PMID- 18672928
OWN - NLM
STAT- MEDLINE
DCOM- 20090917
LR  - 20131121
IS  - 1520-6106 (Print)
IS  - 1520-5207 (Linking)
VI  - 113
IP  - 9
DP  - 2009 Mar 5
TI  - Solvent effects on the fluorescence quenching of tryptophan by amides via 
      electron transfer. Experimental and computational studies.
PG  - 2572-7
LID - 10.1021/jp711513b [doi]
AB  - Hybrid quantum mechanical/molecular mechanics (QM-MM) calculations [Callis and 
      Liu, J. Phys. Chem. B 2004, 108, 4248-4259] make a strong case that the large 
      variation in tryptophan (Trp) fluorescence yields in proteins is explained by 
      ring-to-backbone amide electron transfer, as predicted decades ago. Quenching 
      occurs in systems when the charge transfer (CT) state is brought below the 
      fluorescing state (1L(a)) as a result of strong local electric fields. To further 
      test this hypothesis, we have measured the fluorescence quantum yield in solvents 
      of different polarity for the following systems: N-acetyl-L-tryptophanamide 
      (NATA), an analogue for Trp in a protein; N-acetyl-L-tryptophan ethyl ester 
      (NATE), wherein the Trp amide is replaced by an ester group, lowering the CT 
      state energy; and 3-methylindole (3MI), a control wherein this quenching 
      mechanism cannot take place. Experimental yields in water are 0.31, 0.13, and 
      0.057 for 3MI, NATA, and NATE, respectively, whereas, in the nonpolar aprotic 
      solvent dioxane, all three have quantum yields near 0.35, indicating the absence 
      of electron transfer. In alkyl alcohols the quantum yield for NATA and NATE is 
      between that found for water and that found for dioxane, and it is surprisingly 
      independent of chain length (varying from methanol to decanol), revealing that 
      microscopic H-bonding, and not the bulk dielectric constant, dictates the 
      electron transfer rate. QM-MM calculations indicate that, when averaged over the 
      six rotamers, the greatly increased quenching found in water relative to dioxane 
      can be attributed mainly to the larger fluctuations of the energy gap in water. 
      These experiments and calculations are in complete accord with quenching by a 
      solvent stabilized charge transfer from ring to amide state in proteins.
FAU - Muino, Pedro L
AU  - Muino PL
AD  - Department of Chemistry, Saint Francis University, Loretto, Pennsylvania 15940, 
      USA. pmuino@francis.edu
FAU - Callis, Patrik R
AU  - Callis PR
LA  - eng
PT  - Journal Article
PT  - Research Support, U.S. Gov't, Non-P.H.S.
PL  - United States
TA  - J Phys Chem B
JT  - The journal of physical chemistry. B
JID - 101157530
RN  - 0 (Amides)
RN  - 0 (Proteins)
RN  - 0 (Solvents)
RN  - 059QF0KO0R (Water)
RN  - 6519-66-0 (tryptophan ethyl ester)
RN  - 8DUH1N11BX (Tryptophan)
SB  - IM
MH  - Amides/*chemistry
MH  - Biophysics/methods
MH  - Computational Biology/*methods
MH  - Computer Simulation
MH  - Electrons
MH  - Hydrogen Bonding
MH  - Microscopy, Fluorescence/*methods
MH  - Models, Chemical
MH  - Proteins/chemistry
MH  - Quantum Theory
MH  - Software
MH  - Solvents/*chemistry
MH  - Tryptophan/analogs & derivatives/*chemistry
MH  - Water/chemistry
EDAT- 2008/08/05 09:00
MHDA- 2009/09/18 06:00
CRDT- 2008/08/05 09:00
PHST- 2008/08/05 09:00 [pubmed]
PHST- 2009/09/18 06:00 [medline]
PHST- 2008/08/05 09:00 [entrez]
AID - 10.1021/jp711513b [doi]
PST - ppublish
SO  - J Phys Chem B. 2009 Mar 5;113(9):2572-7. doi: 10.1021/jp711513b.