is almost identical to that in DMSO, indicating that the azido group is not exposed to water but to the hydrophobic environment. We believe that beta-azidoalanine will be used as an effective IR probe for providing site-specific information about the local electrostatic environments of proteins.

Switching regioselectivity in crossed acyloin condensations between aromatic aldehydes and acetaldehyde by altering n -heterocyclic carbene catalysts

An unprecedented high level of regioselectivities (up to 96%) in the intermolecular crossed acyloin condensations of various aromatic aldehydes with acetaldehyde was realized by an appropriate choice of N-heterocyclic carbene catalysts.

Enantioselective syntheses of decursinol angelate and decursin

Abstract The practical enantioselective syntheses of decursinol angelate and decursin were achieved in eight steps from resorcinol. The stereochemistry was addressed using the catalytic asymmetric epoxidation of 7-acetoxy-2,2-dimethylchromene by chiral (salen)Mn complexes as the key step.

Azido Gauche Effect on the Backbone Conformation of β-Azidoalanine Peptides

To study the azido gauche effect on the backbone conformation of β-azidoalanine (Aza) dipeptide (AAD, Ac-Aza-NHMe) and tripeptide (AAT, Ac-Aza-Aza-NH(2)), we used spectroscopic methods in combination with quantum chemistry calculations and molecular dynamics (MD) simulations. From the (1)H NMR coupling constants and (1)H,(1)H NOESY experimental data, we found that AAD in water mainly adopts a seven-membered cyclic (C(7)) rather than polyproline II (P(II)) backbone conformation and prefers the gauche- (g(-)) side-chain conformer. From the amide I IR absorption and circular dichroism (CD) spectra, the backbone conformation of AAD in water is found to deviate from P(II) but is rather close to C(7). Thus, the backbone conformation of AAD differs from that of alanine dipeptide (AD, Ac-Ala-NHMe), which is mainly P(II) in water. The underlying origin of the backbone conformational difference between AAD and AD in water was elucidated by quantum chemistry calculations with density functional theory (DFT). It was found that the C(7)/g(-) conformer is the lowest energy structure of an isolated AAD. Here, the β-azido group forms intramolecular electrostatic interactions with two neighboring peptide bonds, which are facilitated by the azido gauche effect. Thus, the β-azido group appears to be responsible for directing the peptide backbone conformation toward the C(7) structure. The quantum mechanical/molecular mechanical (QM/MM) MD simulations show that AAD in water adopts neither P(II) nor right-handed α-helix (α(R)) and prefers the g(-) conformer. Thus, the intramolecular electrostatic interactions between the β-azido group and two nearby peptide bonds are also found even in the aqueous solution structure of AAD. Consequently, the β-azido group appears to be an effective C(7)-conformation-directing element, which may also be useful for tuning the structures of other amino acids and polypeptides.

Synthesis and structure-activity relationships of tri-substituted thiazoles as RAGE antagonists for the treatment of Alzheimer's disease.

A series of thiazole derivatives were designed, and prepared to develop RAGE antagonist for the treatment of Alzheimers disease (AD). SAR studies were performed to optimize inhibitory activity on Aβ-RAGE binding. SAR studies showed that introducing an amino group at part A was essential for inhibitory activity on Aβ-RAGE binding. Compounds selected from Aβ-RAGE binding screening displayed inhibitory activity on Aβ transport across BBB. They also showed inhibitory activity against Aβ-induced NF-κB activation. These results indicated that our derivatives had a potential as therapeutic agent for the treatment of AD.

Small molecules that protect against β-amyloid-induced cytotoxicity by inhibiting aggregation of β-amyloid

Aggregated β-amyloid (Aβ) plays crucial roles in Alzheimers disease (AD) pathogenesis, therefore blockade of Aβ aggregation is considered as a potential therapeutic target. We designed and synthesized small molecules to reduce Aβ-induced cytotoxicity by inhibiting Aβ aggregation. The small molecules were screened via ThT, MTT, and cell-based cytotoxicity assay (Aβ burden assay). Selected compounds 1c, 1d, 1e, and 1f were then investigated by evaluating their effects on cognitive impairment of acute AD mice model. Learning and memory dysfunction by injection of Aβ(1-42) was recovered by administration of these molecules. Especially, 1d showed the best recovery activity in Y-maze task, object recognition task, and passive avoidance task with dose dependent manner. These results suggest that 1d has high potential as a therapeutic agent for AD.

