Warner L. Peticolas

On the quantitative interpretation of biomembrane structure by Raman spectroscopy

The now well-established use of Raman spectroscopy to examine the structure of biomembranes is extended through an examination of the origins of the structure-sensitive features of phospholipid spectra and the development of quantitative order-parameters. One parameter gives a quantitative measure of the fraction of all-trans bonds in the hydrocarbon chains while the other provides a semiquantitative estimate of the lateral crystal-like order between the chains. The parameters are used to study the differences between vesicles and dispersions of dipalmitoyl phosphotidylcholine, dimyristoylcholine and egg lecithin. We find that the vesicles of dipalmitoyl phosphotidylcholine are substantially less ordered than the dispersions in terms of both longitudinal and lateral order which are greatly decreased. A very careful measurement of the order as a function of temperature shows that there is a pre-melting transition in the dispersions of dipalmitoyl phosphotidylcholine which does not exist in the vesicles. Remarkable agreement is obtained between the Raman technique and that previously reported by calorimetric measurements and theoretical calculations.

Reorientation and Vibrational Relaxation as Line Broadening Factors in Vibrational Spectroscopy

The theory of Gordon which relates the shape of the depolarized components of certain Raman bands to a molecular reorientation correlation function is extended to a number of types of molecular vibrations not previously covered. In addition, the contribution to the line shape for vibrational relaxation and reorientation is obtained for the three types of vibrational spectroscopy, infrared, Raman, and hyper‐Raman, and it is shown that the form of the vibrational relaxation contribution is the same for each spectroscopy. Conditions and assumptions are given under which the line broadening contributions of reorientation and vibrational relaxation can be separated and experimentally determined.

Raman active vibrations in long-chain fatty acids and phospholipid sonicates.

The Raman active vibrations of saturated and unsaturated fatty acids are examined. The all-trans chain lengths of the saturated carbon chains in both types of homogeneous fatty acid samples can be determined in this way. The side-chain melting transition of l-α-dioleoyl lecithin suspension is studied and it is found that although the interior of the multilayer is more mobile than the region closer to the polar surface, both regions are in a random configuration at room temperature.

Quantum Theory of the Intensities of Molecular Vibrational Spectra

Formulas for the transition probabilities and hence the absolute intensities of molecular vibrational spectra are obtained from a unified quantum field treatment. The theory of infrared, Raman, and hyper‐Raman spectroscopy of molecular vibrations is developed by assuming these processes occur as time‐ordered steps involving the creation or destruction of one quantum of vibrational energy and changes in the occupation number of one, two, or three photons, respectively. The formulas obtained by this method for ir transitions become equivalent to the earlier treatment of Jones and Simpson if the energy difference of the ground and first excited electronic energy levels are very large relative to that of the vibrational quantum. The formulas obtained for Raman transitions are very similar to those obtained by the method originated by Albrecht and developed further by Savin; we get not only the original terms of Albrecht but also the trace terms obtained by Savin. Furthermore by using third‐order time‐dependen...

Interpretation of biomembrane structure by Raman difference spectroscopy. Nature of the endothermic transitions in phosphatidylcholines.

Raman difference spectroscopy has been applied to aqueous dispersions of dipalmitoyl phosphatidylcholine (DPPC). Difference spectra have been created by computer subtraction of absolute Raman spectra taken in each of three different temperature ranges: below the endothermic pretransition at 34 +/- 2 degrees C; between this temperature and the melting transition at 42 degrees C; and above the melting temperature. The resultant difference spectra are both quantitatively and qualitatively different, indicating that a distinct phospholipid conformation occurs in each of the three temperature ranges. Furthermore, the difference spectra show details of Raman spectral changes with greater clarity than is possible with conventional Raman techniques. A description of the lateral interchain order and the longitudinal chain order is given for each of the three temperature ranges. In addition to obtaining a more precise quantitative measurement of the changes in the Raman spectra, we observed some significant and previously unreported changes. It is suggested that distortion in the hexagonal lattice below the pretransition temperature previously reported by X-ray diffraction techniques may be responsible for interchain interactions which give rise to a Raman band observed only in the triclinic lattice of even-numbered n-alkanes.

