Carbonyl Lone Pair Research Articles

Electronic Control of Polyproline II Helix Stability via the Identity of Acyl Capping Groups: the Pivaloyl Group Particularly Promotes PPII.

The type II polyproline helix (PPII) is a fundamental secondary structure of proteins, important in globular proteins, in intrinsically disordered proteins, and at protein-protein interfaces. PPII is stabilized in part by n→π* interactions between consecutive carbonyls, via electron delocalization between an electron-donor carbonyl lone pair (n) and an electron-acceptor carbonyl (π*) on the subsequent residue. We previously demonstrated that changes to the electronic properties of the acyl donor can predictably modulate the strength of n→π* interactions, with data from model compounds, in solution in chloroform, in the solid state, and computationally. Herein, we examined whether the electronic properties of acyl capping groups could modulate the stability of PPII in peptides in water. In X-PPGY-NH2 peptides (X=10 acyl capping groups), the effect of acyl group identity on PPII was quantified by circular dichroism and NMR spectroscopy. Electron-rich acyl groups promoted PPII relative to the standard acetyl (Ac-) group, with the pivaloyl and iso-butyryl groups most significantly increasing PPII. In contrast, acyl derivatives with electron-withdrawing substituents and the formyl group relatively disfavored PPII. Similar results, though lesser in magnitude, were also observed in X-APPGY-NH2 peptides, indicating that the capping group can impact PPII conformation at both proline and non-proline residues. The pivaloyl group was particularly favorable in promoting PPII. The effects of acyl capping groups were further analyzed in X-DfpPGY-NH2 and X-ADfpPGY-NH2 peptides, Dfp=4,4-difluoroproline. Data on these peptides indicated that acyl groups induced order Piv- > Ac- > For-. These results suggest that greater consideration should be given to the identity of acyl capping groups in inducing structure in peptides.

Excited State Decay Pathways of 2'-Deoxy-5-methylcytidine and Deoxycytidine Revisited in Solution: A Comprehensive Kinetic Study by Femtosecond Transient Absorption.

Methylated cytosine is proved to have an important role as an epigenetic signal in gene regulation and is often referred to "the fifth base of DNA". A comprehensive understanding of the electronic excited state relaxation in cytosine and its methylated derivatives is crucial for revealing UV-induced photodamage to the biological genome. Because of the existence of multiple closely lying "bright" and "dark" excited states, the decay pathways in these DNA nucleosides are the most complex and the least understood so far. In this study, femtosecond transient absorption with different excitation wavelengths (240-296 nm) was used to study the relaxation of excited electronic states of 5-methylcytosine (5mC) and 2'-deoxy-5-methylcytidine (5mdCyd) in phosphate buffered aqueous solution and in acetonitrile solution. Two distinct nonradiative decay channels were directly observed. The first one is a several picosecond internal conversion channel that involves two bright ππ* states (ππ*2 and ππ*1) when ππ*2 state is initially populated. The second channel contains the lower energy ππ*1 state and a so far experimental unidentified long-lived state which exhibits a several nanosecond lifetime. The long-lived state can only be accessed by the initially excited ππ*1 state. Inspired by this new discovery in 5mC and 5mdCyd, we revisited the decay of excited state of 2'-deoxycytidine (dCyd), revealing very similar decay pathways. Additionally, a well-known dark nOπ* state (carbonyl lone pair) with ∼30 ps lifetime is present in both decay channels in dCyd. With our detailed experimental results, we successfully reconcile the long history debate of cytosine excited state relaxation mechanism by pointing out that the reason for the complex dynamics under traditional 266 nm excitation is mixed signals from the above-mentioned two distinct decay pathways. Our findings lead to a dramatically different and new picture of electronic energy relaxation in 5mdCyd/dCyd and could help to understand photostability as well as UV-induced photodamage of these nucleotides and related DNAs.

Photophysics of Deoxycytidine and 5-Methyldeoxycytidine in Solution: A Comprehensive Picture by Quantum Mechanical Calculations and Femtosecond Fluorescence Spectroscopy.