Phosphorylation alters backbone conformational preferences of serine and threonine peptides

Despite the notion that a control of protein function by phosphorylation works mainly by inducing its conformational changes, the phosphorylation effects on even small peptide conformation have not been fully understood yet. To study its possible effects on serine and threonine peptide conformations, we recently carried out pH‐ and temperature‐dependent circular dichroism (CD) as well as 1H NMR studies of the phosphorylated serine and threonine peptides and compared them with their unphosphorylated analogs. In the present article, by performing the self‐consistent singular value decomposition analysis of the temperature‐dependent CD spectra and by analyzing the 3J(HN,Hα) coupling constants extracted from the NMR spectra, the populations of the polyproline II (PPII) and β‐strand conformers of the phosphorylated Ser and Thr peptides are determined. As temperature is increased, the β‐strand populations of both phosphorylated serine and threonine peptides increase. However, the dependences of PPII/β‐strand population ratio on pH are different for these two cases. The phosphorylation of the serine peptide enhances the PPII propensity, whereas that of the threonine peptide has the opposite effect. This suggests that the serine and threonine phosphorylations can alter the backbone conformational propensity via direct but selective intramolecular hydrogen‐bonding interactions with the peptide NH groups. This clearly indicates that the phosphoryl group actively participates in modulating the peptide backbone conformations. Proteins 2011;

Phosphorylation effect on the GSSS peptide conformation in water: Infrared, vibrational circular dichroism, and circular dichroism experiments and comparisons with molecular dynamics simulations

The phosphorylation effect on the small peptide conformation in water has not been clearly understood yet, despite the widely acknowledged notion that control of protein activity by phosphorylation works mainly by inducing conformational change. To elucidate the detailed mechanism, we performed infrared (IR) absorption and vibrational and electronic circular dichroism studies of both unphosphorylated and phosphorylated tetrapeptides, GSSS 1 and GSSpS 2. The solution structure of the tetrapeptide is found to be little dependent on the presence of the neutral or negatively charged phosphoryl group, and to be a mixture of extended structures including polyproline II (PII) and beta-sheet conformations. The additional band at 1598 cm(-1) in the amide I IR spectrum of the phosphorylated peptide GSSpS at neutral pD appears to be clear spectroscopic evidence for direct intramolecular hydrogen-bonding interaction between the side chain dianionic phosphoryl group and the backbone amide proton. On the basis of amide I IR band analyses, the authors found that the probability of finding the phosphoryl group strongly H bonded to the backbone proton in GSSpS is about 43% at pD 7.0 and 37 degrees C. Such a H-bonding interaction in GSSpS has the biological standard enthalpy and entropy of -15.1 kJ/mol and -51.2 J/K mol, respectively. Comparisons between the experimentally measured IR and VCD spectra and the numerically simulated ones suggested that the currently available force field parameters need to be properly modified. The results in this paper may shed light on an unknown mechanism of controlling the peptide conformation by phosphorylation.

Infrared Probing of 4-Azidoproline Conformations Modulated by Azido Configurations

4-Azidoproline (Azp) can tune the stability of the polyproline II (P(II)) conformation in collagen. The azido group in the 4R and 4S configurations stabilizes and destabilizes the P(II) conformation, respectively. To obtain insights into the dependence of the conformational stability on the azido configuration, we carried out Fourier transform (FT) IR experiments with four 4-azidoproline derivatives, Ac-(4R/S)-Azp-(NH/O)Me. We found that the amide I and azido IR spectra are different depending on the azido configuration and C-terminal structure. The origin of such spectral differences between 4R and 4S configurations and between C-terminal methylamide and ester ends was elucidated by quantum chemistry calculations in combination with (1)H NMR and time- and frequency-resolved IR pump-probe spectroscopy. We found that the azido configurations and C-terminal structures affect intramolecular interactions, which are responsible for the ensuing conformational and thereby IR spectral differences. Consequently, 4-azidoproline conformations modulated by azido configurations can be probed by IR spectroscopy. These findings suggest that 4-azidoproline can be both a structure-control and -probing element, which enables the infrared tracking of proline roles in protein structure, function, and dynamics.

The 2-(4-trifluoromethylphenylsulfonyl)ethoxycarbonyl (Tsc) amino-protecting group: use in the solid-phase synthesis of pyrrole-imidazole polyamides

The development of the 2-(4-trifluoromethylphenylsulfonyl)ethoxycarbonyl (Tsc) function, a novel base-sensitive amino-protecting group, and its application to the preparation of DNA-binding polyamides are described. Pyrrole–imidazole polyamides were synthesized by an efficient solid-phase method under conditions compatible with Fmoc chemistry using two Tsc-protected amino acids, Tsc-Py-OH 1a and Tsc-Im-OH 1b.