A relationship between amide hydrogen bond strength and quinone reduction potential: Implications for photosystem I and bacterial reaction center quinone function

Article abstract of DOI:10.1016/j.bmcl.2007.06.041

A series of 11 simple phylloquinone derivatives, each lacking the extended phytyl side chain but featuring H-bond donor amides at one or both peri positions, were prepared and some salient physical properties were measured. A correlation between both IR frequency and NMR peak position, as indicators of internal H-bond strength, and the quinone half-wave reduction potential, was observed. These data are consistent with the prevailing hypothesis that quinone carbonyl H-bonding in general, and stronger H-bonds in particular, favorably bias the endogenous quinone's electrochemical potential toward easier reduction.

Full text of DOI:10.1016/j.bmcl.2007.06.041

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4891–4894
A relationship between amide hydrogen bond strength and
quinone reduction potential: Implications for photosystem I
and bacterial reaction center quinone function
Ken S. Feldman,a,* D. Keith Hester, IIa and John H. Golbecka,b
aDepartment of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
bDepartment of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA
Received 14 February 2007; revised 11 June 2007; accepted 12 June 2007
Available online 14 June 2007
Abstract—A series of 11 simple phylloquinone derivatives, each lacking the extended phytyl side chain but featuring H-bond donor
amides at one or both peri positions, were prepared and some salient physical properties were measured. A correlation between both
IR frequency and NMR peak position, as indicators of internal H-bond strength, and the quinone half-wave reduction potential,
was observed. These data are consistent with the prevailing hypothesis that quinone carbonyl H-bonding in general, and stronger
H-bonds in particular, favorably bias the endogenous quinone’s electrochemical potential toward easier reduction.
Ó 2007 Elsevier Ltd. All rights reserved.
The complete structural characterization of the photo-
synthetic reaction centers photosystem I (PS I)1 and bac-
terial reaction center (bRC)2 has enabled unparalleled
advances in understanding the mechanistic underpin-
nings of photosynthesis.3 Even as Angstrom-level fea-
tures have come into focus, the relationship between
structure and function, in many cases, has yet to be clar-
ified. One pivotal feature of the light-driven electron
transfer from each system’s primary chlorophyll accep-
tor to the terminal repository, a quinone (bRC) or
Fe4S4 cluster (PS I), involves reaction through an inter-
mediate quinone transfer junction (ubiquinone (1) in
bRC and phylloquinone (2) in PS I), Figure 1. The crit-
ical reduction potentials of these quinones are thought
to be responsive to local electrostatic effects, with prox-
imal negatively charged amino acid side chains and a
juxtaposed negatively charged iron–sulfur cluster con-
tributing to a lower value (harder to reduce) for phyllo-
quinone in PS I compared with an adjacent and
reduction-facilitating Fe2+ site in bRC.4 In addition to
these charge effects, both H-bonding and p-stacking also
are cited as influential factors in determining the qui-
none reduction potential. Ubiquinone is pinioned by
the protein matrix between two H-bonds as shown,
and so it is not surprising that its estimated reduction
H
O
Ala M260
H
Leu A722
N
N
O
H
H
Trp A697
H3CO
H3CO
His M219
O
O
N
NH
C50H81
phytyl
O
HN
O
1 ubiquinone
~ 190 mV8
2 phylloquinone
reduction potential
in vivo10
~ - 360 mV8 in vitro (DMF)10
~ - 810 mV9
~ - 480 mV9
Figure 1. Structures and figures of merit for the quinones of bRC (left)
and PS I (right). All reduction potentials are reported versus the
normal hydrogen electrode.10 See above-mentioned references for
further information.
potential in vivo is higher than (more easily reduced
than) the same quinone in the non-protic media DMF.
Whereas the inexact modeling of a protein interior by
DMF does not allow too firm of a conclusion to be
drawn, it is likely that at least some of the diminished
reduction potential of the in vivo version can be attrib-
uted to dual H-bond activation of the quinone’s carbo-
nyls. It is quite surprising, then, that the analogous
quinone 2 of PS I exhibits a reduction potential that is
substantially lower than various phylloquinone models
in DMF solution, given that it, too, presumably benefits
Keywords: Quinone; Reduction; Photosynthesis; H-bonding.
*
Corresponding author. E-mail: ksf@chem.psu.edu
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.06.041