The study concerns the relaxation of electronic excited states of the DNA nucleoside deoxycytidine (dCyd) and its methylated analogue 5-methyldeoxycytidine (5mdCyd), known to be involved in the formation of UV-induced lesions of the genetic code. Due to the existence of four closely lying and potentially coupled excited states, the deactivation pathways in these systems are particularly complex and have not been assessed so far. Here, we provide a complete mechanistic picture of the excited state relaxation of dCyd/5mdCyd in three solvents-water, acetonitrile, and tetrahydrofuran-by combining femtosecond fluorescence experiments, addressing the effect of solvent proticity on the relaxation dynamics of dCyd and 5mdCyd for the first time, and two complementary quantum mechanical approaches (CASPT2/MM and PCM/TD-CAM-B3LYP). The lowest energy ππ* state is responsible for the sub-picosecond lifetime observed for dCyd in all the solvents. In addition, computed excited state absorption and transient IR spectra allow one, for the first time, to assign the tens of picoseconds time constant, reported previously, to a dark state (nOπ*) involving the carbonyl lone pair. A second low-lying dark state, involving the nitrogen lone pair (nNπ*), does significantly participate in the excited state dynamics. The 267 nm excitation of dCyd leads to a non-negligible population of the second bright ππ* state, which affects the dynamics, acting mainly as a "doorway" state for the nOπ* state. The solvent plays a key role governing the interplay between the different excited states; unexpectedly, water favors population of the dark states. In the case of 5mdCyd, an energy barrier present on the main nonradiative decay route explains the 6-fold lengthening of the excited state lifetime compared to that of dCyd, observed for all the examined solvents. Moreover, C5-methylation destabilizes both nOπ* and nNπ* dark states, thus preventing them from being populated.

The excited state behavior of cytosine in the gas phase: A TD-DFT study

We report a TD-DFT study of the three lowest energy tautomers of Cytosine in the gas phase, focussing on the Franck–Condon region and on the excited state minima and comparing the performances of three different density functionals, namely CAM-B3LYP, M052X and PBE0. For what concerns the oxo-amino tautomer, the one present in water and within DNA, CAM-B3LYP and M052X provide a picture close to that of recent CCSD(T) calculations. In particular, in the Franck–Condon region the electronic transition involving the Lone Pair of the Nitrogen atom corresponds to S2 and is the lowest energy nπ∗ transition (SNπ∗), being significantly more stable than that involving the Carbonyl Lone Pair (SOπ∗), whose stability is instead overestimated by PBE0. CAM-B3LYP and M052X also provide minima fully consistent with those of the most recent MCSCF methods: different relative minima are present on the lowest energy adiabatic state and the most stable corresponds to the SNπ∗ diabatic state. PBE0 predicts instead a strong coupling of SOπ∗ with the lowest energy ππ∗ state. Calculations of the vibrationally resolved absorption spectra of the three lowest enegy tautomers, including Duschinsky effects, provide spectra in very good agreement with the experiments.

Kinetic Solvent Effects on Hydrogen Abstraction Reactions from Carbon by the Cumyloxyl Radical. The Importance of Solvent Hydrogen-Bond Interactions with the Substrate and the Abstracting Radical

A kinetic study of the hydrogen atom abstraction reactions from propanal (PA) and 2,2-dimethylpropanal (DMPA) by the cumyloxyl radical (CumO•) has been carried out in different solvents (benzene, PhCl, MeCN, t-BuOH, MeOH, and TFE). The corresponding reactions of the benzyloxyl radical (BnO•) have been studied in MeCN. The reaction of CumO• with 1,4-cyclohexadiene (CHD) also has been investigated in TFE solution. With CHD a 3-fold increase in rate constant (k(H)) has been observed on going from benzene, PhCl, and MeCN to TFE. This represents the first observation of a sizable kinetic solvent effect for hydrogen atom abstraction reactions from hydrocarbons by alkoxyl radicals and indicates that strong HBD solvents influence the hydrogen abstraction reactivity of CumO•. With PA and DMPA a significant decrease in k(H) has been observed on going from benzene and PhCl to MeOH and TFE, indicative of hydrogen-bond interactions between the carbonyl lone pair and the solvent in the transition state. The similar k(H) values observed for the reactions of the aldehydes in MeOH and TFE point toward differential hydrogen bond interactions of the latter solvent with the substrate and the radical in the transition state. The small reactivity ratios observed for the reactions of CumO• and BnO• with PA and DMPA (k(H)(BnO•)/k(H)(CumO•) = 1.2 and 1.6, respectively) indicate that with these substrates alkoxyl radical sterics play a minor role.

