Infrared intensities of amide modes in N‐methylacetamide and poly(glycine I) from ab initio calculations of dipole moment derivatives of N‐methylacetamide

The infrared intensities of the amide modes in N‐methylacetamide (NMA) and poly(glycine I) (PGI) have been studied using ab initio dipole moment derivatives obtained for the peptide group in NMA and an empirical force field refined for PGI. Good agreement is found between the calculated transition moment magnitudes and directions of the amide I and II modes and experimental intensity and dichroism data. By analyzing the separate contributions of each internal coordinate to the total intensity, we are able to understand in detail the origins of the IR intensities of the amide modes. Besides demonstrating one approach by which IR intensities can be studied in complex molecules and polymers, our results also provide a basis for using IR intensities in structural studies of peptides and polypeptides.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70763/2/JCPSA6-82-4-1631-1.pd

Ab initio force fields of glycine dipeptide in C5 and C7 conformations

Schafer and coworkers have previously derived, with the 4-21 Gaussian basis set ab initio optimized geometries for the glycine dipeptide, CH3CONHCH2CONHCH3, in several conformations. Using their geometries for the C5 and C7 conformations, respectively with five- and seven-membered intramolecular NH...OC hydrogen-bonded rings, we have computed the vibrational force fields and dipole moment derivatives at the 4-21 level. Scale factors for the quadratic force constants were transferred, with two changes, from the set derived by Fogarasi and Balazs from a study of small amides. The differences in the force constants, harmonic frequencies, dipole derivatives, and infrared intensities between the conformations and within each conformation are discussed and related, where possible, to differences in hydrogen bonding and structure. In particular, the NH and CO stretch and bend force constants and dipole derivatives show trends that can be correlated with the C5 and C7 hydrogen bonds. The amide modes are compared with some available data on glycine dipeptide in argon matrix, and the comparison supports the empirically-based conclusion that C5 and C7 conformations are both present in the matrix-isolated sample.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28060/1/0000499.pd

Ab initio force fields of alanine dipeptide in four non-hydrogen bonded conformations

We have previously calculated the force fields and dipole-moment derivatives of the L-alanyl dipeptide, CH3CONHCHCH2CONHCH3, in the C5, C7eq, and C7ax conformations with intramolecular NH...OC hydrogen bonding. We have now extended these studies with similar ones on four open, non-hydrogen-bonded conformers optimized by Scarsdale et al. the [beta]2, [alpha]R, [alpha]L and [alpha]' structures. The force constants were derived using the 4-21 Gaussian basis set and were scaled as before. The present results help to isolate the effects of conformation on the force fields and dipole derivatives, and also provide further insight into the effects of hydrogen bonding by comparison with the previous results. Additional effects can also be seen, such as those of the non-planarity of the peptide group, and the influence on the CO bond when the adjacent NH forms a hydrogen bond, and vice versa.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28700/1/0000520.pd

Vibrational analysis of crystalline diketopiperazine--II. Normal mode calculations

Based on our Raman and i.r. [1] data and assignments for DKP and five of its isotopic derivatives, we have refined an intramolecular force field in a non-redundant basis. This analysis shows that the cis peptide group may be best differentiated from the trans by the presence of an NH out-of-plane bend mode in the 800 cm-1 region. Several atom--atom potentials have been evaluated for their ability to explain observed lattice frequencies and internal mode splittings. Observed splittings in CO stretch modes can be accounted for by dynamical charge transfer and transition dipole coupling interactions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25013/1/0000440.pd

Ab initio force fields of alanine dipeptide in C5 and C7 conformations

We have followed up our previous ab initio calculations of the force fields and dipole-moment derivatives of glycine dipeptide in C5 and C7 conformations with similar studies of the -alanyl compound [CH3CONHCH(CH3)CONHCH3]. We have done, with the 4-21 Gaussian basis set, calculations of the three lowest-energy optimized conformations with intramolecular hydrogen bonding that were derived by Scarsdale et al.: the C5, C7eq, and C7ax structures. The quadratic force constants were scaled, as in our previous work, using the set of scale factors derived by Fogarasi and Balazs for small amides, with slight modifications. The differences in the force constants, harmonic frequencies, dipole derivatives, and infrared intensities among the conformations and within each conformation are discussed and related, where possible, to differences in hydrogen bonding and structure. The results are also compared and integrated with those on Gly dipeptide. The force constants and dipole derivatives show that the C7ax hydrogen bond is the strongest, in agreement with the NH[middle dot]O geometries, even though the SCF energy of this conformer is the highest. The NH stretch and the amide I and II modes are compared with available infrared data on Ala dipeptide in argon matrix, and the comparison supports the empirically based conclusion that C5 and C7 conformations are both present in the matrix-isolated sample. The problem of distinguishing between the C7eq and C7ax forms is also discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27891/1/0000308.pd

