Stereocontrolled Synthesis of 19'-Deoxyperidinin.

The stereocontrolled convergent synthesis of 19'-deoxyperidinin, 2, which might be a useful peridinin analog to understand the ICT characteristics, was efficiently achieved by sequential Pd-catalyzed cross-coupling reactions using bidirectionally extensible conjugated C5 olefin segments. The crucial 5(2H)-ylidenedihydrofuran function of 2 was successfully constructed by the Au-catalyzed regio- and stereoselective 5-exo-dig etherification.

Strategies to enhance the excitation energy-transfer efficiency in a light-harvesting system using the intra-molecular charge transfer character of carotenoids.

Fucoxanthin is a carotenoid that is mainly found in light-harvesting complexes from brown algae and diatoms. Due to the presence of a carbonyl group attached to polyene chains in polar environments, excitation produces an excited intra-molecular charge transfer. This intra-molecular charge transfer state plays a key role in the highly efficient (∼95%) energy-transfer from fucoxanthin to chlorophyll a in the light-harvesting complexes from brown algae. In purple bacterial light-harvesting systems the efficiency of excitation energy-transfer from carotenoids to bacteriochlorophylls depends on the extent of conjugation of the carotenoids. In this study we were successful, for the first time, in incorporating fucoxanthin into a light-harvesting complex 1 from the purple photosynthetic bacterium, Rhodospirillum rubrum G9+ (a carotenoidless strain). Femtosecond pump-probe spectroscopy was applied to this reconstituted light-harvesting complex in order to determine the efficiency of excitation energy-transfer from fucoxanthin to bacteriochlorophyll a when they are bound to the light-harvesting 1 apo-proteins.

Excited state properties of ß-carotene analogs incorporating a lactone ring.

Carotenoids possessing a carbonyl group along their polyene backbone exhibit unique excited state properties due to the occurrence of intramolecular charge transfer (ICT) in the excited state. In fact, the ICT characteristics of naturally occurring carbonyl carotenoids play an essential role in the highly efficient energy transfer that proceeds in aquatic photosynthetic antenna systems. In the present study, we synthesized two short-chain polyene carotenoids incorporating a lactone ring, denoted as BL-7 and BL-8, having seven and eight conjugated double bonds (n = 7 and 8), respectively. The excited state properties of these compounds were directly compared to those of their non-carbonyl counterparts to clarify the role of the carbonyl group in the generation of ICT. The energies of the optically allowed S2 states for BL-7 and BL-8 were found to be more than 0.3 eV (2400 cm-1) below those of non-carbonyl short ß-carotene homologs. Ultrafast spectroscopic data demonstrated various solvent polarity-induced effects, including the appearance of stimulated emission in the near-IR region in the case of BL-7, and significant lifetime shortening of the lowest-lying singlet S1 excited states of both BL-7 and BL-8. These results suggest that these compounds exhibit ICT characteristics.

Spectroscopic Investigation of the Carotenoid Deoxyperidinin: Direct Observation of the Forbidden S0 → S1 Transition.

This paper presents a spectroscopic investigation of deoxyperidinin, a synthetic peridinin analogue in which the carbonyl functional group in peridinin was replaced by a nonconjugated methylene group. Steady-state and ultrafast time-resolved absorption and fluorescence spectroscopic experiments are carried out on deoxyperidinin in n-hexane and acetonitrile at room temperature and in 2-methyltetrahydrofuran at 77 K. The spectra of deoxyperidinin have higher vibronic resolution compared to those of peridinin. The higher resolution is due to a substantial reduction in both molecular conformational disorder and inhomogeneous broadening of the spectra of deoxyperidinin compared to peridinin. Features in the steady-state absorption spectrum of deoxyperidinin that are not evident in the spectrum of peridinin are unambiguously assigned to the forbidden S0 (1(1)Ag(-)) → S1 (2(1)Ag(-)) absorption transition. The characteristics of both the steady-state and time-resolved spectra are interpreted using EOM-CCSD, SAC-CI, and MNDO-PSDCI quantum computational formalisms that provided a theoretical framework for understanding the photophysical properties of the molecules.

Formation of an ordered phase by ceramides and diacylglycerols in a fluid phosphatidylcholine bilayer--Correlation with structure and hydrogen bonding capacity.