Solid-State Changes in Ligand-to-Metal Charge-Transfer Spectra of (NH3)5RuIII(2,4-dihydroxybenzoate) and (NH3)5RuIII(xanthine) Chromophores

Five distorted-octahedral complexes containing (NH3)5Ru(III)L ions, where L = 2,4-dihydroxybenzoate or a xanthine, have been studied using a combination of X-ray crystallography, solution and polarized single-crystal electronic absorption spectroscopy, and first principles electronic structure computational techniques. Both yellow (2) and red (3) forms of the complex (NH3)5Ru(III)L, where L = 2,4-dihydroxybenzoate, as well as three xanthine complexes in which L = hypoxanthine-kappaN(7) (4), 7-methylhypoxanthine-kappaN(9) (5), and 1,3,9-trimethylxanthine-kappaN(7) (6) were examined. In the solid state, some of these complexes exhibit split low-energy ligand-to-metal charge-transfer (LMCT) bands. Traditional solid-state effects, such as ligand pi-pi overlap or hydrogen bonding that might lead to splitting of electronic absorption bands, were probed in an attempt to identify the origins of these unusual observations. For comparison, companion studies were carried out for spectroscopically normal reference complexes of the same ligands. Time-dependent density-functional theory (TD-DFT) calculations, employing modified B3LYP-type functionals with increased contributions of exact exchange, attribute the color change in the crystalline complexes 2 and 3 to pi(ligand) --> Ru[d(pi)] LMCT bands which, in the red form (3), arise from ligand donor pi-orbitals split by strongly overlapping phenyl moieties in centrosymmetric (NH3)5Ru(III)(2,4-dihydroxybenzoate) dimers. Complex 5 does not show split visible absorptions, whereas both the polarizations and energies of the split visible absorptions shown by 4 and 6 also suggest assignment as LMCT. No support is found for relating the split absorptions of 4 and 6 to the details of pi-pi xanthine overlap in the solid state; indeed, complex 4 enjoys considerably less pi-stacking overlap than does 5. We feel compelled to attribute the split absorptions in crystalline 4 and 6 to the emergence of a LMCT transition originating in the carbonyl lone pair, potentially deriving intensity from the significant intramolecular N-H...O hydrogen bonding present in both 4 and 6 (but not in 5). The electronic structure calculations suggest an O(n) --> Ru[d(sigma*)] LMCT transition; however, this novel assignment must be considered tentative.

Theoretical study of formic acid: A new look at the origin of the planar Z conformation and C–O rotational barrier

The E and Z rotamers of formic acid (HCOOH) and its barrier to internal rotation about the C–O bond were computationally explored at the HF/6-311 + G ∗∗, B3LYP/cc-pVTZ, and CCSD(T)/cc-pVTZ levels of theory. All calculations yielded similar results consistent with experimental observations. Subsequent analysis of the interaction between formate ion (HCOO −) and proton (H +) within formic acid demonstrated a direct correlation between the changes in fragment interaction energy and the total energy of formic acid upon rotation. To obtain further insights into the interaction, energy decomposition analysis based on the reactive bond orbital (RBO) method was carried out using the 6-311 + G ∗∗ basis set. The analysis showed the electrostatic effect constitutes a major component that gives rise to the interaction energy variation along the rotation path. Thus, the electrostatic environment of HCOO − can be viewed as the key factor determining the Z ground state and C–O rotational barrier of formic acid. The anisotropic electrostatic environment of formate that favors planar conformations of formic acid may be due to the in-plane distribution of carbonyl lone pairs, and the larger electrostatic attraction in the Z form appears to come from a secondary electrostatic interaction between the proton and the distal oxygen. At the rotational transition state, the O–H bond was not exactly perpendicular to the molecular plane, but slightly tilted toward the E side, which can also be explained by the electrostatic hypothesis. Charge-transfer stabilization was smallest in the Z conformation, but it gradually increased upon rotation to a maximum at the E conformation. Therefore, charge - transfer does not explain the geometry of formic acid. The important role of the electrostatic effect was also observed in in-plane rotation of the O–H bond.