Vibrational analysis of cyclo(-Ala--Ala) in two crystalline forms. Effect of structure on peptide group and CH modes

Cyclo(-Ala--Ala) has been found by X-ray diffraction to crystallize in two forms with different hydrogen bond patterns and strengths and different conformations of the C[alpha]H[alpha]C[beta]H3 group. We have done a comprehensive analysis of the Raman and i.r. spectra of both forms and their N-deuterated derivatives. Spectra were taken of oriented single crystals, polycrystalline powders, and aqueous solutions, allowing a virtually complete vibrational assignment. Systematic, distinct differences were observed in the modes of the CONH group and in the CH stretching and bending modes. These spectral differences have been correlated with the different molecular and crystal structures. In particular, the width and sub-band structure of the NH stretch mode and the splittings of the CO stretch are shown to be related to the hydrogen bond pattern, and the NH bend modes are found to be relatively as sensitive to the hydrogen bond strength as is the NH stretch. The differences in the CH[alpha] bend modes show that both conformations exist in solution. Ab initio molecular orbital calculations have been done to understand the frequency shifts of the NH and CH[alpha] stretch modes in the two forms. Normal mode calculations were also done.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27499/1/0000543.pd

Vibrational analysis of crystalline diketopiperazine--I. Raman and i.r. spectra

Raman and i.r. spectra have been obtained of diketopiperazine and five of its isotopic derivatives: (CONDCH2)2, (CONHCD2)2, (CONDCD2)2, (13CONHCH2)2 and (13CONDCH2)2. In addition to solution spectra and powder spectra at room and liquid nitrogen temperatures, polarized Raman spectra were obtained on single crystals. These data have permitted an essentially complete assignment to be made of the Raman and i.r. modes of the crystal, a necessary step in the refinement of an intramolecular force field for this molecule [13].Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25010/1/0000437.pd

C=O stretch mode splitting in the formic acid dimer: Electrostatic models of the intermonomer interaction

The physical origin of the large (74 cm-1) splitting between the symmetric (Ag) and antisymmetric (Bu) components of the C=O stretch mode in the formic acid dimer has previously been attributed to tautomerism effects, transition dipole--dipole coupling, or dynamical charge transfer through the hydrogen bonds. We show that an electrostatic model involving atomic charge--charge interactions can account for a splitting of 56 cm-1, provided the atomic partial charges are allowed to vary in magnitude during vibrational motion. The charges and charge derivatives have been obtained from ab initio Hartree--Fock calculations up to the 6-31G** level. An additional 13 cm-1 of the remaining discrepancy in the splitting of 69 cm-1, compared to the observed value of 74 cm-1.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26690/1/0000237.pd

