Salting Out near the Critical Point
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- 作者：Robert M. Mazo
- 刊名：Journal of Physical Chemistry B
- 出版年：2008
- 出版时间：May 8, 2008
- 年：2008
- 卷：112
- 期：18
- 页码：5879 - 5879
- 全文大小：10K
- 年卷期：v.112,no.18(May 8, 2008)
- ISSN：1520-5207
文摘
The active form of vitamin B6, pyridoxal 5'-phosphate (PLP), is an important cofactor for numerous enzymesin amine and amino acid metabolism. Presented here is the first femtosecond transient absorption study offree PLP and two Schiff bases, PLP-valine and PLP-
-aminoisobutyric acid (AIB), in solution. Photoexcitationof free PLP leads to efficient triplet formation with an internal conversion rate that increases with increasingpH. The measured excited-state kinetics of the PLP-valine Schiff base exhibits a dramatic deuterium dependenceas a result of excited-state proton transfer (ESPT) of the C hydrogen in the amino acid substrate. This isconsistent with formation of the key reaction carbanionic intermediate (quinonoid), which is resonance stabilizedby the electron-deficient, conjugated 
system of the Schiff base/pyridine ring. The transient absorption signalsof the PLP-Schiff base with -methylalanine (2-aminoisobutyric acid), which does not have a C proton,lack an observable deuterium effect, verifying ESPT formation of the quinonoid intermediate. In contrast toprevious studies, no dependence on the excitation wavelength of the femtosecond kinetics is observed withPLP or PLP-valine, which suggests that a rapid (<250 fs) tautomerization occurs between the enolimine(absorbing at 330 nm) and ketoenamine (absorbing at 410 nm) tautomers in solution.
-aminoisobutyric acid (AIB), in solution. Photoexcitationof free PLP leads to efficient triplet formation with an internal conversion rate that increases with increasingpH. The measured excited-state kinetics of the PLP-valine Schiff base exhibits a dramatic deuterium dependenceas a result of excited-state proton transfer (ESPT) of the C hydrogen in the amino acid substrate. This isconsistent with formation of the key reaction carbanionic intermediate (quinonoid), which is resonance stabilizedby the electron-deficient, conjugated system of the Schiff base/pyridine ring. The transient absorption signalsof the PLP-Schiff base with -methylalanine (2-aminoisobutyric acid), which does not have a C proton,lack an observable deuterium effect, verifying ESPT formation of the quinonoid intermediate. In contrast toprevious studies, no dependence on the excitation wavelength of the femtosecond kinetics is observed withPLP or PLP-valine, which suggests that a rapid (<250 fs) tautomerization occurs between the enolimine(absorbing at 330 nm) and ketoenamine (absorbing at 410 nm) tautomers in solution.