20. The impact of solution-state equilibria on the rate of alpha-acid isomerization
Paul Hughes (1); (1) Oregon State University, Corvallis, OR, U.S.A.
Technical Session 6: Hops I
Sunday, August 14 • 3:30–5:15 p.m.
Plaza Building, Concourse Level, Governor’s Square 15
The recent revision of the absolute configurations of naturally
occurring alpha-acids and their isomerized counterparts in some ways
simplifies the rationalized mechanism for the isomerization mechanism.
Essentially, the revised stereochemistry invokes conservation of the
stereocenter in the alpha-acids, so that the tertiary carbinol retains
the same S-configuration and partial racemization at the “5” position,
resulting in the formation of cis- and trans-iso-alpha-acids.
Nevertheless, there are some unanswered questions regarding alpha-acid
isomerization. Firstly, alpha-acids exist as a set of keto-enol
tautomers, which influences the electronic distribution around the
alpha-acid system. Secondly, alpha-acids are dibasic, so they can
effectively exist as free acids, monoanions and dianions, which is
presumably important as isomerization is much more efficient at higher
pH values. Thirdly, certain metal cations, such as magnesium(II) can
accelerate isomerization rates substantially. Here we have carried out a
thorough theoretical evaluation of the solution state of alpha-acids
and their mono- and dianions, as well as the various possible magnesium
chelates, using well-established computational chemistry procedures. We
have identified a small population of free acids, monoanions and
dianions that form the basis of alpha-acids in solution, and we have
further tentatively identified a number of key features in these
populations that point to potential opportunities to increase the rate
and specificity of the isomerization reaction.
Paul gained bachelor and Ph.D. degrees in chemistry before joining
the U.K. Health and Safety Executive in 1988. In 1990 Paul moved to the
Brewing Research Foundation, where he worked on and managed a wide
range of research projects, including the sensory and foaming properties
of hop acids, LOX control in malts and malting and protein–polyphenol
interactions. In 1999 Paul moved to Heineken Technical Services in the
Netherlands as principal scientist. He was responsible for a number of
projects and aspects of product safety and integrity. In 2005 Paul took
up the position of professor of brewing at Heriot-Watt University,
before assuming the role of director of the International Centre for
Brewing and Distilling in 2006 and director of research for the School
of Life Sciences in 2008. Today Paul is assistant professor of
distilling at Oregon State University, where he leads research,
education and outreach on distilled spirits. Paul holds an MBA from the
University of Surrey and the IBD diploma in brewing. He is the author of
more than 60 research papers and of four patents and is co-author of
two textbooks on beer quality and whisky. He is a frequent speaker at
international conferences and runs international courses, principally
focused on spirit production and quality.