Basic catalysts for making carbonyl compounds

The adsorption and reaction of methanoi (CH3OH), methyl formate (CH3OCHO) and formaldehyde (H2CO) on clean and oxygen-covered Cu(110) surfaces has been studied with EELS, UPS and thermal desorption spectroscopy (TDS). The clean surface is relatively unreactive but adsorbed oxygen readily attacks the hydroxyl proton and formyl carbon atoms to generate the intermediate methoxy (CH3O) and formate (HCOO). Methyl formate is split into two intermediates, methoxy and formate. By correlating the three techniques we analyse (a) the condensed multilayer at 90 K; (b) the weakly bound molecular monolayer states prior to dissociation or reaction and (c) the reactive intermediates at higher temperatures. Formaldehyde forms the surface polymer polyoxymethylene [(CH2O)n] in the monolayer on Cu(110) which subsequently reacts with oxygen to generate formate. No molecular formaldehyde was observed above 120 K. Correlation of the EELS and UPS results for polyoxymethylene shows that an earlier interpretation by Rubloff et al. [Phys. Rev. B14 (1976) 1450] of anomalous shifts in the formaldehyde UPS spectrum on surfaces is incorrect, and due simply to the new polymeric structure of surface formaldehyde. Methyl formate coordinates to copper via the carbonyl lone pair orbital and methanol via the oxygen lone pair orbital. No evidence was found for methyl formate synthesis by dimerization of formaldehyde (the Tischenko reaction) or dehydrogenation of methanol on the clean Cu(110) surface. These latter reactions are facile over copper catalysts at atmospheric pressure. The success of the oxidation experiments and the failure of the synthesis reactions in UHV is a consequence of the pressure dependence of the equilibrium constants for the different reactions. As found previously in Fischer-Tropsch studies, condensation reaction equilibria are pressure dependent and product formation is considerably suppressed at UHV pressures.

Infrared Study of the Hydrogen Bonding Association in Polyamides Plasticized by Benzenesulfonamides. Part I: Self-Association in Amide and Sulfonamide Systems; Part II: Amide—Sulfonamide Interaction

The molecular associations of N-( n-hexyl)hexanamide (PA-12MC) and N-( n-butyl)benzenesulfonamide (BBSA), taken as model compounds for polydodecamide (PA-12) and benzenesulfonamide plasticizers, respectively, were examined by Fourier transform infrared spectroscopy. In solution, the amide is distributed between a self-associated form, involving intermolecular hydrogen bonding, and isolated species. Dissociation is favored in an electron donor solvent due to hydrogen bonding between the amide and the solvent. Comparatively, BBSA has much less tendency to dissociate. Molecular modeling suggests that BBSA dimer associations exist in the condensed state thanks to intermolecular hydrogen bonds, while gas-phase infrared spectroscopy supports a stabilizing intramolecular interaction between the sulfonamide proton and the sulfonyl lone pairs for isolated molecules. Mixtures of the amide model compound with BBSA show the creation of a strong S-N-H … O=C hydrogen bond between the sulfonamide proton and the amide's carbonyl lone pairs. The amide N-H groups liberated from the former amide-amide interaction find themselves involved in a weaker C-N-H … O=S hydrogen bond (“free N-H”) with the plasticizer's sulfonyl lone pairs, the concentration of these bonds being maximum at mid-composition. For polyamide/BSAs mixtures, the accessibility of the amide and sulfonamide groups can restrict these associations. Mixtures of AAPA, an aliphatic amorphous polyamide, with a plasticizer bearing a branched alkyl chain, generate a low free N-H concentration and cause phase separation to occur, which confirms the steric sensibility of their interaction. A bifunctional benzenesulfonamide plasticizer appears to be less efficient than BBSA and leads to an increased dispersion in hydrogen bond distribution, both of which could be ascribed to the bulk of this molecule. Incorporation of BBSA in semicrystalline PA-12 leads to a behavior identical to that of AAPA/BBSA mixtures.