Raman Spectra and the Phonon Dispersion of Polyglycine

The Raman spectra of the two crystalline modifications of polyglycine, I and II, and of N‐deuterated polyglycine II have been recorded. The results of a normal‐coordinate analysis of polyglycine II and N‐deuterated polyglycine II are presented along with the phonon dispersion curves of polyglycine II. Assignments are made on a number of bands not observed in the infrared. Some previous band assignments are found to be inconsistent with the Raman data.

Polarized Laser Raman Studies of Biological Polymers

A group‐theoretical analysis of the Raman tensor for helical polymers is given and illustrated for the Pauling α helix which possesses 18 residues in five turns. A detailed determination of the polarized Raman spectra is given for oriented α‐helical poly‐l‐alanine fibers and for four polyribonucleotides in solution. Tentative assignments to A, E1, and E2 symmetry are made on certain Raman bands of poly‐l‐alanine based on the polarization of the Raman light. A detailed comparison of monomer and polymer bands are given for the four polynucleotides. A theory for the angular dependence of Raman light scattered from helical molecules is given.

Ab initio calculations of the ultraviolet resonance Raman spectra of uracil

An equation been derived to calculate, ab initio, the frequencies and intensities of a resonant Raman spectrum from the transform theory of resonance Raman scattering. This equation has been used to calculate the intensities of the ultraviolet resonance Raman spectra from the first π‐π* excited state of uracil and 1,3‐dideuterouracil. The protocol for this calculation is as follows: (1) The force constant matrix elements in Cartesian coordinate space, the vibrational frequencies, and the minimum energy ground and excited state geometries of the molecule are calculated ab initio using the molecular orbital program Gaussian 92, (2) the force constants in Cartesian coordinates are transformed into force constants in the space of a set of 3N – 6 nonredundant symmetrized internal coordinates, (3) the G matrix is constructed from the energy minimized ground state Cartesian coordinates and the GFL = LΛ eigenvalue equation is solved in internal coordinate space, (4) the elements of the L and L−1 matrices are calculated, (5) the changes in all of the internal coordinates in going from the ground to the excited state are calculated, and (6) these results are used in combination with the transform theory of resonance Raman scattering to calculate the relative intensities of each of the 3N – 6 vibrations as a function of the exciting laser frequency. There are no adjustable parameters in this calculation, which reproduces the experimental frequencies and intensities with remarkable fidelity. This indicates that the Dushinsky rotation of the modes in the excited state of these molecules is not important and that the simplest form of the transform theory is adequate.

Ultraviolet resonance Raman excitation profiles of pyrimidine nucleotides

A semiempirical theoretical method for calculating the resonant Raman excitation profiles (RREP) from the measured absorption profile is developed and applied to measurements on pyrimidine mononucleotides. The method is general and provides a way of checking the consistency of a measured RREP with a measured absorption profile. From this treatment one obtains Δej, the shift in the equilibrium position of the normal coordinate Qj when the molecule is excited to the eth (resonant) electronic state. This quantity is useful in the estimation of the excited state geometry. Some error in our calculated values of Δej may result because changes in the excited state normal‐mode frequencies have not been included in our evaluation of the Franck–Condon factors.

The use of resonant Raman intensities in refining molecular force fields for Wilson G–F calculations and obtaining excited state molecular geometries

In this paper equations are developed for the quantitative refinement of molecular force fields for Wilson G–F calculations through the use of molecular orbital (MO) calculations of the bond order change between the resonant excited electronic state and the ground state together with the observed intensity of the resonant Raman spectrum (RRS). The method should permit an unambiguous choice between two or more force fields which give equally good fit of the observed frequencies but which give different potential energy distributions among the internal coordinates. We have used this method to study methyl uracil, a planar ring molecule of low symmetry, where partitioning of the matrices is not possible. A least‐squares calculation of the difference between the MO bond order changes and the bond order changes calculated from the force field together with the measured intensities in the RRS allows a choice of the correct sign for the excited state changes in the normal coordinates of modes which are active in...