Toksičnost talija u humanoj populaciji

Thallium is a naturally occurring trace element, widely distributed in the earth’s crust, but at very low concentrations. It does not have a known biological use and does not appear to be an essential element for life. It has been considered one of the most toxic heavy metals. Occasionally, there are reports on thallium poisoning as results of suicide or murder attempt or accident. The main threat to humans is through occupational exposure, environmental contamination, and accumulation in food, mainly in vegetables grown on contaminated soil. Increasing use in emerging new technologies and demanding high-tech industry constantly raise concern about exposure risk to all living organisms. Thallium is considered a cumulative poison that can cause adverse health effects and degenerative changes in many organs. The effects are the most severe in the nervous system. The exact mechanism of thallium toxicity still remains unknown, although impaired glutathione metabolism, oxidative stress, and disruption of potassium-regulated homeostasis may play a role. The lack of data about mutagenic, carcinogenic, or teratogenic effects of thallium compounds in humans calls for further research.Talij je u prirodi široko rasprostranjen teški metal, prisutan u vrlo niskim koncentracijama pa ga stoga ubrajamo u elemente u tragovima. Budući da organizmima nije potreban ni u jednoj razvojnoj fazi, ne ubrajamo ga u grupu esencijalnih elemenata. Talij zbog njegovih svojstava ubrajamo među najtoksičnije teške metale. Povremeno se još uvijek pojavljuju slučajevi u kojima je talij upotrijebljen kao sredstvo za pokušaj ubojstva, odnosno samoubojstva, ali i slučajevi nenamjernog, slučajnog trovanja talijem. U današnje vrijeme potencijalna o asnost od trovanja talijem postoji zbog profesionalne izloženosti, izbijanja ekološke katastrofe ili zbog akumulacije u hranidbenim lancima, uglavnom zbog uzgoja hrane na onečišćenom tlu. Sve češća uporaba talija u visokotehnološkoj industriji kao odgovor na zahtjeve moderne tehnologije neprestano povećava rizik od izloženosti svih živih organizama štetnim utjecajima talija u okolišu. Talij ima izuzetno negativan učinak na različite organske sustave, a osobito na živčani sustav. Mehanizmi toksičnosti talija još uvijek nisu u potpunosti razjašnjeni, premda važnu ulogu imaju poremećaji metabolizma glutationa, oksidativni stres i narušavanje homeostaze posredovane ionima kalija. Nedostatak podataka o mutagenim, kancerogenim ili teratogenim učincima talija i njegovih spojeva u ljudi opravdava buduća istraživanja ovog vrlo toksičnog metala

Toksičnost talija u humanoj populaciji

Thallium is a naturally occurring trace element, widely distributed in the earth’s crust, but at very low concentrations. It does not have a known biological use and does not appear to be an essential element for life. It has been considered one of the most toxic heavy metals. Occasionally, there are reports on thallium poisoning as results of suicide or murder attempt or accident. The main threat to humans is through occupational exposure, environmental contamination, and accumulation in food, mainly in vegetables grown on contaminated soil. Increasing use in emerging new technologies and demanding high-tech industry constantly raise concern about exposure risk to all living organisms. Thallium is considered a cumulative poison that can cause adverse health effects and degenerative changes in many organs. The effects are the most severe in the nervous system. The exact mechanism of thallium toxicity still remains unknown, although impaired glutathione metabolism, oxidative stress, and disruption of potassium-regulated homeostasis may play a role. The lack of data about mutagenic, carcinogenic, or teratogenic effects of thallium compounds in humans calls for further research.Talij je u prirodi široko rasprostranjen teški metal, prisutan u vrlo niskim koncentracijama pa ga stoga ubrajamo u elemente u tragovima. Budući da organizmima nije potreban ni u jednoj razvojnoj fazi, ne ubrajamo ga u grupu esencijalnih elemenata. Talij zbog njegovih svojstava ubrajamo među najtoksičnije teške metale. Povremeno se još uvijek pojavljuju slučajevi u kojima je talij upotrijebljen kao sredstvo za pokušaj ubojstva, odnosno samoubojstva, ali i slučajevi nenamjernog, slučajnog trovanja talijem. U današnje vrijeme potencijalna o asnost od trovanja talijem postoji zbog profesionalne izloženosti, izbijanja ekološke katastrofe ili zbog akumulacije u hranidbenim lancima, uglavnom zbog uzgoja hrane na onečišćenom tlu. Sve češća uporaba talija u visokotehnološkoj industriji kao odgovor na zahtjeve moderne tehnologije neprestano povećava rizik od izloženosti svih živih organizama štetnim utjecajima talija u okolišu. Talij ima izuzetno negativan učinak na različite organske sustave, a osobito na živčani sustav. Mehanizmi toksičnosti talija još uvijek nisu u potpunosti razjašnjeni, premda važnu ulogu imaju poremećaji metabolizma glutationa, oksidativni stres i narušavanje homeostaze posredovane ionima kalija. Nedostatak podataka o mutagenim, kancerogenim ili teratogenim učincima talija i njegovih spojeva u ljudi opravdava buduća istraživanja ovog vrlo toksičnog metala