Ceramides and diacylglycerols are lipids with a large hydrophobic part (acyl chains and long-chain base) whereas their polar function (hydroxyl group) is small. They need colipids with large head groups to coexist in bilayer membranes. In this study, we have determined how saturated and unsaturated ceramides and acyl-chain matched diacylglycerols form ordered domains in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers as a function of bilayer concentration. The formation of ordered domains was determined from lifetime analysis of trans-parinaric acid. Ceramides formed ordered domains with equal average tPA lifetime at lower bilayer concentration when compared to acyl-chain matched diacylglycerols. This was true for both saturated (16:0) and mono-unsaturated (18:1) species. This finding suggested that hydrogen bonding among ceramides contributed to their more efficient ordered phase formation, since diacylglycerols do not form similar hydrogen bonding networks. The role of hydrogen bonding in ordered domain formation was further verified by using palmitoyl ceramide analogs with 2N and 3OH methylated long-chain bases. These analogs do not form hydrogen bonds from the 2NH or the 3OH, respectively. While methylation of the 3OH did not affect ordered phase formation compared to native palmitoyl ceramide, 2NH methylation markedly attenuated ceramide ordered phase formation. We conclude that in addition to acyl chain length, saturation, molecular order, and lack of large head group, also hydrogen bonding involving the 2NH is crucial for efficient formation of ceramide-rich domains in fluid phosphatidylcholine bilayers.

Excited state properties of a short π-electron conjugated peridinin analogue.

C29-peridinin is a synthetic analogue of the important, naturally-occurring carotenoid, peridinin, found in several marine algal species. C29-peridinin has five conjugated carbon-carbon double bonds compared to eight possessed by peridinin and also lacks the methyl group functionalities typically present along the polyene chain of carotenoids. These structural modifications lead to unique excited state properties and important insights regarding the factors controlling the photophysics of peridinin and other carbonyl-containing carotenoids, which are critical components of the light-harvesting systems of many photosynthetic organisms.

Elucidation and Control of an Intramolecular Charge Transfer Property of Fucoxanthin by a Modification of Its Polyene Chain Length.

Fucoxanthin is an essential pigment for the highly efficient light-harvesting function of marine algal photosynthesis. It exhibits excited state properties attributed to intramolecular charge transfer (ICT) in polar environments due to the presence of the carbonyl group in its polyene backbone. This report describes the excited state properties of fucoxanthin homologues with four to eight conjugated double bonds in various solvents using the femtosecond pump-probe technique. The results clarified that fucoxanthin homologues with longer polyene chains did not possess pronounced ICT spectroscopic signatures, while the shorter fucoxanthin homologues had a strong ICT character, even in a nonpolar solvent. On the basis of the observations, we quantitatively correlated the ICT character in the excited state to the conjugated polyene chain lengths of fucoxanthin molecules.

Excited states energies and dynamics of peridinin analogues and the nature of the intramolecular charge transfer state in carbonyl-containing carotenoids.

The lifetime of the lowest excited singlet state of carbonyl-containing carotenoids typically depends on the polarity of the solvent, an effect that has been attributed to the presence of an intramolecular charge transfer (ICT) state. The nature of this ICT state has yet to be clarified. In the present work, steady-state and ultrafast time-resolved optical spectroscopic experiments have been performed on peridinin and three synthetic analogues, C33-peridinin, C35-peridinin, and C39-peridinin, which have different extents of π-electron conjugation. Steady-state absorption at cryogenic temperatures revealed new absorption bands on the long-wavelength side of the strongly allowed S0 (1(1)Ag(-)) → S2 (1(1)Bu(+)) transition that can be assigned to S0 (1(1)Ag(-)) → S1 (2(1)Ag(-)) absorption. Analysis of the time-resolved absorption and fluorescence data sets revealed that the influence of polarity of the solvent on the excited state lifetime is unique for each molecule, leading to subtle differences in the values in highly polar solvents. Measurements in the most polar solvent, acetonitrile, demonstrated that the ICT state lifetime is shortest at 6.4 ps for C39-peridinin and gradually increases as the extent of π-electron conjugation decreases, becoming 10.6 ps for C33-peridinin. This suggests that the energy of the ICT state is dependent on the number of conjugated carbon-carbon double bonds.

Total synthesis of (-)-hippodamine by stereocontrolled construction of azaphenalene skeleton based on extended one-pot asymmetric azaelectrocyclization.

The first asymmetric total synthesis of (-)-hippodamine has been accomplished via the concise construction of its azaphenalene core, which is featured by the 2,4,6-chiral piperidine synthesis based on one-pot asymmetric azaelectrocyclization in the partially activated substituent system and the subsequent intramolecular Mannich reaction.

