Tyrosyl rotamer interconversion rates and the fluorescence decays of N-acetyltyrosinamide and short tyrosyl peptides.
It has long been recognized that the fluorescence lifetimes of amino acid residues such as tyrosine and tryptophan depend on the rotameric configuration of the aromatic side chain, but estimates of the rate of interchange of rotameric states have varied widely. We report measurements of the rotameric populations and interchange rates for tyrosine in N-acetyltyrosinamide (NATyrA), the tripeptide Tyr-Gly-Gly (YGG), and the pentapeptide Leu-enkephalin (YGGFL). The fluorescence lifetimes were analyzed to determine the rotameric interchange rates in the context of a model incorporating exchange among three rotameric states. Maximum entropy method analysis verified the presence of three fluorescence decay components for YGGFL and two for YGG and NATyrA. Rotameric exchange between the gauche(-) and trans states occurred on the nanosecond time scale, whereas exchange with the gauche(+) state occurred on a longer time scale. Good agreement was obtained with rotameric populations and exchange rates from molecular dynamics simulations. Quenching by iodide was used to vary the intrinsic fluorescence lifetimes, providing additional constraints on the determined interchange rates. The temperature dependence was measured to determine barriers to exchange of the two most populated rotamers of 3, 5, and 7 kcal/mol for NATyrA, YGG, and YGGFL, respectively.
Unruh
Jay R
JR
Department of Chemistry, University of Kansas, 1251 Wescoe Drive, Lawrence, Kansas 66044, USA.
Liyanage
Mangala Roshan
MR
Johnson
Carey K
CK
D005453:Fluorescence; D007454:Iodides / Q000737:chemistry; D007700:Kinetics; D015394:Molecular Structure; D010455:Peptides / Q000737:chemistry*; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry*
D007454:Iodides; D010455:Peptides; C019142:N-acetyltyrosylamide; D014443:Tyrosine