4892
K. S. Feldman et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4891–4894
from the single H-bond shown.5 The role that the enve-
loping protein plays in modulating the reduction poten-
tial of this quinone has been a matter of speculation,
and one theory in current ascendancy focuses on the
protein-mediated modulation of a p-stacking interac-
tion with the adjacent indole ring of TrpA697.6 From
this perspective, the p-stacking interaction is viewed
as energetically favorable for the parent quinone, but
destabilizing (and yet structurally unavoidable) for
the reduced species. Nevertheless, the contributions of
the singular H-bond to 2 cannot be ignored, especially
when circumstantial evidence suggests that this H-
bond, in contrast to the dual H-bonded arrangement
exhibited for the analogous quinone 1 in bRC, is par-
ticularly strong as a consequence of an extended H-
bonding network provided by the protein matrix and
associated waters.7 Thus, the overarching question
emerges, how can amide H-bonding, and more pre-
cisely, the strength of amide H-bonding, influence a
quinone’s reduction potential?
O
R
H
R1
MnO2 (20 equiv.)
NHBOC
N
O
O
(R2)3C
AcCl
or TFAA
N
O
O
4
Et2O
reflux, 24 h 75%
OH
8a R1 = R2 = H
100%
100%
TFA
5 R = BOC
6 R = H
7 R = CH3
100%
8b R1 = H, R2 = F
3
9a R1 = CH3, R2 = H 98%
9b R1 = CH3, R2 = F 98%
(CH3)2SO4,
NaOH, Bu4NBr, 90%
OH
1) MnO2 (8 equiv.), Et2O
R
R
H
H
TFA
N
N
O
O
2)
11 R = BOC
12 R = H
10
NHBOC
BOCHN
96%
AcCl or
tol
reflux, 27 h; DDQ,
24h reflux
TFAA
O
90%
H
R3C
N
O
R3C
N
O
H
The relationship between H-bond strength and quinone
reduction potential11 was probed using simple model
5-amido and 5,8-diamidonaphthoquinones. This
approach is precedented by a long history of studying
the relationship between the reduction potential of ortho
hydroxy- and amino substituents on benzo- and naph-
thoquinones,12 and peri-positioned hydroxy, and in a
few examples amino,13a groups on naphtho- and 9,10-
anthroquinones.13 The upshot of these earlier studies is
that a peri-substituted hydroxyl (i.e., 5-hydroxy or 5,8-
dihydroxy in the naphthoquinones series) renders the
quinone more easily reduced as a consequence of the
LUMO-lowering effect of a geometrically favorable14
internal H-bond. The ortho-hydroxylated (aminated)
benzoquinones, on the other hand, do not consistently
follow this trend. Unfortunately, the H-bond strength
of the peri-hydroxyls cannot be modulated, and so these
types of substrates cannot be employed to test the
hypothesis that reduction potential scales with H-bond
strength. However, the use of a series of naphthoqui-
nones bearing peri-positioned amide units with deliber-
ately varying H-bond donor capabilities can overcome
this limitation and represents one approach by which
the effect of varying H-bond strength on quinone reduc-
tion potential can be probed.
O
13a R = H ~ 100%
13b R = F ~ 100%
thermal elipsoid plot of 8a
Scheme 1. Synthesis of the target naphthoquinones 5–9, and 11–13;
X-ray structure of 8a. The thermal ellipsoid diagram (50% probability)
was obtained with Bruker XSHELL software.
ucts 5 and 11. These Boc-protected intermediates then
served as branch points for the preparation of all of
the other aminonaphthoquinone derivatives. All
quinone products were characterized fully,18 including
19
single crystal X-ray for 8a (Supplementary data).
The X-ray derived structure clearly indicated the domi-
nating effect of the N–H–O@C H-bond, which held the
amide side chain in a coplanar orientation with the qui-
none framework.
Measurement of half-wave reduction potentials of the
substituted naphthoquinones in CH2Cl2 led to the data
shown in Table 1. These values were obtained under
the following standard conditions: Pt disc electrode
(1.6 mm diameter), Ag/AgNO3 reference (0.01 M in
CH3CN), 2 mM in quinone, 100 mM in TBAP, 3 V/s,
0 to À2496 mV range, T = 20 °C. Under these condi-
tions, ferrocenium ion exhibited a reduction potential
of +200 mV (lit. 206 mV vs Ag/AgPF6).20 The IR fre-
quency of the H-bond, which is well known to scale with
H-bond strength, and the 1H NMR H-bond signal posi-
tion, a value also related to H-bond strength,21 are
reported in Table 1 as well. Concentration studies with
8a over the span 1–100 mM revealed that neither the
This premise was explored by synthesis of both unsym-
metrically and symmetrically peri-functionalized 2,3-
dimethylnaphthoquinones 5–9 and 11–13, respectively,
and correlation of salient molecular attributes reflecting
H-bond strength with their half-wave reduction poten-
tials. The syntheses of the target quinones were accom-
plished by modification of Diels–Alder/oxidation
chemistry reported by Fillion15 using the mono
amidobutadiene 416 and the bis amidobutadiene 10,17
Scheme 1 (see Supplementary data for details). In this
chemistry, the efficiency of naphthoquinone formation
was increased over prior work by consolidating the oxi-
dation–cycloaddition–oxidation sequence into a single
pot operation. Simply exposing the dihydroquinone 3
and the requisite diene to excess oxidant (MnO2) fur-
nished good yields of the desired naphthoquinone prod-
1
IR nor the H NMR signal values varied significantly
( 1 cmÀ1, and 0.05 ppm, respectively) throughout the
experimental range.
Examination of these data reveals a clear correlation
between the half-wave reduction potential and both
H-bond strength metrics. However, a dissection of the
two related effects that result when an amide is intro-
duced onto the naphthoquinone framework is necessary
in order to make further sense out of the data.

