0000000000194218
AUTHOR
Vesa M. Helenius
showing 7 related works from this author
Molecular structures of chlorophyll a aggregates: spectroscopic and molecular modeling study
1993
Molecular structures of chlorophyll a aggregates have been studied. Spectroscopic properties of these aggregates have been studied by absorption and fluorescence spectroscopy in hydrocarbon solution at various temperatures. Observed spectroscopic shifts were interpreted in terms of simple exciton theory. Exciton splittings were estimated from computer optimized models of previously suggested Chl a aggregate structures.
Rotation correlation time as a measure of microviscosity of excited state isomerization reactions of three cyanine dyes in n-alcohol solutions
1994
Abstract Rotation correlation times of three chemically similar cyanine dyes of different sizes in n -alcohol solutions have been recorded at several temperatures by using polarized picosecond spectroscopy. For all three dyes the linear temperature dependencies of τ or on η/ T were observed to be independent of solvent up to viscosities of about 60 cP. The rotational motion of the dyes proceeds at much slower rates than the excited state isomerization in viscous solutions of the same fluidity. Isomerization seems to depend on special solvent-induced changes of the force field of the reactant and clearly proceeds faster, especially for the two larger dyes, than predicted by Kramers' theory a…
Pico: A data acquisition program for picosecond laser spectroscopy
1992
Abstract A data acquisition and control program for picosecond pump and probe experiments has been developed. The program PICO is written in C-programming language for maximum efficiency. The software which runs on an IBM PC compatible microcomputer controls a stepper motor driven optical delay, and at the same time collects data from a digital lock-in amplifier. PICO can be used to control any experiment utilizing two-beam pump probe technique. The modular structure of the software allows for easy implementation with different hardware configurations. The program includes: measurement option, manual control of the delay line using a joystick and functions for file retrieval and editing. Ad…
Molecular aggregates of quinuclidine and chlorophyll a
1991
A slightly polar molecule quinuclidine seems to form weakly bound aggregates in concentrated water solutions. Molecular dynamics simulation of a 6 mol water solution indicates clustering of quinuclidine molecules into an almost spherical structure with polar ends of the molecules pointing towards the solvent. Experimental evidence of aggregation was obtained by observing a small but obvious red shift of the o'' a absorption and a dramatic shortening of the fluorescence lifetime of the c n transition of concentrated solutions. The observed self-quenching is interpreted in terms of Frster model for energy transfer in the cluster. It is estimated that excitation may migrate on the cluster surf…
Excitation Transport in Helical Proteins
1994
Recent results for excitation dynamics in and IR-absorption spectrum of helical polypeptides are briefly reviewed.
Anomalous temperature dependence of the IR spectrum of polyalanine
1997
Abstract We have studied the temperature dependence of the infrared spectra of acetanilide (ACN), tryptophan–(alanine) 15 , and tyrosine–(alanine) 15 . No sidebands of the amide-I vibration were observed in the polypeptides, but two anomalous sidebands of the NH stretch with a similar temperature dependence as that of the anomalous amide-I vibrational mode at 1650 cm −1 of crystalline ACN were detected. Fermi resonance combined with the appearance of a red-shifted sideband of NH stretch through coupling to lattice modes seems to explain this band structure. Observations are indicative of excitons that may occur in polypeptides as well as in single crystals of ACN.
CHLOROPHYLL a AGGREGATES IN HYDROCARBON SOLUTION, A PICOSECOND SPECTROSCOPY AND MOLECULAR MODELING STUDY
1993
Chlorophyll a aggregates in 3-methylpentane solution have been studied by using picosecond absorption and fluorescence spectroscopy and molecular modeling. Chlorophyll a aggregates give rise to reversible temperature changes in the absorption and fluorescence spectra. Time-dependent anisotropies were used to estimate rotational correlation times of the aggregates. These were indicative of the sizes of the aggregates. The rotational diffusion of the monomer and the two identified aggregates was hydrodynamic over the viscosity range studied (0.29–1.8 cP). Molecular mechanics calculations were used to predict the minimum energy structures of several chlorophyll a dimers suggested earlier in th…