Theoretical Studies on the Continuum Solvation of Some N,N‘-Dimethyl- and N-Methyl,N‘-acetyl-Guanidine and Guanidinium Conformers

The conformational properties of neutral and protonated N,N‘-dimethyl and N-methyl,N‘-acetyl-guanidines have been studied in vacuo and in solution, considering either water or chloroform as solvents. Using the ab initio MP2/6-31G* optimized geometries obtained in vacuo, single-point HF/6-311++G**//MP2/6-31G* and MP2/6-311++G**//MP2/6-31G* calculations have been performed in vacuo and continuum solvent free energy calculations in solution for most of the syn (S) and anti (A) conformers (with respect to the unsubstituted nitrogen) for different tautomeric structures. The solvation free energy is considerably less favorable, as expected, for neutral than for protonated guanidines (about −11 vs −63 kcal/mol in water and about −4 vs −41 kcal/mol in chloroform for dimethyl-guanidines at the HF level; methyl-acetyl-guanidines show a slightly larger spread around those average values). The correlation contribution to the solvent effect is feeble for the protonated derivatives in solution, while it is larger (up to 4 kcal/mol in water) for the neutral conformers. The cooperative effect among the carbonyl lone pairs and the nitrogen in-plane lone pair is responsible for the fairly large solvent stabilization of the conformers with these polar groups facing each other. Several different conformers/tautomers of neutral methyl-acetyl-guanidine are within a 0.5 kcal/mol free energy range in vacuo, whereas in solution both the neutral (with a cis CN double bond with respect to the acetyl CO) and protonated SS methyl-acetyl conformers turn out to be slightly stabilized over the others, especially at the HF level.

Synthesis and Solution and Solid-State Structures of Tris(pentafluorophenyl)borane Adducts of PhC(O)X (X = H, Me, OEt, NPri2)

Reaction of the highly electrophilic borane B(C6F5)3 with the carbonyl Lewis bases benzaldehyde, acetophenone, ethyl benzoate, and N,N-diisopropylbenzamide led to isolation of the crystalline adducts 1-H, 1-Me, 1-OEt, and 1-NPr, respectively, in good to excellent yields (63−89%). Equilibrium measurements and exchange experiments indicated that the order of basicity (from highest to lowest) of these bases toward B(C6F5)3 follows the order N,N-diisopropylbenzamide > benzaldehyde > acetophenone > ethyl benzoate. The solution and solid-state structures were probed to rationalize these observations. In solution, the borane coordinates to the carbonyl lone pair syn to H and Me in the aldehyde and ketone adducts, as indicated by 1H/19F NOE difference experiments. The same coordination geometry was observed in the solid state upon X-ray diffraction analysis of the two adducts. The added front strain associated with the ketone adduct (C−O−B =133.6(3)° vs 126.7(5)° for the benzaldehyde complex) accounts for the observed order of basicity with these two bases. For ethyl benzoate and N,N-diisopropylbenzamide, the borane coordinates syn to the phenyl group in both solution and the solid state. In addition to the carbonyl oxygen−boron interaction, the two complexes engage in a π-stacking interaction between one of the borane C6F5 rings and the syn phenyl group. In addition to the structural proof of this interaction in the solid state, variable-temperature 19F NMR experiments suggest it is important in the solution structures of these adducts as well.

Spectroscopic investigations of dehydroepiandrosterone. Part II: Photoelectron spectrum and electronic structure

The gas phase electronic structure of dehydroepiandrosterone (3β-hydroxy-5α-androsten-17-one, DHEA) is elucidated from its Hel photoelectron spectrum, quantum chemical calculations, and correlation with spectra of the related molecules 5α-androsten-17-one (1) and epiandrosterone (3β-hydroxy-5α-androstan-17-one) (2). The lowest ionization energy event in DHEA is shown to correspond to a π-ionization energy of ∼8.5 eV, followed closely and overlapped in part by the carbonyl lone pair ionization at 8.7 eV.

