Linked Life Data

7766600

Source:http://linkedlifedata.com/resource/pubmed/id/7766600

Statements in which the resource exists as a subject.
PredicateObject
rdf:type
lifeskim:mentions
pubmed:issue
21
pubmed:dateCreated
1995-7-3
pubmed:abstractText
We present spectroscopic evidence for the presence of low-barrier hydrogen bonds (LBHBs) in molecular complexes composed of carboxylic acids and 1-methylimidazole (1-MeIm) dissolved in aprotic organic solvents. A plot of the values of the low-field proton NMR chemical shifts versus the aqueous pKa of the carboxylic acid exhibits a positive slope for pKa values below 2.1 and a negative slope for higher pKa values. The chemical shifts for protons near the maximum in this plot are 18 ppm, similar to that of 18.3 ppm for His57-Asp102 in the protonated catalytic triad of chymotrypsin. The chemical shifts for the proton bonded to C2 of 1-MeIm in these complexes also vary with the pKa of the carboxylic acid and reveal a gradual change from neutral, hydrogen-bonded 1-MeIm in complexes of weaker acids to hydrogen-bonded 1-methylimidazolium ion in complexes of stronger acids. The midpoint chemical shift for the C2 proton corresponds to a carboxylic aqueous pKa of about 2.1. FTIR spectra of the 1-MeIm-carboxylic acid complexes in CHCl3 indicate that hydrogen bonding is strong and that the complexes are of three types: (a) neutral complexes with the weaker acids (pKa > or = 2.2) in which the antisymmetric carbonyl stretching frequencies are lowered relative to the free acids and the ethyl esters of the same acids; (b) ionic complexes of stronger acids (pKa < or = 2.1) in which the carbonyl stretching frequencies are slightly lower than those for the tetrabutylammonium salts of the same acids; (c) ionic complexes of the same acids (pKa < or = 2.1) coexisting with type b, in which the carbonyl stretching frequencies are intermediate between those for the tetrabutylammonium salts (bond order 1.5) and those of the same acids or their esters (bond order 2.0). The latter complexes appear to incorporate a low-barrier hydrogen bond and are presented as models for the protonated triad of chymotrypsin and other serine proteases. These enzymes have been postulated to utilize a low-barrier hydrogen bond between His57 and Asp102 to facilitate the abstraction of the beta-OH proton from Ser195 in the course of catalysis [Frey, P.A., Whitt, S.A., & Tobin, J.B. (1994) Science (Washington, D.C.)264,1927-1930].
pubmed:grant
pubmed:language
eng
pubmed:journal
pubmed:citationSubset
IM
pubmed:chemical
pubmed:status
MEDLINE
pubmed:month
May
pubmed:issn
0006-2960
pubmed:author
pubmed:issnType
Print
pubmed:day
30
pubmed:volume
34
pubmed:owner
NLM
pubmed:authorsComplete
Y
pubmed:pagination
6919-24
pubmed:dateRevised
2007-11-14
pubmed:meshHeading
pubmed:year
1995
pubmed:articleTitle
Low-barrier hydrogen bonding in molecular complexes analogous to histidine and aspartate in the catalytic triad of serine proteases.
pubmed:affiliation
Institute for Enzyme Research, University of Wisconsin, Madison 53705, USA.
pubmed:publicationType
Journal Article, Research Support, U.S. Gov't, P.H.S., Research Support, U.S. Gov't, Non-P.H.S., Research Support, Non-U.S. Gov't