2NH and 3OH are crucial structural requirements in sphingomyelin for sticholysin II binding and pore formation in bilayer membranes.

Sticholysin II (StnII) is a pore-forming toxin from the sea anemone Stichodactyla heliantus which belongs to the large actinoporin family. The toxin binds to sphingomyelin (SM) containing membranes, and shows high binding specificity for this lipid. In this study, we have examined the role of the hydrogen bonding groups of the SM long-chain base (i.e., the 2NH and the 3OH) for StnII recognition. We prepared methylated SM-analogs which had reduced hydrogen bonding capability from 2NH and 3OH. Both surface plasmon resonance experiments, and isothermal titration calorimetry measurements indicated that StnII failed to bind to bilayers containing methylated SM-analogs, whereas clear binding was seen to SM-containing bilayers. StnII also failed to induce calcein release (i.e., pore formation) from vesicles made to contain methylated SM-analogs, but readily induced calcein release from SM-containing vesicles. Molecular modeling of SM docked to the phosphocholine binding site of StnII indicated that the 2NH and 3OH groups were likely to form a hydrogen bond with Tyr135. In addition, it appeared that Tyr111 and Tyr136 could donate hydrogen bonds to phosphate oxygen, thus stabilizing SM binding to the toxin. We conclude that the interfacial hydrogen bonding properties of SM, in addition to the phosphocholine head group, are crucial for high-affinity SM/StnII-interaction.

Effects of sphingomyelin headgroup size on interactions with ceramide.

Sphingomyelins (SMs) and ceramides are known to interact favorably in bilayer membranes. Because ceramide lacks a headgroup that could shield its hydrophobic body from unfavorable interactions with water, accommodation of ceramide under the larger phosphocholine headgroup of SM could contribute to their favorable interactions. To elucidate the role of SM headgroup for SM/ceramide interactions, we explored the effects of reducing the size of the phosphocholine headgroup (removing one, two, or three methyls on the choline moiety, or the choline moiety itself). Using differential scanning calorimetry and fluorescence spectroscopy, we found that the size of the SM headgroup had no marked effect on the thermal stability of ordered domains formed by SM analog/palmitoyl ceramide (PCer) interactions. In more complex bilayers composed of a fluid glycerophospholipid, SM analog, and PCer, the thermal stability and molecular order of the laterally segregated gel domains were roughly identical despite variation in SM headgroup size. We suggest that that the association between PCer and SM analogs was stabilized by ceramide's aversion for disordered phospholipids, by interfacial hydrogen bonding between PCer and the SM analogs, and by attractive van der Waals' forces between saturated chains of PCer and SM analogs.

Cholesterol's interactions with serine phospholipids - a comparison of N-palmitoyl ceramide phosphoserine with dipalmitoyl phosphatidylserine.

In this study we have prepared ceramide phosphoserine (CerPS) and examined its sterol-interacting properties. CerPS is a hydrogen-bonding sphingolipid, but its head group differs from that found in sphingomyelin (SM). Based on diphenylhexatriene steady-state anisotropy measurements, we observed that fully hydrated N-palmitoyl CerPS had a gel-to-liquid crystalline phase transition temperature of about 51°C in 50mM sodium phosphate buffer (pH 7.4). This was close to the T(m) measured for 1,2-dipalmitoyl-sn-glycero-3-phosphoserine (DPPS) bilayers (T(m) 50.5°C). Based on cholestatrienol (CTL) quenching experiments in liquid disordered ternary bilayers (containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphcholine; POPC), cholesterol/CTL formed sterol-enriched ordered domains with CerPS. These had similar thermostability as the sterol domains formed with N-palmitoyl SM. Cholesterol failed to form sterol-enriched ordered domains with DPPS under comparable conditions. Based on the equilibrium partitioning of CTL, we observed that the affinity of sterol for bilayers containing POPC/CerPS/cholesterol (6:3:1 by mol) was much higher than the affinity measured for control fluid POPC/cholesterol (9:1 by mol) bilayers, but slightly less than seen for comparable PSM-containing bilayers. We conclude that the phosphoserine head group was less efficient than the phosphocholine head group in stabilizing sterol/sphingolipid interaction. However, hydrogen bonding apparently can overcome some of the negative effects of the phosphoserine head group, since CerPS interacted more favorably with cholesterol compared to DPPS.

Effect of Molecular Symmetry on the Spectra and Dynamics of the Intramolecular Charge Transfer (ICT) state of peridinin.