Donor-acceptor interfaces in langmuir-blodgett films. Structural and optical features related to nonlinear optical properties

The linear optical properties of Langmuir-Blodgett films of alternating layers of the electron acceptor, octadecylthiobenzoquinone, and the electron donor 7-( N-octadodecylaminomethyl)-8-16-dioxadibenzo [f,g]perylene indicate that only a weak intermolecular contact is formed at the interface between donors and acceptors. Structural data based on linear dichroism and X-ray diffraction show that the intermolecular contact between donor and acceptor layers is possible due to the carbonyl lonepairs in the acceptor, which are directed towards the interface. The prospects for donor-acceptor interfaces in nonlinear optical materials are discussed.

Photoelectron spectroscopy of biologically active molecules. 20.Para-quinones, semiquinones, and aromatic ketones

We have recorded and analyzed the HeI PE spectra of the following molecules: 9,10-dihydroanthracene (1); 9,10(H)-anthracenone (2); 9,10-anthracenedione (3); 1,2,3,4-tetrahydro-9,10-anthracenedione (4); 1,4,1a,4a-tetrahydro 9,10-anthracenedione (5); 10-methylene-9,10(H)-anthracenone (6); and 10-(phenylmethylene)-9,10(H)-anthracenone (7). The PE spectra are assigned by comparison with those of the composite parts (i.e., by employing an orbital interaction model such as the composite molecule method). This approach, which works surprisingly well in the present instance, indicates that the carbonyl lone pair and the carbonyl π electrons interact negligibly with the outer π electrons of the aromatic unit(s). If no change in conformation of the component aromatic parts occurs, the spectrum of the composite molecule exhibits the additivity property. This result agrees with previous studies of benzophenones [3]. However, it is argued that the ordering of the strongly overlapped, low energy ionization bands of benzaldehyde and acetophenone should be changed to I1(π), I2(π) ≈︁ I3(no) and I1(π), I2(no), I3(π), respectively.

A spectroscopic study of the adsorption and reactions of methanol, formaldehyde and methyl formate on clean and oxygenated Cu(110) surfaces

The adsorption and reaction of methanoi (CH3OH), methyl formate (CH3OCHO) and formaldehyde (H2CO) on clean and oxygen-covered Cu(110) surfaces has been studied with EELS, UPS and thermal desorption spectroscopy (TDS). The clean surface is relatively unreactive but adsorbed oxygen readily attacks the hydroxyl proton and formyl carbon atoms to generate the intermediate methoxy (CH3O) and formate (HCOO). Methyl formate is split into two intermediates, methoxy and formate. By correlating the three techniques we analyse (a) the condensed multilayer at 90 K; (b) the weakly bound molecular monolayer states prior to dissociation or reaction and (c) the reactive intermediates at higher temperatures. Formaldehyde forms the surface polymer polyoxymethylene [(CH2O)n] in the monolayer on Cu(110) which subsequently reacts with oxygen to generate formate. No molecular formaldehyde was observed above 120 K. Correlation of the EELS and UPS results for polyoxymethylene shows that an earlier interpretation by Rubloff et al. [Phys. Rev. B14 (1976) 1450] of anomalous shifts in the formaldehyde UPS spectrum on surfaces is incorrect, and due simply to the new polymeric structure of surface formaldehyde. Methyl formate coordinates to copper via the carbonyl lone pair orbital and methanol via the oxygen lone pair orbital. No evidence was found for methyl formate synthesis by dimerization of formaldehyde (the Tischenko reaction) or dehydrogenation of methanol on the clean Cu(110) surface. These latter reactions are facile over copper catalysts at atmospheric pressure. The success of the oxidation experiments and the failure of the synthesis reactions in UHV is a consequence of the pressure dependence of the equilibrium constants for the different reactions. As found previously in Fischer-Tropsch studies, condensation reaction equilibria are pressure dependent and product formation is considerably suppressed at UHV pressures.