K. S. Feldman et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4891–4894
4893
Table 1. Half-wave reduction potentials, salient IR frequencies, and 1H NMR signal positions for the naphthoquinones 14
R
O
R1
O
14
a
Red. Pot. CH2Cl2 (mV)
Entry
Compound
R
R1
N–H IR CCl4 (cmÀ1
)
N–H 1H NMR C6D6 (d)
a
b
c
d
e
f
H
NH2
H
À623
À721
À697
À551
À484
À347
À560
À554
À819
À419
À359
À108
—
—
6
H
3339
3304
3263
3262
3109
—
Not observed
9.29
11.77
7
NHCH3
NHBoc
NHAc
H
5
H
8a
8b
9a
9b
12
11
13a
13b
H
11.92
13.08
NHTFA
N(CH3)Ac
N(CH3)TFA
NH2
H
g
h
i
H
—
—
H
—
NH2
NHBoc
NHAc
NHTFA
3288
3240
3201
3076
Not observed
12.06
12.18
j
NHBoc
NHAc
NHTFA
k
l
13.18
a All reduction potentials have been converted to NHE values via the formula: E(NHE) = E(AgNO ) + 541 mV; see Handbook of Analytical Chemistry;
3
Meites, L., Ed.; McGraw-Hill: New York, 1963.
Both the (1) strong and directional H-bond between
N–H and O@C and the (2) contributions of the nitro-
gen’s lone pair via resonance with the carbonyl can play
a role. Much precedent from the peri-hydroxylated cases
suggest that H-bond activation of the carbonyl exerts
the dominant influence, between these two interactions,
on quinone reduction potential.12a,13a–c,e In an effort to
probe the relative contributions of these two features
in the amide-substituted series examined herein, the con-
trols 6, 7, 9a, and 9b were evaluated. In the absence of
any electron-engaging carbonyl (amide) functions, the
nitrogen’s lone pair certainly has the expected effect on
the quinone’s reduction potential, making it harder to
reduce by 98 mV; compare entry a (R = H) with entry
b (6, R = NH2). In this instance, the N–H–O@C bond
(IR absorption at 3339 cmÀ1) can only be described as
modest at best, and so this comparison is closest to a
case where the H-bonding is turned off whereas the
nitrogen lone pair contribution is turned on. Designing
the complementary control (H-bonding turned off,
nitrogen lone pair turned on) cannot be assured in this
system due to uncertainty in the degree of overlap
between the nitrogen’s lone pair and the quinone, but
entries 9a and 9b, where H-bonding has been abolished
by methyl incorporation, come closest to approximating
this goal. In these instances, the quinone reduction po-
tential is depressed compared to the H-bonding ana-
logues 8a/8b by 76 mV (8a vs 9a) and 207 mV (8b vs
9b), respectively. In fact, the large difference between
these values appears to be attributable primarily to the
increased strength of the H-bond in the NHTFA case
8b, as the reduction potentials of the H-bond incapable
species 9a and 9b are essentially equivalent. From these
data, it is apparent that whereas the nitrogen lone pair
definitely contributes to depression of the quinone’s
reduction potential, its effects are relatively smaller than
the H-bond effect and consistent throughout the rele-
vant substrates. Therefore, the differences in measured
reduction potential between the different acylated
aminoquinone substrates should to a large measure
reflect the impact of the H-bond, much as they do in
the more well-studied peri-hydroxylated series.
Overall, the data for substrates 5, 8a, 8b, 11, 13a, and
13b do support the contention that quinone reduction
potential correlates with the strength of the H-bond,
but the imperfect measures of H-bond strength used
for this correlation do little to encourage linearity. Dou-
ble activation of the quinone function leads to increases
in the reduction potential, as expected, but the amount
of increase does not scale with increased H-bond
strength. Thus, introducing an additional H-bond in
the Boc species leads to an increase in reduction poten-
tial of 132 mV (entries d vs j), whereas in the NHAc ser-
ies (stronger H-bond), the difference is 125 mV (entries e
vs k) and in the NHTFA analogues (strongest H-bond),
239 mV (entries f vs l). Extrapolation from these model
compounds to photosynthetically relevant quinones
embedded in protein matrices must be viewed with cau-
tion, but at the very least, the following hypotheses are
supported by the data: (1) stronger N–H–O@C hydro-
gen bonds, as might be provided by H-bond networks
in the enveloping proteins, make the quinones more eas-
ily reduced, and (2) simultaneous N–H–O@C hydrogen-
bonding to both quinone carbonyls makes the quinones
even more easily reduced than simple additivity might
suggest.
Acknowledgments
Funding via the National Institutes of Health, General
Medical Sciences Division (GM 37681) and the
Molecular Biochemistry/Biophysics Program of the
National Science Foundation (MCB-0519743) is
gratefully acknowledged. In addition, the National
Science Foundation is acknowledged for the X-Ray
structural analysis facility (CHE-013111).

4894
K. S. Feldman et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4891–4894
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