The spectroscopic properties and dynamics of the excited states of two different synthetic analogues of peridinin were investigated as a function of solvent polarity using steady-state absorption, fluorescence, and ultrafast time-resolved optical spectroscopy. The analogues are denoted S-1- and S-2-peridinin and differ from naturally occurring peridinin in the location of the lactone ring and its associated carbonyl group, known to be obligatory for the observation of a solvent dependence of the lifetime of the S(1) state of carotenoids. Relative to peridinin, S-1- and S-2-peridinin have their lactone rings two and four carbons more toward the center of the π-electron system of conjugated carbon-carbon double bonds, respectively. The present experimental results show that as the polarity of the solvent increases, the steady-state spectra of the molecules broaden, and the lowest excited state lifetime of S-1-peridinin changes from ∼155 to ∼17 ps which is similar to the magnitude of the effect reported for peridinin. The solvent-induced change in the lowest excited state lifetime of S-2-peridinin is much smaller and changes only from ∼90 to ∼67 ps as the solvent polarity is increased. These results are interpreted in terms of an intramolecular charge transfer (ICT) state that is formed readily in peridinin and S-1-peridinin, but not in S-2-peridinin. Quantum mechanical computations reveal the critical factors required for the formation of the ICT state and the associated solvent-modulated effects on the spectra and dynamics of these molecules and other carbonyl-containing carotenoids and polyenes. The factors are the magnitude and orientation of the ground- and excited-state dipole moments which must be suitable to generate sufficient mixing of the lowest two excited singlet states.

Palladium-catalyzed asymmetric 6-endo cyclization of dienamides with substituent-driven activation.

Chiral 2-piperidinone compounds with various C-6 substituents were successfully synthesized via a Pd-catalyzed asymmetric 6-endo cyclization of dienamides, which were evidently activated by both N-p-toluenesulfonyl and C-3 ester substituents.

Stereocontrolled total synthesis of fucoxanthin and its polyene chain-modified derivative.

Fucoxanthin exhibits high energy transfer efficiencies to Chlorophyll a (Chl a) in photosynthesis in the sea. In order to reveal how each characteristic functional group, such as the length of the polyene chain, allene, and conjugated carbonyl groups, of this marine natural product are responsible for its remarkably efficient ability, the total synthesis of fucoxanthin by controlling the stereochemistry was achieved. The method established for fucoxanthin synthesis was successfully applied to the synthesis of the C42 longer chain analogue.

Stereocontrolled synthesis of substituted chiral piperidines via one-pot asymmetric 6π-azaelectrocyclization: asymmetric syntheses of (-)-dendroprimine, (+)-7-epidendroprimine, (+)-5-epidendroprimine, and (+)-5,7-epidendroprimine.

The asymmetric one-pot 6π-azaelectrocyclization of alkenyl vinyl stannane, ethyl (Z)-2-iodo-4-oxobutenoate, and (-)-7-isopropyl-cis-aminoindanol in the presence of a Pd(0) catalyst stereoselectively produced the tetracyclic aminoacetal compounds, resulting from the four-bond formation accompanying by controlling the stereochemistry at the two asymmetric centers. The produced cyclic aminoacetals can be regarded as synthetic precursors of substituted chiral piperidines, and the syntheses of 2,4- and 2,4,6-substituted piperidines were realized from the obtained aminoacetals by the stereoselective hydrogenation of the double bond conjugated with the C-4 ester group and alkylation at the aminoacetal moiety. In addition, the stereoselective synthesis of an indolizidine alkaloid, (-)-dendroprimine, and its three stereoisomers, (+)-7-epidendroprimine, (+)-5-epidendroprimine, and (+)-5,7-epidendroprimine, were achieved.

Structure-activity relationship of sphingomyelin analogs with sphingomyelinase from Bacillus cereus.

The aim of this study was to examine how structural properties of different sphingomyelin (SM) analogs affected their substrate properties with sphingomyelinase (SMase) from Bacillus cereus. Using molecular docking and dynamics simulations (for SMase-SM complex), we then attempted to explain the relationship between SM structure and enzyme activity. With both micellar and monolayer substrates, 3O-methylated SM was found not to be degraded by the SMase. 2N-methylated SM was a substrate, but was degraded at about half the rate of its 2NH-SM control. PhytoPSM was readily hydrolyzed by the enzyme. PSM lacking one methyl in the phosphocholine head group was a good substrate, but PSM lacking two or three methyls failed to act as substrates for SMase. Based on literature data, and our docking and MD simulations, we conclude that the 3O-methylated PSM fails to interact with Mg(2+) and Glu53 in the active site, thus preventing hydrolysis. Methylation of 2NH was not crucial for binding to the active site, but appeared to interfere with an induced fit activation of the SMase via interaction with Asp156. An OH on carbon 4 in the long-chain base of phytoPSM appeared not to interfere with the 3OH interacting with Mg(2+) and Glu53 in the active site, and thus did not interfere with catalysis. Removing two or three methyls from the PSM head group apparently increased the positive charge on the terminal N significantly, which most likely led to ionic interactions with Glu250 and Glu155 adjacent to the active site. This likely interaction could have misaligned the SM substrate and hindered proper catalysis.