Surface intermediates in the reaction of methanol, formaldehyde and methyl formate on copper (110)

The adsorption and reactions of methanol, methyl formate and formaldehyde on clean and oxygen-covered copper (110) surfaces has been studied with EELS, UPS and temperature programmed desorption (TPD). We report spectroscopic data for (a) the condensed multilayers at 90 K, (b) the surface monolayers and (c) the reaction intermediates formate (HCOO) and methoxy (CH 3O) generated by reaction with surface atomic oxygen. On the clean surface methanol and methyl formate bond via the oxygen and carbonyl lone pair orbitals, whereas formaldehyde always polymerizes to surface paraformaldehyde above 120 K. Atomic oxygen generates methoxy by reaction with methanol and formate by reaction with formaldehyde as reported previously. Methyl formate undergoes nucleophilic attack to form both methoxy and formate in the presence of atomic oxygen, in a similar fashion to the reaction on Ag (110). Whereas the oxidation reactions of these molecules on copper catalysts can be explained with reference to the observed intermediates, no evidence is found for the dimerization of formaldehyde or the dehydrogenation of methanol to form methyl formate: reactions which are quite facile under higher pressure conditions.

Modification of pK values caused by change in H-bond geometry.

The competition between various groups for a proton is studied by ab initio molecular orbital methods. It is found that reorientations of the two groups involved in a H-bond can reverse the equilibrium position of the proton shared between them. Specifically, the carbonyl and hydroxyl groups were modeled by H2CO and HOH. In the H-bond between these two groups, association of the proton with the carbonyl (H2COH...OH2)+ is favored over the hydroxyl (H2CO...HOH2)+ when the latter group is situated along a lone pair of the carbonyl oxygen. However, displacement of the water to the C = O axis between the two carbonyl lone pairs reverses the situation and (H2CO...HOH2)+ is more stable. A similar reversal of stability is observed in the H-bond involving a Schiff base (modeled by CH2NH) and amine (NH3). In one arrangement where the lone pairs of the two groups point toward one another, the proton prefers the Schiff base to the amine--i.e., (H2CHNH...NH3)+ is more stable than (H2CHN...HNH3)+. On the other hand, rotation of the lone pair of the amine away from the Schiff base nitrogen results in proton transfer across to the amine. These shifts in stability correspond to reversal of relative pK of the groups involved. A fundamental principle emerging from the calculations is that ion-dipole electrostatic interactions favor transfer of a proton to the group that is positioned as closely as possible to the negative end of the dipole moment vector of the other. The ideas developed here suggest a number of means by which conformational changes may be utilized to shift protons from residue to residue within a protein molecule such as an enzyme or bacteriorhodopsin.

INTERACTION BETWEEN THE CARBONYL GROUP AND A SULFUR ATOM. PART XI PHOTOELECTRON SPECTRA OF SOME 2-ALKYLTHIOCYCLOPENTANONES

Abstract The helium I photoelectron spectra of some 2-alkylthio-cyclopentanones are reported. The IP′s related to the sulfur and carbonyl lone pairs are compared to those in the corresponding cyclopentanones and cyclopentyl alkyl sulfide. The observed stabilization of both orbitals cannot be explained by the mutual inductive effects of the sulfur and carbonyl group. The conformational dependence of this effect is discussed.

Photoelectron and electronic spectra of acenaphthenequinone, naphthalic anhydride, and naphthalimide

The u.v. photoelectron spectra of acenaphthenequinone, naphthalic anhydride, naphthalimide, and N-methyl-naphthalimide are reported and interpreted employing correlative arguments assisted by results of semiempirical calculations. The core ionisation energies (I.E.s) of the atoms of the carbonyl groups are also reported. For a series of cyclic anhydrides and imides it is shown that there is a fair correlation between the average I.E.s of the carbonyl lone pair orbitals and the average CO stretching frequencies, while there is no direct correlation between the average n I.E.s and the 13C carbonyl chemical shifts.The main features of the u.v.–visible spectra of these compounds have been analysed theoretically by means of SCF–INDO/S–Cl calculations and an elucidation of the electronic bands is given in terms of n→π* carbonyl transitions and π→π* transitions of specifically naphthalene origin.

On the photoelectron and ultraviolet spectra of molecules containing a divalent sulfur atom and a carbonyl group separated by a methylene group

Photoelectron spectra show that in CH3SCH2COCH3 the sulfur and carbonyl lone pair orbitals [Formula: see text] are both stabilized with respect to the compounds containing S or CO only. This is in keeping with the observed values of the basicity constants. The ultraviolet absorption spectra contain the expected [Formula: see text] and [Formula: see text] bands and give evidence for a significant amount of mixing between the excited orbitals.