Molecular electronic spectral broadening in liquids and glasses.

The magnitudes, time scales, and underlying mechanisms responsible for broadening the electronic spectra of molecules in liquid solutions and glasses are reviewed. The emphasis is on experimental results from hole-burning, single-molecule, photon echo, and resonance Raman and fluorescence studies. The influence of the time scale of the measurement in distinguishing between homogeneous broadening (electronic dephasing) and inhomogeneous broadening is discussed, and the role of coupling of solvent phonons to the solute's electronic transition is stressed.

Plant movements caused by differential growth--unity or diversity of mechanisms?

A very wide range of plant organ movements have been described and yet it is not clear how each of them is related to the others. This uncertainty has had two undesirable consequences. Firstly, some workers have accepted the idea of a vague unity in the area and have subsequently been misled by information obtained in one system and applied without adequate justification to a study of a quite different system. Secondly, some researchers have evoked a possible diversity to explain why a particular mechanistic explanation may continue to be valid even when the model fails to explain events of a very similar nature in a slightly different system. We argue that this confusion has resulted from a classification of organ movements which has been based on functional rather than on mechanistic considerations. Mechanistic unity is to be expected on evolutionary grounds. This unity, however, may apply only to certain elements of the stimulus-response chain, at certain levels of organization. It follows from this that in seeking this unity, comparisons should be made between equivalent elements of the stimulus-response chain at the same level of organization in different systems. Only when this is done will theories built around the concept of unity provoke meaningful discussion.

Vibrational analysis of the all-trans retinal protonated Schiff base.

We have obtained Raman spectra of a series of all-trans retinal protonated Schiff-base isotopic derivatives. 13C-substitutions were made at the 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 positions while deuteration was performed at position 15. Based on the isotopic shifts, the observed C--C stretching vibrations in the 1,100-1,400 cm-1 fingerprint region are assigned. Normal mode calculations using a modified Urey-Bradley force field have been refined to reproduce the observed frequencies and isotopic shifts. Comparison with fingerprint assignments of all-trans retinal and its unprotonated Schiff base shows that the major effect of Schiff-base formation is a shift of the C14--C15 stretch from 1,111 cm-1 in the aldehyde to approximately 1,163 cm-1 in the Shiff base. This shift is attributed to the increased C14--C15 bond order that results from the reduced electronegativity of the Schiff-base nitrogen compared with the aldehyde oxygen. Protonation of the Schiff base increases pi-electron delocalization, causing a 6 to 16 cm-1 frequency increase of the normal modes involving the C8--C9, C10--C11, C12--C13, and C14--C15 stretches. Comparison of the protonated Schiff base Raman spectrum with that of light-adapted bacteriorhodopsin (BR568) shows that incorporation of the all-trans protonated Schiff base into bacterio-opsin produces an additional approximately 10 cm-1 increase of each C--C stretching frequency as a result of protein-induced pi-electron delocalization. Importantly, the frequency ordering and spacing of the C--C stretches in BR568 is the same as that found in the protonated Schiff base.

Determination of retinal Schiff base configuration in bacteriorhodopsin.

Resonance Raman spectra of the BR(568), BR(548), K(625), and L(550) intermediates of the bacteriorhodopsin photocycle have been obtained in (1)H(2)O and (2)H(2)O by using native purple membrane as well as purple membrane regenerated with 14,15-(13)C(2) and 12,14-(2)H(2) isotopic derivatives of retinal. These derivatives were selected to determine the contribution of the C(14)-C(15) stretch to the normal modes in the 1100- to 1400-cm(-1) fingerprint region and to characterize the coupling of the C(14)-C(15) stretch with the NH rock. Normal mode calculations demonstrate that when the retinal Schiff base is in the C[unk]N cis configuration the C(14)-C(15) stretch and the NH rock are strongly coupled, resulting in a large ( approximately 50-cm(-1)) upshift of the C(14)-C(15) stretch upon deuteration of the Schiff base nitrogen. In the C[unk]N trans geometry these vibrations are weakly coupled and only a slight (<5-cm(-1)) upshift of the C(14)-C(15) stretch is predicted upon N-deuteration. In BR(568), the insensitivity of the 1201-cm(-1) C(14)-C(15) stretch to N-deuteration demonstrates that its retinal C[unk]N configuration is trans. The C(14)-C(15) stretch in BR(548), however, shifts up from 1167 cm(-1) in (1)H(2)O to 1208 cm(-1) in (2)H(2)O, indicating that BR(548) contains a C[unk]N cis chromophore. Thus, the conversion of BR(568) to BR(548) (dark adaptation) involves isomerization about the C[unk]N bond in addition to isomerization about the C(13)[unk]C(14) bond. The insensitivity of the native, [14,15-(13)C(2)]-, and [12,14-(2)H(2)]K(625) and L(550) spectra to N-deuteration argues that these intermediates have a C[unk]N trans configuration. Thus, the primary photochemical step in bacteriorhodopsin (BR(568) --> K(625)) involves isomerization about the C(13)[unk]C(14) bond alone. The significance of these results for the mechanism of proton-pumping by bacteriorhodopsin is discussed.