[1,5]-H shift of aldehyde hydrogen in dienal compounds to produce ketenes.

In 3-ethoxycarbonyl-2,4-dienal compounds, a thermal [1,5]-H shift of aldehyde hydrogen easily proceeded to produce the corresponding vinyl ketenes due to the remarkable substituent effect caused by the C3 ester group. The produced ketenes were captured by an alcohol and olefins to afford the corresponding esters and four-membered ring compounds, respectively.

Singlet and triplet state spectra and dynamics of structurally modified peridinins.

The peridinin-chlorophyll a-protein (PCP) is a light-harvesting pigment-protein complex found in many species of marine algae. It contains the highly substituted carotenoid peridinin and chlorophyll a, which together facilitate the transfer of absorbed solar energy to the photosynthetic reaction center. Photoexcited peridinin exhibits unorthodox spectroscopic and kinetic behavior for a carotenoid, including a strong dependence of the S(1) excited singlet state lifetime on solvent environment. This effect has been attributed to the presence of an intramolecular charge transfer (ICT) state in the molecule. The present work explores the effect of changing the extent of π-electron conjugation and attached functional groups on the nature of the ICT state of peridinin and how these factors affect the excited singlet and triplet state spectra and kinetics of the carotenoid. In this investigation three peridinin analogues denoted C-1-R-peridinin, C-1-peridinin, and D-1-peridinin were synthesized and studied using steady-state absorption and fluorescence techniques and ultrafast time-resolved transient absorption spectroscopy. The study explores the effect on the singlet and triplet state spectra and dynamics of removing the allene group from the peridinin structure and either replacing it with a rigid furanoid ring, replacing it with an epoxide group, or extending the polyene chain into the ß-ionylidine ring.

N-cholesteryl sphingomyelin-A synthetic sphingolipid with unique membrane properties.

A sphingomyelin chimera in which the amide-linked acyl chain was replaced with cholesterol carbamate was prepared and its properties examined. The sphingomyelin/cholesterol chimera (N-cholesterol-D-erythro-sphingomyelin) was able to form unilamellar vesicles of defined size when extruded through 200nm pore size membranes. These N-cholesteryl sphingomyelin bilayers were resistant to solubilization by Triton X-100. When N-cholesteryl sphingomyelin was added to N-palmitoyl sphingomyelin (N-palmitoyl-d-erythro-sphingomyelin) bilayers, it increased acyl chain order as determined by 1,6-diphenyl-1,3,5-hexatriene fluorescence anisotropy. N-cholesteryl sphingomyelin was, however, not as good an inducer of membrane order compared to cholesterol on a molar basis. Differential scanning calorimetry studies further showed that the miscibility of N-cholesteryl sphingomyelin with N-palmitoyl-d-erythro-sphingomyelin bilayers was non-ideal, and the effect of N-cholesteryl sphingomyelin on the N-palmitoyl-d-erythro-sphingomyelin gel-fluid transition enthalpy differed from that seen with cholesterol. Together with N-palmitoyl-d-erythro-sphingomyelin, the N-cholesteryl sphingomyelin chimera was able to form sterol-enriched ordered domains in a fluid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer. N-cholesteryl sphingomyelin in the absence of N-palmitoyl-d-erythro-sphingomyelin was unable to form such sterol-enriched ordered domains in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer. However, N-cholesteryl sphingomyelin markedly increased the affinity of cholestatrienol for N-cholesteryl sphingomyelin containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers, suggesting that N-cholesteryl sphingomyelin was able to somehow stabilize sterol interaction in fluid bilayers. Based on our results, we conclude that N-cholesteryl sphingomyelin behaved more like a cholesterol than a sphingolipid in fluid bilayer membranes. Because N-cholesteryl sphingomyelin increased bilayer order, conferred resistance against detergent solubilization, and is not degradable by phospholipases A(2), it could constitute a good lipocomplex matrix for drug delivery vehicles.